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The spatio-temporal characteristics of ULF waves driven by substorm injected particles
Abstract
A previous case study observed a ULF wave with an eastward and equatorward phase propagation (an azimuthal wave number m, of ∼13) generated during the expansion phase of a substorm. The eastward phase propagation of the wave suggested that eastward drifting energetic electrons injected during the substorm were responsible for driving that particular wave. In this study, a population of 83 similar ULF wave events also associated with substorm-injected particles have been identified using multiple Super Dual Auroral Radar Network radars in Europe and North America between June 2000 and September 2005. The wave events identified in this study exhibit azimuthal wave numbers ranging in magnitude from 2 to 92, where the direction of propagation depends on the relative positions of the substorm onsets and the wave observations. We suggest that azimuthally drifting energetic particles associated with the substorms are responsible for driving the waves. Both westward drifting ions and eastward drifting electrons are implicated with energies ranging from ∼1 to 70 keV. A clear dependence of the particle energy on the azimuthal separation of the wave observations and the substorm onset is seen, with higher energy particles (leading to lower m-number waves) being involved at smaller azimuthal separations.
... We cannot precisely identify the substorm location with only ground magnetometers and also considering that the Atlantic lies to the west with no magnetometers in contrast to, for example, James et al. (2013), where they used substorms located by their auroral signatures. However, Figure 9 depicting a positive and negative bay in the midlatitude X-and Y-component magnetic field respectively, suggests that the substorm is near and to the west of the IMAGE magnetometers. ...
... However, Figure 9 depicting a positive and negative bay in the midlatitude X-and Y-component magnetic field respectively, suggests that the substorm is near and to the west of the IMAGE magnetometers. The variations of magnetic bays in the components of the geomagnetic field close to the substorm current wedge (SCW) can be used to deduce the location of the substorm (e.g., James et al., 2013). A decrease in the X-component coinciding with the Pi2 burst at higher latitude as shown previously in the top panels of Figure 8 indicates a westward electrojet. ...
... The dissimilarities in the driving mechanism ascribed to previous observations of intermediate-m waves (e.g., Yeoman et al., 2010;James et al., 2013;Hao et al., 2014;Mager et al., 2019) observed by radar and spacecraft, illustrates the complexities in predicting processes driving such a wave. ...
An ultralow frequency (ULF) wave was simultaneously observed in the ionosphere by the Super Dual Auroral Radar Network (SuperDARN) radar at Hankasalmi, Finland and on the ground by the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers with close proximity to the radar. The onset time of the wave event was around 03:00 magnetic local time. Fourier wave analysis of the event suggests a wave period of about 1,340 s with an equatorward latitudinal and eastward longitudinal wave phase propagation, and an effective azimuthal wave number of 17 ± 1, in the intermediate range of those observed in ULF waves. This wave has been interpreted as resulting from drifting electrons of energies of 13 ± 5 keV in a drift resonance condition linked to energetic particle populations during a magnetospheric substorm. The latitudinal phase characteristics of this wave experienced temporal evolution, believed to be caused by additional injected particle populations associated with the same substorm driving the wave, which resulted in an observed loss of HF backscatter. This observation of a unique type of temporal evolution in the phase propagation characteristics of ULF waves enhances current understanding about the structure, dynamics and source of these types of ULF waves.
... Substorms associated with HSS can also contribute to the production of Ultra Low Frequency -ULF waves (a few millihertz up to about 5 Hz) (e.g., Claudepierre et al., 2008;Elkington, 2006;Elkington et al., 1999;Kavosi & Raeder, 2015;Kivelson & Southwood, 1985;Mann et al., 1999) through energy deposition (James et al., 2016(James et al., , 2013Yeoman et al., 2010). HSSs' energy transfer to the magnetosphere via Kelvin-Helmholtz instabilities can lead to the generation of the ULF waves, which also have an essential role in accelerating medium-energy electrons up to high-energies (i.e., relativistic) through inward radial diffusion (Da Silva et al., 2019;Jaynes et al., 2018;Souza et al., 2017;Zhao et al., 2018). ...
... High resolution (5-min averaged) AE index (panel g) indicates peaks next to 400 nT (median) and 750 nT (upper quartiles) after the zero epoch, which suggests the presence of the substorms during the global recovery of the high-energy electron flux. Alfvénic fluctuations associated with the HSSs, Bz southward, and substorms can contribute to the occurrences of the low-energy electron particle injections in the outer radiation belt and to the generation of magnetospheric waves in a wide range of frequency (Da Silva et al., 2019;James et al., 2013James et al., , 2016Jauer et al., 2019;Jaynes et al., 2015;Souza et al., 2017;Thorne et al., 2013;Yeoman et al., 2010). ...
... The chorus waves can interact resonantly with the seed electron population to promote local acceleration, the sixth key ingredient. The injection of low-energy and medium-energy electrons into the Earth's inner magnetosphere during the substorms can also drive the generation of ULF waves (see James et al., 2013James et al., , 2016Yeoman et al., 2010), the seventh ingredient. The ULF waves can also interact resonantly with the seed electron population to accelerate the particles through the radial diffusion mechanism (Da Silva et al., 2019 andSouza et al., 2017), as shown in the eighth key ingredient. ...
The coupling response between solar wind structures and the magnetosphere is highly complex, leading to different effects in the outer radiation belt electron fluxes. Most Coronal Mass Ejections cause strong geomagnetic storms with short recovery phases, often 1–2 days. By contrast, High‐Speed Solar Wind Streams lead to moderate and weak storms often with much longer recovery phases, from several to ∼10 days. The magnetosphere receives energy for a long time under the influence of the HSSs, considerably changing its dynamics. This in turn has an effect on the charged particles trapped in the outer radiation belt. Although the high‐energy electron flux enhancements have received considerable attention, the high‐energy electron flux enhancement pattern (L > 4) has not. This paper identifies 37 events with this enhancement pattern in the high‐energy electron flux during the Van Allen Probes era. We find the enhancements coincident with HSS occurrence. The interplanetary magnetic field (IMF) exhibits north/south Bz fluctuations of Alfvénic nature with moderate amplitudes. The high‐energy electron flux enhancements also correspond to long periods of auroral activity indicating a relationship to magnetotail dynamics. However, the AE index only reaches moderate values. Ultra‐Low Frequency waves were present in all of the events and whistler‐mode chorus waves were present in 89.1% of the events, providing a convenient scenario for wave‐particle interactions. The radial gradient of the ULF wave power related to the L, under the influence of the HSSs, is necessary to trigger the physical processes responsible for this type of high‐energy electron flux enhancement pattern.
... The arrival of such solar wind structures can also generate a wide range of magnetospheric oscillation frequencies, among them, the ULF wave modes, which are recognized as efficient accelerators of the seed population particles responsible for the outer radiation belt global recovery (Summers et al., 2002;Horne & Thorne, 2003;Thorne et al., 2005;Elkington, 2006;Elkington et al., 1999;Summers & Ma, 2000;Perry et al., 2005, Ozeke et al., 2014 andothers). James et al. (2013) performed a statistical analysis of 83 substorm events, and they showed that substorm activity can function as an internal (i.e., within the Earth's magnetosphere) source of poloidal-mode (high-m) ULF waves. On the other hand, James et al. (2015) analyzed three individual substorm events and showed that the magnetospheric ULF waves can be driven by sources both internal and external to the magnetosphere. ...
... During the global electron flux recovery, both Alfvénic fluctuations in the three components of the IMF and a southward turning of the average IMF Bz component contributed to provide energy input to the Earth's inner magnetosphere via dayside reconnection, which in turn may drive enhanced substorm activity (see, e.g., Gonzalez et al., 1994). Also, it has been shown in the literature that substorm activity may induce ULF wave generation (see, e.g., James et al., 2013James et al., , 2015. While James et al. (2013) suggest that ULF waves can be driven by energy sources internal to the magnetosphere, James et al. (2015) suggest that the ULF waves can be driven by energy sources coming from both internal and external to the magnetosphere. ...
... Also, it has been shown in the literature that substorm activity may induce ULF wave generation (see, e.g., James et al., 2013James et al., , 2015. While James et al. (2013) suggest that ULF waves can be driven by energy sources internal to the magnetosphere, James et al. (2015) suggest that the ULF waves can be driven by energy sources coming from both internal and external to the magnetosphere. According to James et al. (2015), the external sources can be Kelvin-Helmholtz instability on the magnetopause or solar wind buffeting. ...
Energy coupling between the solar wind and the Earth's magnetosphere can affect the electron population in the outer radiation belt. However, the precise role of different internal and external mechanisms that leads to changes of the relativistic electron population is not entirely known. This paper describes how ultralow frequency (ULF) wave activity during the passage of Alfvénic solar wind streams contributes to the global recovery of the relativistic electron population in the outer radiation belt. To investigate the contribution of the ULF waves, we searched the Van Allen Probes data for a period in which we can clearly distinguish the enhancement of electron fluxes from the background. We found that the global recovery that started on 22 September 2014, which coincides with the corotating interaction region preceding a high‐speed stream and the occurrence of persistent substorm activity, provides an excellent scenario to explore the contribution of ULF waves. To support our analyses, we employed ground‐ and space‐based observational data and global magnetohydrodynamic simulations and calculated the ULF wave radial diffusion coefficients employing an empirical model. Observations show a gradual increase of electron fluxes in the outer radiation belt and a concomitant enhancement of ULF activity that spreads from higher to lower L‐shells. Magnetohydrodynamic simulation results agree with observed ULF wave activity in the magnetotail, which leads to both fast and Alfvén modes in the magnetospheric nightside sector. The observations agree with the empirical model and are confirmed by phase space density calculations for this global recovery period.
... The ULF waves in each event exhibit westward phase propagation and are likely to be driven by westward-drifting populations of recently injected protons. This study aims to extend the work of James et al. [2013], where 83 ULF pulsations were each associated with substorm activity, and whose characteristics were found to vary depending upon azimuthal distance from the location of the particle injections. This study makes use of SuperDARN (Super Dual Auroral Radar Network, Chisham et al., 2007]) data supplemented by ground magnetometer and auroral images to study each multiple ULF wave event in detail. ...
... In contrast, wave energy sources internal to the magnetosphere, such as driftbounce resonances and other wave-particle interactions, often generate waves with high-m numbers, or small scale sizes. It should be noted that there is also evidence that some low-m and intermediate-m ULF waves may also be driven by wave-particle interactions within the magnetosphere [Yeoman et al., 2010;James et al., 2013]. ...
... A statistical study of substorm-driven ULF waves similar to that analyzed by Yeoman et al. [2010] was later undertaken by James et al. [2013] in order to examine the suggestion that the proximity of the wave to the substorm had an effect upon the wave's phase characteristics. James et al. [2013] examined 83 events, in each of which a ULF pulsation was observed shortly after the onset of a substorm. ...
A recent statistical study of ULF waves driven by substorm-injected particles observed using SuperDARN [James et al., 2013] found that the phase characteristics of these waves varied depending on where the wave was observed relative to the substorm. Typically, positive azimuthal wave numbers, m, were observed in waves generated to the east of the substorms, and negative m to the west. The magnitude of m typically increased with the azimuthal separation between the wave observation and the substorm location. The energies estimated for the driving particles for these 83 waves were found to be highest when the waves were observed closer to the substorm and lowest farther away. Each of the 83 events studied by James et al. [2013] involved just a single wave observation per substorm. Here a study of three individual substorm events are presented, with associated observations of multiple ULF waves using various different SuperDARN radars. We demonstrate that a single substorm is capable of driving a number of wave events characterised by different azimuthal scale lengths and wave periods, associated with different energies in the driving particle population. We find that similar trends in m and W exist for multiple wave events with a single substorm as was seen in the single wave events of James et al. [2013]. The variety of wave periods present on similar L-shells present in this study may also be evidence for the detection of both poloidal Alfvén and drift-compressional mode waves driven by substorm-injected particles.
... It had been suggested that such eigenmodes might explain observed quasi-periodic oscillations of the magnetopause (e.g., Archer et al., 2013;Hartinger et al., 2015;Plaschke et al., 2009Plaschke et al., , 2013 and plasmapause (Goldstein et al., 2004(Goldstein et al., , 2007. However, short-timescale impulsive drivers result in complicated superpositions of ultralow frequency (ULF) wave modes (e.g., James et al., 2013;Miyashita et al., 2021;Villante et al., 2016), hindering subsequent progress. Only in recent years have direct, conclusive observational evidence of magnetopause surface eigenmodes (MSE; Archer et al., 2019Archer et al., , 2021 and plasmapause surface waves (PSW; Hao et al., 2023;He et al., 2020;Zhou et al., 2021Zhou et al., , 2022 been captured through combined space-borne and ground-based instruments. ...
... In addition to the transition in wave modes, it is noteworthy that the PSW displays a consistently increasing frequency variation in correspondence with its spatial evolution, that is, ∼0.7 mHz (B r V r ; MMS-1), ∼0.9 mHz (B a ; ERG), ∼1 mHz (B r ; THEMIS-E), ∼1.1 mHz (B r ; THEMIS-A), ∼1.3 mHz (V r ; VAP-A), and ∼1.5 mHz (V a ; VAP-B) along the midnight-dusk-afternoon plasmapause boundary. This intriguing phenomenon likely corresponds well with the presence of waves propagating along increasingly stretched magnetic field lines from the afternoon to the midnight plasmapause region, as documented in previous studies (e.g., James et al., 2013;Samson et al., 1992;Yeoman et al., 2010). Noteworthy, other ULF waves may complicate the PSW frequency and cause frequency uncertainty to some extent in Figure 4, such as the ULF waves generated by the feedback instability driven by the electric field in the ionosphere (Streltsov & Mishin, 2020) and the spatially localized enhanced ULF waves in the plasmaspheric plume (Sandhu et al., 2021). ...
Plain Language Summary
Much like the rhythmic beating of a drum, boundaries within the terrestrial system, which delimit distinct plasmas with varying temperatures and densities, can oscillate due to external impulses or internal instabilities, giving rise to surface waves. These surface waves travel along the boundary surface and establish standing wave structures in magnetic field lines. In the terrestrial space environment, two types of surface waves have been identified: the magnetopause surface wave and the plasmapause surface wave (PSW). These waves typically have frequencies ranging from fractions of milli Hertz to a few milli Hertz, with periods spanning from several to tens of minutes. They are believed to play a vital role in mass, energy, and momentum transport within Earth's magnetosphere. However, the exact source that excites the waves and the manner in which the waves evolve along the boundary remain elusive. In this study, we, for the first time, unveil the spatial evolutionary signatures of the PSW during a moderate geomagnetic storm accompanied by substorms, utilizing data from six magnetospheric spacecrafts positioned near the plasmapause. This research contributes to a better understanding of the physical mechanisms underlying the widespread existence of surface waves in the universe, shedding light on the processes of magnetosphere‐plasmasphere‐ionosphere energy couplings and wave‐particle interactions associated with surface waves.
... • The morning/dusk flanks of the outer magnetosphere are regions with high-β plasma which is favorable for excitation of plasma instabilities. The injection of energetic electrons to the morning flank and hot protons to the dusk flank during substorms was found to be a trigger of long-period wave excitation [James et al., 2013]. Magnetospheric pulsations driven by instability of the injected particles may contribute to Pi3 activity [Vaivads et al., 2001]. ...
... The pulsations were propagating in the sunward direction and were often related to substorm activity. Also, Pi3 waves with an intermediate azimuthal wave number m=2-90, were recorded during the expansion phase of a substorm using multiple Super Dual Auroral Radar Network radars [James et al., 2013]. Azimuthally drifting energetic particles associated with the substorms were suggested to be responsible for driving the waves. ...
We consider interhemispheric properties of fine structure of substorm – quasi-periodic geomagnetic fluctuations, Pi3 pulsations, using data from conjugate magnetometers in Antarctica and Greenland. Pi3 pulsations are found to accompany both the substorm expansion/recovery phases and the steady magnetospheric convection (SMC) events. The epicenter of Pi3 power is at the same latitude as maximal amplitude of magnetic bays. The interhemispheric properties of Pi3 pulsations are not consistent: in some events, coherent in-phase magnetic oscillations are observed in both hemispheres, in others, periodic variations are observed in one hemisphere only. When Pi3 pulsations are observed in both conjugate sites, their H-components are in-phase, which corresponds to the fundamental mode of field line oscillations between high-conductive ionospheres. Conjugate observations have provided an additional information on an elusive mechanism of Pi3 pulsations.
... The amplitude of the AE index is higher at t 0 and remains with auroral activity up to 3 days after, for both ICME and HSS, indicating that, as expected, the substorm activity (Tsurutani et al., 1995) increased during Alfvenic fluctuations (Da Silva et al., 2019;Jaynes et al., 2015). Prolonged periods of southern fluctuating IMF Bz (Zhang et al., 2014) can trigger the dayside reconnection process in the Earth's magnetosphere (Gonzalez et al., 1994), which can drive ULF waves (James et al., 2013(James et al., , 2016. We indeed observe high values of IPSD B (Figure 7-left panels) concomitant with the electron flux enhancement period after t 0 (Figure 5a). ...
... Thus, the higher compressional and poloidal modes seem to contribute to enhancement cases and persist during 3-4 days, following the behavior of the AE index and IMF Bz. During the reduction cases, both IPSD B and IPSD E are lower after t 0 than at t 0 , which suggests an absence of seed populations that could be accelerated in the subsequent hours (Bingham et al., 2018;James et al., 2013James et al., , 2016. ...
The ultra‐low frequency (ULF) waves can stochastically accelerate radiation belt electrons. Radial diffusion is a well‐established mechanism that can enhance or reduce the electron population in combination with other processes. Using data from the Van Allen Probes, we investigated the response of the 2.10 MeV energy electrons and ULF waves to two types of solar wind structures interacting with Earth's magnetosphere, namely, interplanetary coronal mass ejections (ICMEs) and High‐Speed solar wind streams (HSS). We use measured electron differential flux and ULF waves in the Pc4–Pc5 frequency range from October 2012 to May 2019. We examine 155 events with changes in the outer radiation belt electron differential flux. Results considering all ICMEs and HSSs during the Van Allen Probes era show that for both solar wind structures, solar wind interplanetary magnetic field Bz, solar wind proton density, and speed are related to the outer radiation belt relativistic electrons' response. The persistent ULF power is present during enhancement cases, while for reduction, the ULF waves power is concentrated at the initial reduction on the outer radiation belt electron flux.
... ULF wave power in these sectors is weakly linked to solar wind driving (Takahashi et al., 2012), and the sources of this wave power can instead be attributed to internal sources of ULF wave activity. Coupling of ULF waves and westward drifting ring current ions can drive enhancements in narrowband ULF wave activity predominantly in the afternoon and dusk MLT sectors (Baddeley et al., 2005;Engebretson & Cahill, 1981;Hughes, 1983;James et al., 2013James et al., , 2016Nosé et al., 1998;Woch et al., 1990). Referring to Figure 2, afternoon sector enhancements during the main and recovery phase relative to the initial phase are apparent and are particularly prominent for the magnetic field component. ...
... Studies show that externally driven waves that dominate the dayside MLT sectors typically have low m numbers (Rae et al., 2005;Tu et al., 2012), whereas ULF waves on the nightside have markedly higher m numbers (Barani et al., 2019;Fenrich et al., 1995;Sarris & Li, 2017). The high m ULF waves can originate from substorm-related activity, where they can be driven by drift-bounce resonances with injected ions (James et al., 2013;Walker et al., 1982;Yeoman et al, 2010). The presence of these high m number waves has implications for our estimations of radial diffusion coefficients. ...
The impact of radial diffusion in storm time radiation belt dynamics is well‐debated. In this study we quantify the changes and variability in radial diffusion coefficients during geomagnetic storms. A statistical analysis of Van Allen Probes data (2012–2019) is conducted to obtain measurements of the magnetic and electric power spectral densities for Ultra Low Frequency (ULF) waves, and corresponding radial diffusion coefficients. The results show global wave power enhancements occur during the storm main phase, and continue into the recovery phase. Local time asymmetries show sources of wave power are both external solar wind driving and internal sources from coupling with ring current ions and substorms. Wave power enhancements are also observed at low L values (L < 4). The accessibility of wave power to low L is attributed to a depression of the Alfvén continuum. The increased wave power drives enhancements in both the magnetic and electric field diffusion coefficients by more than an order of magnitude. Significant variability in diffusion coefficients is observed, with values ranging over several orders of magnitude. A comparison to the Kp parameterized empirical model of Ozeke et al. (2014) is conducted and indicates important differences during storm times. Although the electric field diffusion coefficient is relatively well described by the empirical model, the magnetic field diffusion coefficient is approximately ∼10 times larger than predicted. We discuss how differences could be attributed to data set limitations and assumptions. Alternative storm‐time radial diffusion coefficients are provided as a function of L* and storm phase.
... Arrays of longitudinally separated ground magnetometers offer much better coverage and have also been used for mode number estimation (Chisham & Mann, 1999). However, for high m values, significant amplitude attenuation is expected for ULF waves propagating down to the ground due to the ionospheric screening effect (James et al., 2013;Nishida, 1978), which imposes an extra limitation in resolving physical m values of ULF waves from ground measurements. Due to the difficulties in resolving the actual mode structure of ULF waves, simplified assumptions have been made in previous estimations of the radial diffusion coefficients. ...
... In contrast, during the storm main phase and early recovery phase when the solar wind P dyn is low and AE is high, the XWT results show that the wave power spreads almost evenly over all different mode numbers from low to high. The strong correlation between the appearance of the high-mode ULF waves and high AE in Figure 5 suggests that these waves may be driven by small-scale substorm injections from the nightside (e.g., James et al., 2013;Yeoman et al., 2010). By investigating the MLT-distribution of the mode structure during the high AE period from multiple pairs of GOES satellites, we find that the evenly distributed mode structure or the significant contribution from high modes is not limited to localized regions close to midnight, as generally assumed, and can spread all the way to the MLT regions on the dayside. ...
Characterizing the azimuthal mode number, m , of ultralow‐frequency (ULF) waves is necessary for calculating radial diffusion of radiation belt electrons. A cross‐spectral technique is applied to the compressional Pc5 ULF waves observed by multiple pairs of GOES satellites to estimate the azimuthal mode structure during the 28‐31 May 2010 storm. We find that allowing for both positive and negative m is important to achieve a more realistic distribution of mode numbers and to resolve wave propagation direction. During the storm commencement when the solar wind dynamic pressure is high, ULF wave power is found to dominate at low‐mode numbers. An interesting change of sign in m occurred around noon, which is consistent with the driving of ULF waves by solar wind buffeting around noon, creating antisunward wave propagation. The low‐mode ULF waves are also found to have a less global coverage in magnetic local time than previously assumed. In contrast, during the storm main phase and early recovery phase when the solar wind dynamic pressure is low and the auroral electrojet index is high, wave power is shown to be distributed over all modes from low to high. The high‐mode waves are found to cover a wider range of magnetic local time than what was previously assumed. Furthermore, to reduce the 2nπ ambiguity in resolving m , a cross‐pair analysis is performed on satellite field measurements for the first time, which is demonstrated to be effective in generating more reliable mode structure of ULF waves during high auroral electrojet periods.
... The variation of resonating magnetospheric field lines with changing geomagnetic and solar wind conditions. Toroidally polarized Alfvén waves usually have a large azimuthal scale size and are thought to be excited by sources external to the magnetosphere on the dayside (e.g., Agapitov et al., 2009;James et al., 2013). These external sources generate fast mode waves at or beyond the magnetopause. ...
... This could be because the Hall conductivity at night is too low to generate a detectable magnetic field. Much of the wave activity on the nightside is driven by substorms too, which drive poloidal mode waves (e.g., James et al., 2013;Wright & Yeoman T. K., 1999;Yeoman et al., 2010;). However, all of these observations were of the toroidal mode. ...
Standing ultralow frequency waves redistribute energy and momentum around the Earth's magnetosphere. The eigenfrequencies of these standing waves can be measured by applying the cross‐phase technique to ground magnetometer data. To make a detection, the flux tubes in the vicinity of the magnetometers must all be driven at their local eigenfrequencies by a source with a sufficient frequency width. Therefore, successful measurement of the local eigenfrequencies indicates that a broadband source is exciting the flux tubes. We have analyzed 10 years of magnetometer data with an automated cross‐phase algorithm and used correlations with the OMNI data set to understand under what conditions broadband excitation occurs and how the conditions affect the eigenfrequency values. This is the largest such survey of its kind to date. We found that lower eigenfrequencies at higher latitudes (L>5) and higher eigenfrequencies at lower latitudes (L<4) were excited under different conditions. It was also possible to directly compare the first and third harmonics at midlatitudes. The lower eigenfrequencies were excited during more disturbed conditions, and we suggest that these harmonics are driven by solar wind pressure pulses or the Kelvin‐Helmholtz instability at the magnetopause. The higher eigenfrequencies were excited when the magnetosphere was relatively quiet, and we suggest that the cause was waves generated upstream of the Earth's bow shock. The eigenfrequencies were observed to decrease in the middle magnetosphere during disturbed intervals. This is because the intensification of the ring current weakens the magnetic field. Variations in magnetic local time and latitude were also investigated.
... The Super Dual Auroral Radar Network (SuperDARN) is a global network of ground-based high frequency (HF: 3-30 MHz) radars designed primarily for studying ionospheric plasma convection [Chisham et al., 2007]. The line-of-sight (LOS) velocity measured by SuperDARN radars can be used to detect and monitor ionospheric ULF wave signatures with both high-and low-m [Fenrich et al., 1995;James et al., 2013] and with a total geographical coverage area that cannot be achieved with any other ground-or space-based instrumentation. Based on a new data display technique developed by Ponomarenko et al. [2003], SuperDARN detected ULF wave signatures have been characterized in a few recent studies [e.g., Sakaguchi et al., c 2018American Geophysical Union. ...
... This indicates a connection between the occurrence of Pc4 events with prior substorm activity and the decay of the ring current. Numerous studies have shown that poloidal ULF waves are capable of efficiently interacting with energetic particles in the ring current and the radiation belt during both geomagnetically quiet and active times [Hudson et al., 2004;James et al., 2013;Chi and Le, 2015;Dai et al., 2015]. A small convection electric field at quiet times and plasmaspheric refilling during the recovery phase of geomagnetic storms might play roles in the generation of internal instabilities that drive these waves. ...
Ionospheric signatures of ultralow frequency (ULF) wave in the Pc3-5 band (1.7–40.0 mHz) were surveyed using ∼ 6-s resolution data from Super Dual Auroral Radar Network (SuperDARN) radars in the Northern Hemisphere from 2010 to 2016. Numerical experiments were conducted to derive wave period-dependent thresholds for automated detection of ULF waves using the Lomb-Scargle periodogram technique. The spatial occurrence distribution, frequency characteristics, seasonal effects, solar wind condition, and geomagnetic activity level dependence have been studied. Pc5 wave events were found to dominate at high and polar latitudes with a most probable frequency of 2.08 ± 0.07 mHz, while Pc3-4 waves were relatively more common at midlatitudes on the nightside with a most probable frequency of 11.39 ± 0.14 mHz. At high latitudes, the occurrence rate of Pc4-5 waves maximizes in the dusk sector and during winter. These events tend to occur during low geomagnetic activity and northward interplanetary
magnetic field. For the category of radially bounded but longitudinally extended Pc4 events in the duskside ionosphere, an internal driving source is suggested. At midlatitudes, the poloidal Pc3-4 occurrence rate maximizes premidnight and during equinox. This tendency becomes more prominent with increasing auroral electrojet (AE) index and during southward interplanetary magnetic field, which suggests that
many of these events are Pi2 and Pc3-4 pulsations associated with magnetotail dynamics during active geomagnetic intervals. The overall occurrence rate of Pc3-5 wave events is lowest in summer, which suggests that the ionospheric conductivity plays a role in controlling ULF wave occurrence.
... The excited poloidal standing Alfvén waves in the Pc 4-5 band then interact with energetic particles via drift-bounce resonance moderate m-number in the second harmonic. Fundamental mode poloidal waves are possibly related to eastward-drifting injected electrons (James et al. 2013). As shown in Fig. 12, the observed dispersionless energetic electron injections may excite poloidal mode ULF waves (first harmonic) through the drift-bounce resonance mechanism. ...
... If so, is energization of radiation belt electrons caused by ring current ion-excited second-harmonic poloidal ULF waves of moderate m-number ? Alternatively, do fundamental mode poloidal ULF waves excited by eastward-drifting injected electrons (James et al. 2013) accelerate ring current ions? 5. In the inner magnetosphere, the ULF waves are usually localized both radially and azimuthally. ...
One of the most important issues in space physics is to identify the dominant processes that transfer energy from the solar wind to energetic particle populations in Earth’s inner magnetosphere. Ultra-low-frequency (ULF) waves are an important consideration as they propagate electromagnetic energy over vast distances with little dissipation and interact with charged particles via drift resonance and drift-bounce resonance. ULF waves also take part in magnetosphere-ionosphere coupling and thus play an essential role in regulating energy flow throughout the entire system. This review summarizes recent advances in the characterization of ULF Pc3-5 waves in different regions of the magnetosphere, including ion and electron acceleration associated with these waves.
... Both fundamental [Takahashi et al., 2011;Dai et al., 2013Dai et al., , 2015 and second harmonic [Liu et al., 2013;Leonovich et al., 2015] of the high-m waves are observed, which are explained in terms of various kinds of kinetic plasma instabilities [e.g., Southwood, 1980;Baddeley et al., 2005]. Besides, the high-m Alfvén waves can be generated by alternating currents of substorm injected particles Zolotukhina et al., 2008;James et al., 2013]. ...
... However, it does not mean that all high-m Pc5 waves must be identified with this mode. Probably, a majority of high-m Pc5 waves observed with radars represent fundamental or second harmonic Alfvén waves with a poloidal polarization [Fenrich et al., 1995;Yeoman et al., 2012;James et al., 2013]. ...
A case study of shortwave radar observations of magnetospheric Pc5 ULF waves (wave periods of 150–600 s) that occurred on 26 December 2014 in the nightside magnetosphere during substorm activity is presented. The radar study of waves in the magnetosphere is based on analysis of scattering from field-aligned irregularities of the ionospheric F-layer. Variations of their drift velocity at F-layer heights are associated with the wave electric field. Analysis of the observations from the Ekaterinburg (EKB) radar shows that the frequency f of the observed wave depends on the azimuthal wave number m(positive correlation of about 0.90): an increase in frequency from 2.5 to 5 mHz corresponds to increased m number from 20 to 80. Of the known types of waves in the magnetosphere corresponding to the Pc5 range, only drift-compressional waves have such azimuthal dispersion: the frequency of the drift-compressional mode is directly proportional to the azimuthal wave number and the gradient-curvature drift velocity of energetic particles in the magnetic field. This wave has a kinetic nature and represents the most common kind of the compressional modes, demanding for its existence only finite pressure and plasma inhomogeneity across magnetic shells.
... Therefore, high-m Alfvén waves deep in the magnetosphere can only be excited by sources located at the same magnetic shells where these waves can propagate. Currents in the ionosphere are proposed as such a source in Leonovich and Mazur [1993] and magnetospheric nonsteady currents produced by high-energy particles drifting in the magnetosphere in Pilipenko et al. [2001], Mager and Klimushkin [2008], and James et al. [2013]. If the conditions exist for a high-m Alfvén wave instability, the wave amplitude grows in their propagation area in the magnetosphere. ...
... If the conditions exist for a high-m Alfvén wave instability, the wave amplitude grows in their propagation area in the magnetosphere. These conditions may be due to developing MHD instabilities [Cheng and Lui, 1998;Cheremnykh and Parnowski, 2004;Leonovich and Kozlov, 2014] or to nonequilibrium populations of high-energy charged particles [Southwood, 1980;Chen and Hasegawa, 1991;Klimushkin and Mager, 2011;James et al., 2013;Liu et al., 2013]. ...
A transformation of a monochromatic standing poloidal Alfvén wave into a toroidal wave has been observed. A theoretical interpretation is presented of Alfvén-type monochromatic oscillations observed by the Radiation Belt Storm Probes (RBSP)-A satellite on 23 October 2012 when crossing the plasmapause at 21.45–22.30 UT. It is shown that the process involves a poloidal Alfvén wave transforming into a toroidal Alfvén wave. This transformation can serve as proof that the registered oscillations are a single azimuthally small-scale Alfvén mode. As follows from the results of previous theoretical studies, such Alfvén oscillations are generated on the poloidal resonance shell as poloidal standing Alfvén waves. Next, this wave propagates to the toroidal resonance surface across magnetic shells and transforms into a toroidal standing Alfvén wave. The oscillations are absorbed completely near the toroidal resonance surface due to dissipation of their energy in the ionospheric conductive layer. It is shown that the RBSP-A satellite probably crossed two transformation areas of Alfvén waves, located near the plasmapause transition layer.
... SME index shows high latitude disturbances associated with substorm activity, including plasma cloud injections into the inner magnetosphere from the magnetotail. Substorm injected particles are often thought to be an energy source for ULF waves (James et al., 2013(James et al., , 2016Rubtsov et al., 2018;Zolotukhina et al., 2008). Such clouds of protons with energies from 1 to 100 keV drifting westward cause an unstable proton distribution function ("bump-on-tail") or a positive radial gradient of plasma temperature, resulting in the inner plasma instability (Southwood et al., 1969). ...
Magnetic storms and substorms cause global disturbances in the Earth's magnetosphere. Plasma clouds injected from the magnetotail during storm or substorm drift around the Earth and generate ultra‐low frequency (ULF) waves via various mechanisms. At the same time, the inner part of the magnetosphere called plasmasphere is filled with cold particles and its characteristics are sensitive to the geomagnetic activity level. Previous theoretical and some observational studies suggested plasmasphere and its boundary, plasmapause, are special regions for ULF waves to interact with charged particles. We present a statistical analysis of ULF waves during different geomagnetic conditions. We utilized Arase satellite magnetic field and electron density measurements from March 2017 to December 2020 to investigate spatial distribution of ULF waves and its dependence on the plasmapause location. A 1–2 RE gap between the plasmapause and a region of high transverse waves occurrence rate was found. This gap keeps during quiet geomagnetic conditions when plasmasphere expands, and we concluded that the plasmapause controls the ULF wave distribution in the magnetosphere. ULF wave occurrence rate significantly decreases at quiet time, but dayside and dawnside maxima still occur for poloidal and compressional, and toroidal waves, respectively. Thus, we can distinguish internally and externally excited waves. Average wave frequency distribution revealed field‐line resonance character of toroidal waves as frequency increases toward the Earth. Poloidal and compressional waves distributions clearly distinguish low frequency externally excited waves and high frequency storm‐time pulsations.
... These waves can be excited by a moving source, for example, a substorm-injected cloud of ions (Mager & Klimushkin, 2007, 2008. Eastward drifting ULF waves reported in previous studies were observed at dawn and night sectors (James et al., 2013(James et al., , 2016Mikhailova et al., 2022;Yamamoto et al., 2019;Zolotukhina et al., 2008), but its occurrence rate is small according to Figure 4a. Poloidal and toroidal waves may be observed at the same time, but different energy sources result in different spatial distribution. ...
Ultra‐low frequency waves interact with different particle populations all over the magnetosphere. Some interaction mechanisms are associated with certain wave modes, but is it really so and what about waves interaction between each other? We present a statistical analysis of Pc4 and Pc5 waves in the magnetosphere of the Earth that were observed by Arase satellite from March 2017 to December 2020. These waves were classified by polarization into toroidal, poloidal, and compressional waves. Toroidal and poloidal waves are thought to be Alfvén waves that are eigenoscillations of Earth's magnetic field lines. The former are believed to be generated by external sources, while the latter one—by internal sources. We compared spatial distribution features with well‐known case studies to reveal their nature for all three polarizations. A high inclination of Arase orbit supported a wave field‐aligned structure research. We found that toroidal waves are mostly odd harmonics and poloidal waves are both even and odd harmonics of Alfvén waves, while compressional waves were observed in a narrow equatorial region only. Different wave generation mechanisms that cause a clear difference in spatial distributions of toroidal, poloidal, and compressional waves could excite a specific wave polarization. Surprisingly, the statistics of wave polarization has a normal distribution without separate clusters. We suggest that polarization change and mode coupling processes make mixed polarization the most common type of polarization in the magnetosphere. This result raises the question of how the polarization change process affects wave‐particle interactions responsible for energy transport throughout the magnetosphere.
... In all the cases listed above, the resonant interactions of ULF waves with protons were studied. Interactions with electrons are observed much less frequently Hao et al., 2020;Ren et al., 2017Ren et al., , 2018Ren et al., , 2019, although it was shown (Chelpanov et al., 2019;James et al., 2013) that up to 20% of the observed ULF waves can have positive azimuthal wavenumbers. In that case the directions of the wave propagation and the electron drift coincide, which implies the interaction of the wave with electrons via drift interaction: the acceleration of cold plasmaspheric electrons (Hao et al., 2020;Ren et al., 2017Ren et al., , 2018Ren et al., , 2019 and outer radia tion belt electrons (Mann et al., 2016;Su et al., 2015). ...
We report the first observation of the resonant generation of an ultra‐low frequency wave by energetic elections in the magnetosphere. On 27 October 2012 in the morning side of the magnetosphere Van Allen Probe A registered an ULF wave with period of 100 s and an amplitude of 0.7 nT. A cloud of energetic electrons was observed simultaneously with the wave. It is established that the electron cloud was injected into the magnetosphere as a result of a substorm. Electron fluxes in several energy channels were modulated with the frequency of the observed wave. It is shown that these flux modulations are caused by the drift resonance of 38 keV electrons and the wave being a fundamental harmonic of an Alfvén mode with an azimuthal wavenumber m ∼ 110–115 propagating to the east, and generated through the gradient instability due to steep earthward density gradient of the resonant electrons.
... In all the cases listed above, the resonant interactions of ULF waves with protons were studied. Interactions with electrons are observed much less frequently Hao et al., 2020;Ren et al., 2017Ren et al., , 2018Ren et al., , 2019, although it was shown (Chelpanov et al., 2019;James et al., 2013) that up to 20% of the observed ULF waves can have positive azimuthal wavenumbers. In that case the directions of the wave propagation and the electron drift coincide, which implies the interaction of the wave with electrons via drift interaction: the acceleration of cold plasmaspheric electrons (Hao et al., 2020;Ren et al., 2017Ren et al., , 2018Ren et al., , 2019 and outer radia tion belt electrons (Mann et al., 2016;Su et al., 2015). ...
... This also manifested in the corresponding low azimuthal wave number, which was either close to zero or exhibited slightly negative/positive values before/after 13-14 MLT. The ULF waves in the afternoon sector showed fewer signatures of FLRs, but when identified they might result from the impulsive buffeting from the solar wind and/or waveguide mode weakly coupled with FLR (Chisham & Orr, 1997;Fenrich et al., 1995;Rostoker & Sullivan, 1987;Ziesolleck & McDiarmid, 1995) or drif/drift-bounce resonance process (Claudepierre et al., 2013;Glassmeier et al., 1999;James et al., 2013;Yeoman et al., 2010). Note that the wave's azimuthal and latitudinal structure might be also related to the underlying magnetosphere plasma distribution (Archer & Plaschke, 2015;Degeling et al., 2018) In this case study, we have also shown that while dynamic pressure variations at long time scales (≲1 mHz) directly drove ULF waves at similar frequencies, they influenced the properties of waves at a higher frequency, but not their occurrence (Hartinger et al., 2014). ...
Identifying the nature and source of ultra‐low frequencies (ULF) waves (f ⪅ 4 mHz) at discrete frequencies in the Earth's magnetosphere is a complex task. The challenge comes from the simultaneous occurrence of externally and internally generated waves, and the ability to robustly identify such perturbations. Using a recently developed robust spectral analysis procedure, we study an interval that exhibited in magnetic field measurements at geosynchronous orbit and in‐ground magnetic observatories both internally supported and externally generated ULF waves. The event occurred on 9 November 2002 during the interaction of the magnetosphere with two interplanetary shocks that were followed by a train of 90 min solar wind periodic density structures. Using the Wang‐Sheeley‐Arge model, we mapped the source of this solar wind stream to an active region and a mid‐latitude coronal hole just prior to crossing the Heliospheric current sheet. In both the solar wind density and magnetospheric field fluctuations, we separated broad power increases from enhancements at specific frequencies. For the waves at discrete frequencies, we used the combination of satellite and ground magnetometer observations to identify differences in frequency, polarization, and observed magnetospheric locations. The magnetospheric response was characterized by: (a) forced breathing by periodic solar wind dynamic pressure variations below ≈1 mHz, (b) a combination of directly driven oscillations and wave modes triggered by additional mechanisms (e.g., shock and interplanetary magnetic field discontinuity impact, and substorm activity) between ≈1 and 4 mHz, and (c) largely triggered modes above ≈4 mHz.
... Definitive identification of sources is challenging because intense geoelectric fields are sometimes associated with multiple sources simultaneously. For example, ULF waves can be driven by IP shocks and substorm activity (Oliveira et al., 2020;James et al., 2013). The second geoelectric field peak in the E x component in Figure 8c is associated with ULF waves driven by the IP shock. ...
Intense geoelectric fields during geomagnetic storms drive geomagnetically induced currents in power grids and other infrastructure, yet there are limited direct measurements of these storm‐time geoelectric fields. Moreover, most previous studies examining storm‐time geoelectric fields focused on single events or small geographic regions, making it difficult to determine the typical source(s) of intense geoelectric fields. We perform the first comparative analysis of (a) the sources of intense geoelectric fields over multiple geomagnetic storms, (b) using 1‐s cadence geoelectric field measurements made at (c) magnetotelluric survey sites distributed widely across the United States. Temporally localized intense perturbations in measured geoelectric fields with prominences (a measure of the relative amplitude of geoelectric field enhancement above the surrounding signal) of at least 500 mV/km were detected during geomagnetic storms with Dst minima (Dstmin) of less than −100 nT from 2006 to 2019. Most of the intense geoelectric fields were observed in resistive regions with magnetic latitudes greater than 55° even though we have 167 sites located at lower latitudes during geomagnetic storms of −200 nT ≤ Dstmin < −100 nT. Our study indicates intense short‐lived (<1 min) and geoelectric field perturbations with periods on the order of 1–2 min are common. Most of these perturbations cannot be resolved with 1‐min data because they correspond to higher frequency or impulsive phenomena that vary on timescales shorter than that sampling interval. The sources of geomagnetic perturbations inducing these intense geoelectric fields include interplanetary shocks, interplanetary magnetic field turnings, substorms, and ultralow frequency waves.
... Most poloidal waves are the second harmonic of Alfvén modes (asymmetric in wave electric field), generated through the bounce-drift resonance (Hughes & Grard, 1984;Le et al., 2017;Liu et al., 2013;Min et al., 2017;Takahashi et al., 1990;Takahashi, Oimatsu, et al., 2018). The fundamental (symmetric) harmonic of poloidal waves generated through drift resonance is less common (Dai et al., 2013;James et al., 2013;Mager et al., 2018;. As a rule, the waves of the fundamental harmonic are identified as giant magnetic pulsations, Pg, observed on the ground (Glassmeier et al., 1999;Motoba, Takahashi, Rodriguez, & Russell, 2015;Takahashi et al., 2011Takahashi et al., , 2013Thompson & Kivelson, 2001;Wang et al., 2018). ...
The satellites THA, THD, THE of THEMIS mission (Time History of Events and Macroscale Interactions during Substorms) have consistently recorded quasi‐monochromatic poloidal oscillations of the Pc4‐5 range in the nighttime magnetosphere. Wave activity was recorded after a substorm in the ring current region, between L‐shells 6 and 7.3. According to the data of the THA satellite, the fundamental and second harmonics of Alfvén wave were simultaneously recorded at frequencies of about 7 and 18 mHz, respectively. The satellites THD and THE recorded only the second harmonic and oscillating modulations of proton fluxes with energies of about 50 keV at the harmonic frequency. For this second harmonic wave, the azimuthal wave number m=−54 was estimated using the phase delays of the oscillations in the proton fluxes with different gyrophases. It is shown that the wave was in drift‐bounce resonance with 46 keV protons and was generated through resonant instability caused by non‐equilibrium distribution of ring current protons with the significant earthward spatial gradient at the resonance energy.
... As RBSP-B observation shows, ULF waves are excited and frequency-dependent plasmaspheric hiss waves responses appear. In general, the ULF wave could either be caused by particle injections during substorm into the magnetosphere (e.g., James et al., 2013;Yeoman et al., 2010;Zolotukhina et al., 2007) or be generated by Kelvin-Helmholtz surface wave triggered by the sudden increase of solar wind dynamic pressure (e.g., Claudepierre et al., 2008;Zhang et al., 2010;Zong et al., 2017). It is difficult to tell what causes the ULF wave in this particular IP shock event since both the super substorm and the ULF waves are observed immediately after the IP shock arrival. ...
Plain Language Summary
Plasmaspheric hiss waves are whistler‐mode emissions commonly observed in the plasmasphere. They can resonate with radiation belt electrons and significantly influence the radiation belt structure and change the space environment. After an interplanetary shock arrival, the power spectral densities of plasmaspheric hiss may increase or decrease. Our study presents a case of plasmaspheric hiss in response to an interplanetary shock in the inner magnetosphere at ∼23:00 UT on September 7, 2017. After the interplanetary shock arrival, plasmaspheric hiss in different frequency ranges shows various responses. We find that low‐frequency (<0.08fce) hiss waves disappeared while high‐frequency (>∼0.08fce) hiss waves are modulated by the shock‐induced ultralow frequency (ULF) waves. The results suggest that the low‐frequency (<∼0.18fce) hiss waves originate from chorus wave outside the plasmasphere while the high‐frequency (>∼0.18fce) hiss waves are locally generated as implied by previous studies. Our study contributes to revealing the mechanisms of the whistler‐mode wave generation and propagation as well as the modulation by ULF waves.
... The ULF wave activity is controlled by the solar wind-magnetosphere interaction and processes inside the magnetosphere. The common view is that pulsations of extra-magnetospheric origin have longer azimuthal wavelengths than pulsations generated via wave-particle interaction inside the magnetosphere (Baker et al. 2003;James et al. 2013). Small-scale pulsations at auroral latitudes are the result of different kinetic processes (see, e.g., Baddeley et al. 2004;Mager et al. 2013). ...
Geomagnetically induced currents (GICs) in a quasi-meridional power transmission line on the Kola Peninsula are analyzed during the intervals of Pc5/Pi3 (frequency range from 1.5 to 5 mHz) pulsations recorded at the IMAGE magnetometer network. We have analyzed GIC in a transformer at the terminal station Vykhodnoy ( $$68^{\circ }$$ 68 ∘ N, $$33^{\circ }$$ 33 ∘ E) during the entire year of 2015, near the maximum of the 24th Solar cycle. To quantify the efficiency of GIC generation by geomagnetic pulsations, a ratio between power spectral densities of GIC and magnetic field variations is introduced. Upon examination of the geomagnetic pulsation efficiency in GIC generation, the emphasis is given to its dependence on frequency and spatial scale. To estimate pulsation spatial scales in latitudinal and longitudinal directions, the triangle of stations KEV-SOD-KIL has been used. Large-scale pulsations (with a high spectral coherence, low phase difference, and similar amplitudes at latitudinally separated stations) are found to be more effective in GIC generation than small-scale pulsations. The GIC response also depends on the pulsation scale across the electric power line.
... The ULF wave activity is controlled by the solar wind-magnetosphere interaction and processes inside the magnetosphere. The common view is that pulsations of extra-magnetospheric origin have longer azimuthal wavelengths than pulsations generated via wave-particle interaction inside the magnetosphere (Baker et al, 2003;James et al, 2013). Small-scale pulsations at auroral latitudes are the result of different kinetic processes (see, e.g. ...
Geomagnetically induced currents (GICs) in a meridional power transmission line on the Kola Peninsula are analyzed during the intervals of Pc5/Pi3 (frequency range from 1.5 to 5 mHz) pulsations recorded at the IMAGE magnetometer network. We have analyzed GIC in a transformer at the terminal station Vykhodnoj (68 N, 33 E) during the entire year of 2015, near the maximum of 24-th Solar cycle. To quantify the efficiency of GIC generation by geomagnetic pulsations, a ratio between power spectral densities of GIC and magnetic field variations is introduced. Upon examination of the geomagnetic pulsation efficiency in GIC generation, the emphasis is given to its dependence on frequency and spatial scale. To estimate pulsation spatial scales in latitudinal and longitudinal directions, the triangle of stations KEV-SOD-KIL has been used. Large-scale pulsations (with a high spectral coherence, low phase difference, and similar amplitudes at latitudinally separated stations) are found to be more effective in GIC generation than small-scale pulsations. The GIC response also depends on the pulsation scale across the electric power line.
... They have been observed in the recovery phases of geomagnetic storms (Le et al., 2017;Shi et al., 2018), after geomagnetic storms during low geomagnetic activity (Engebretson et al., 1992;Min et al., 2017;Takahashi, Oimatsu, et al., 2018) and in generally quiet times (Kokubun et al., 1989). They can further be driven by substorm-injected particles (James et al., 2013(James et al., , 2016. ...
We investigate how initially high‐m, poloidal Alfvén waves evolve using a numerical model solving the ideal, cold, linear magnetohydrodynamic (MHD) equations in a 2‐D dipole coordinate system. The curved magnetic geometry provides a key difference between the poloidal and toroidal Alfvén frequencies of any one field line. A polarization rotation from poloidal toward toroidal predicted from the Cartesian box model theory still occurs but now with the waves following contours of Alfvén frequency, which moves the Alfvén wave across field lines. The structure of these contours depends on the harmonic mode along the field line and the equilibrium. We find that the amplitude peak of the poloidal mode moves significantly radially outward in time. When the typically observed azimuthal phase motion of such waves is included, hodograms show a polarization rotation from purely poloidal to a mixed poloidal/toroidal polarization at all locations. Such features could be used to help interpret satellite observations of Pc4‐5 poloidal ultralow frequency (ULF) waves in Earth's magnetosphere.
... Dai et al. (2015) suggested that one important energy source for Pc4 poloidal waves is likely attributed to the decay of ring current through internal instabilities. Besides, substorm-injected ions also can provide free energy for the generation of ULF waves (James et al., 2013;Mager & Klimushkin, 2007;Ren, Zong, Miyoshi, et al., 2017), which will drift from the nighside into the dusk sector ( Birn et al., 1998;Reeves et al., 1990). The onset of instabilities may be controlled by the plasma density increases during the refilling process of plasmasphere ( Engebretson et al., 1992;Liu et al., 2013). ...
Six years of Van Allen Probes data are used to investigate cold plasmaspheric electrons affected by ultralow‐frequency (ULF) waves in the inner magnetosphere (L<7) including spatial distributions, occurrence conditions, and resonant energy range. Events exhibit a global distribution within L= 4–7 but preferentially occur at L∼5.5–7 in the dayside, while there is higher occurrence rate in the duskside than dawnside. They can occur under different geomagnetic activities and solar wind velocities (VS), but the occurrence rates are increasing with larger AE, |SYMH|, and VS. These features are closely associated with the generation and propagation of ULF waves in Pc4 (45–150 s) and Pc5 (150–600 s) bands. Combined with electron observations from HOPE instrument, the resonant energies inferred from wave power indicate that cold electrons at ones to hundreds of electron volts can be affected by ULF waves. This study may shed new light on further investigations on the acceleration and transportation of cold plasmaspheric particles that would affect plasmaspheric material release to the Earth's magnetosphere and instabilities for exciting various waves.
... It has been shown ) that westward drifting ions can excite either westward or eastward propagating poloidal mode waves by drift-bounce resonance. The study of James et al. (2013) shows multiple examples of both eastward and westward propagating poloidal mode waves, driven by substorm particle injections. ...
We examine the effect on drift‐resonant particle dynamics of a strongly peaked internally driven poloidal mode field line resonance (FLR; with specified frequency ω in the Pc5 range and azimuthal mode number m≫1). Using an analytic magneto‐hydrodynamic model in a dipole field to describe the ultra low frequency wave mode, we use the bounce‐averaged formalism of Northrop (1963) to obtain equations of motion for charged particles in the wave frame and find an analytic solution for the case of a temporally constant ultra low frequency wave amplitude profile. Focusing on equatorially mirroring electrons in this study, we demonstrate that, for sufficiently peaked radial profiles, multiple drift resonances appear that are associated with the FLR peak. These are in addition to the well‐known zeroth‐order drift resonance location, occurring when the unperturbed drift speed ϕ̇0 satisfies the resonance condition ( mϕ̇−ω=0). The additional resonances arise because the strongly peaked FLR wave field components provide sufficiently strong perturbations to the azimuthal drift speed to cause multiple zero crossings in the resonance condition. These additional resonances have trapping periods much lower than that of the zeroth‐order resonance and considerably complicate the electron dynamics. Further properties of these resonances and their measurable effect on electron dynamics are discussed. For example, their effect on observations of energetic electron flux on board satellites in the vicinity of an FLR is calculated and shown to significantly distort the typical signature associated with drift resonance (modulations in electron flux at the wave frequency with a 180° phase change across the resonant energy).
... Because the frequencies observed are so close to the local ion gyrofrequencies, it has been suggested that the magnetohydrodynamic (MHD) approach to describing ULF waves at Earth used by Dungey (1954) In the terrestrial magnetosphere, ULF waves are often associated with driving by an energy source either internal to the magnetosphere (wave-particle interactions), or external to the magnetosphere (e.g., by solar wind buffeting or KH waves forming on the magnetopause). At Earth, wave-particle interactions include the drift and drift-bounce resonance mechanisms (Southwood et al., 1969), which allow the transfer of energy from gradient-curvature drifting energetic particle populations, such as those injected by substorms, to localized poloidal MHD waves (e.g., James et al., 2013;Yeoman et al., 2008Yeoman et al., , 2010. At Mercury, this instability is unlikely to develop because the magnetosphere may be too small to trap the drifting energetic particles. ...
The first statistical survey of field line resonance (FLR) events is presented using magnetometer data from the entire MErcury Surface, Space ENvironment, GEochemistry and Ranging mission. Ultralow‐frequency waves are an important tool for the magnetoseismology of the Hermean magnetosphere; this study provides a completely new window onto the resonance structures and plasma density distribution in the Hermean magnetosphere. Here we assess resonance events from two categories—toroidal resonances characteristic of the classical picture of FLRs in the terrestrial magnetosphere driven by the Kelvin‐Helmholtz instability and a more comprehensive approach including all observed transverse resonances with more relaxed polarization criteria. Two hundred twenty‐three toroidal FLRs with characteristics consistent with Kelvin‐Helmholtz‐driven FLRs are found in the dayside Hermean magnetosphere. The fundamental frequencies of these waves are used to provide estimates of plasma mass density in the range of ∼ 1–650 amu/cm³. A further 343 transverse resonances are found which provide very similar density estimates to the Earth‐like FLR population. Fundamental and harmonic frequencies from all 566 events are used to fit a power law to plasma mass density along the field lines. The equatorial plasma mass density is predicted to vary approximately with R−7.5. The offset of the Hermean dipole into the northern hemisphere causes significant asymmetries in the standing wave structure. Due to the extreme warping (away from a dipolar configuration) of Mercury's magnetosphere by the solar wind, the fundamental toroidal mode is predicted to oscillate with a notably lower frequency than the fundamental poloidal mode, contrary to relative toroidal and poloidal frequencies modeled for Earth's magnetosphere.
... Interplanetary shocks cause global mode oscillations in the ULF frequency regime (Zong et al., 2009), and high-speed solar wind can increase the energy transfer to the magnetosphere and generate ULF waves via Kelvin-Helmholtz instabilities (Claudepierre et al., 2008;Kavosi & Raeder, 2015;Mathie & Mann, 2001). ULF oscillations can also be produced by particle injections deeper in the magnetosphere (James et al., 2013;Yeoman et al., 2010;Zolotukhina et al., 2008) that occur during elevated substorm activity and can access regions close to Earth. Inward radial diffusion, driven by fluctuating electromagnetic fields in drift resonance with trapped particles, serves to energize particle populations by transporting them to drift paths nearer to Earth (e.g., Barker et al., 2005;Elkington et al., 2003;Hudson et al., 2000;Schulz & Lanzerotti, 1974). ...
Inward radial diffusion driven by ULF waves has long been known to be capable of accelerating radiation belt electrons to very high energies within the heart of the belts, but more recent work has shown that radial diffusion values can be highly event-specific, and mean values or empirical models may not capture the full significance of radial diffusion to acceleration events. Here we present an event of fast inward radial diffusion, occurring during a period following the geomagnetic storm of 17 March 2015. Ultrarelativistic electrons up to ∼8 MeV are accelerated in the absence of intense higher-frequency plasma waves, indicating an acceleration event in the core of the outer belt driven primarily or entirely by ULF wave-driven diffusion. We examine this fast diffusion rate along with derived radial diffusion coefficients using particle and fields instruments on the Van Allen Probes spacecraft mission.
... The substorm-injected westward drifting ion population can act as a source of energy for the excitation of plasma waves due to nonequilibrium phase space particle distributions. In particular, drift-bounce resonance of protons can drive high-m Pc5 pulsations (James et al., 2013;Southwood, 1976;K. Takahashi et al., 1985;Woch et al., 1990;Yeoman & Wright, 2001), which has been previously observed to occur predominantly in the afternoon MLT sector (Anderson et al., 1990;Woch et al., 1990). ...
The substorm process releases large amounts of energy into the magnetospheric system, although where the energy is transferred to and how it is partitioned remains an open question. In this study, we address whether the substorm process contributes a significant amount of energy to the ring current. The ring current is a highly variable region, and understanding the energization processes provides valuable insight into how substorm-ring current coupling may contribute to the generation of storm conditions and provide a source of energy for wave driving. In order to quantify the energy input into the ring current during the substorm process, we analyze Radiation Belt Storm Probes Ion Composition Experiment and Helium Oxygen Proton Electron ion flux measurements for H⁺, O⁺, and He⁺. The energy content of the ring current is estimated and binned spatially for L and magnetic local time. The results are combined with an independently derived substorm event list to perform a statistical analysis of variations in the ring current energy content with substorm phase. We show that the ring current energy is significantly higher in the expansion phase compared to the growth phase, with the energy enhancement persisting into the substorm recovery phase. The characteristics of the energy enhancement suggest the injection of energized ions from the tail plasma sheet following substorm onset. The local time variations indicate a loss of energetic H⁺ ions in the afternoon sector, likely due to wave-particle interactions. Overall, we find that the average energy input into the ring current is ∼9% of the previously reported energy released during substorms.
... The period of the modulation was about 4-5 min (f~3-4 mHz), which belongs to the Pc5 ULF range. Recently, James et al. (2013) have shown that the eastward drifting electrons during substorms can excite the poloidal mode ULF waves in the outer radiation belt. In the 20 March 2016 event, the interplanetary shock with a high dynamic pressure was not observed (Figure 3Sg). ...
To better understand rapid enhancements of the seed populations (hundreds of keV electrons) in the heart of the Earth's outer radiation belt (L* ~ 3.5–5.0) during different geomagnetic activities, we investigate three enhancement events measured by Van Allen Probes in detail. Observations of the fluxes and the pitch angle distributions of energetic electrons are analyzed to determine rapid enhancements of the seed populations. Our study shows that three specified processes associated with substorm electron injections can lead to rapid enhancements of the seed populations, and the electron energy increases up to 342 keV. In the first process, substorm electron injections accompanied by the transient and intense substorm electric fields can directly lead to rapid enhancements of the seed populations in the heart of the outer radiation belt. In the second process, the substorm injected electrons are first trapped in the outer radiation belt and subsequently transported into L* < 4.5 by the convection electric field. In the third process, the lower energy electrons are first injected at L* ~ 5.3 and then undergo drift resonance with ultralow-frequency waves. These accelerated electrons by ultralow-frequency waves are further transported into L* < 4.5 due to the convection electric field. This process is consistent with the radial diffusion. Our results suggest that these specified processes are important for understanding the dynamics of the seed populations in the heart of the outer radiation belt.
... Only a few pulsation events using multiple radars and coordinated magnetometers simultaneously observing a pulsation over an extensive range of longitude have been presented and analysed in the literature (e.g. Samson et al., 1991;Fenrich et al., 1995;Ziesolleck et al., 1998;James et al., 2013;Bland et al., 2014). Spacecraft observations over considerable spatial ranges, such as those by Agatipov et al. (2009), Balasis et al. (2012) and Balasis et al. (2015), have been rare. ...
HF (high-frequency) radars belonging to SuperDARN (Super Dual Auroral Radar Network) receive backscatter over substantial fields of view which, when combined, allow for simultaneous returns over extensive regions of the polar caps and midlatitudes. This makes them ideal instruments for the observation of pulsations in the Pc5 (1–5 mHz) frequency band. Relatively few pulsation events observed by multiple radars have been reported in the literature. Here we describe observations of three such events which extend over more than 120° of magnetic longitude in the Northern Hemisphere and one of which is also detected in the Southern Hemisphere. All three events show characteristics of field line resonances. In one case the pulsation has also been observed by magnetometers under or near the radar fields of view. The extensive longitudinal coverage allows accurate determination of azimuthal wave numbers. These are at the upper end of the lower values associated with external sources such as those in the solar wind. Such sources imply antisunward flow. However, the azimuthal wave number is negative, implying westward propagation at magnetic local times on both sides of noon, as would be expected from drift–bounce resonance with positive particles. Quiet conditions and a very low ring current during the events argue against this. The identification of the source of pulsations from a number of different mechanisms remains a problem of interest.
The high‐energy electron flux enhancement pattern in the outer radiation belt is observed under the influence of the Interplanetary Coronal Mass Ejections (ICMEs). Ten events were selected during the Van Allen Probes era, with the high‐energy electron flux enhancement starting close to L* = 4. A schematic diagram of the main physical processes responsible for this kind of high‐energy electron flux enhancement is presented, considering the energy deposited in the inner magnetosphere under the influence of ICMEs. Superposed Epoch Analysis is applied to the interplanetary medium parameters, the magnetopause standoff distance, the storm‐time geomagnetic activity indices, the Ultra‐Low Frequency (ULF) waves, and the whistler‐mode chorus waves. A compressed magnetopause, Bz component preferentially southward and storm indices considerably high are observed at the beginning of the electron flux enhancements. The modeled parameters of the chorus waves at the dayside/nightside sectors show the high acceleration efficiency at the beginning of the electron flux enhancement pattern II. These results suggest that the local acceleration driven by chorus waves is essential to these electron flux enhancements at low L*. In contrast, although the ULF waves are detected at the beginning of the studied electron flux enhancements, their contribution is insignificant.
The paper reviews the current state of the problem of interaction between long-period ultra-low-frequency (ULF) waves and high-energy particles. We consider elements of the theory of energy exchange between waves and particles, particle transport across magnetic shells under the influence of the electromagnetic field of a wave, the acceleration of radiation belt particles by both resonant and non-resonant mechanisms. We examine the mechanisms of generation of azimuthally-small-scale ULF waves due to instabilities arising from the wave–particle resonance. The cases of Alfvén, drift-compressional, and drift-mirror waves are analyzed. It is noted that due to the lack of a detailed theory of drift-mirror modes, the possibility of their existence in the magnetosphere cannot be taken as a proven fact. We summarize experimental data on the poloidal and compression ULF waves generated by unstable populations of high-energy particles. We investigate the mechanisms of modulation of energetic particle fluxes by ULF waves and possible observational manifestations of such modulation. Methods of studying the structure of waves across magnetic shells by recording fluxes of resonant particles with a finite Larmor radius are discussed.
The paper reviews the current state of the problem of interaction between long-period ultra-low-frequency (ULF) waves and high-energy particles. We consider elements of the theory of energy exchange between waves and particles, particle transport across magnetic shells under the influence of the electromagnetic field of a wave, the acceleration of radiation belt particles by both resonant and non-resonant mechanisms. We examine the mechanisms of generation of azimuthally-small-scale ULF waves due to instabilities arising from the wave–particle resonance. The cases of Alfvén, drift-compressional, and drift-mirror waves are analyzed. It is noted that due to the lack of a detailed theory of drift-mirror modes, the possibility of their existence in the magnetosphere cannot be taken as a proven fact. We summarize experimental data on the poloidal and compression ULF waves generated by unstable populations of high-energy particles. We investigate the mechanisms of modulation of energetic particle fluxes by ULF waves and possible observational manifestations of such modulation. Methods of studying the structure of waves across magnetic shells by recording fluxes of resonant particles with a finite Larmor radius are discussed.
We report global dayside observation of Pc4-5 wave at the late recovery phase of a magnetic storm from seven satellites. Oscillations lasted for 15 hours and revealed the wave appearance before the substorm onset. The continuous observation showed that substorm injections did not affect the wave behavior, and all the wave parameters change was caused by magnetospheric plasma recovery after a magnetic storm. The wave was identified as a second harmonic poloidal Alfvén wave propagating westward with a high azimuthal wave number (|m|∼ 210) and coupled with 100 keV protons through the drift-bounce resonance. A large radial gradient of phase space density was an energy source for the wave.
A review of researches related to ultra low frequency (ULF) waves studied using a midlatitude coherent decameter radar located near Ekaterinburg (EKB), Russia is presented. The radar was constructed similar to SuperDARN radars. During the experiment conducted in 2013–2015 three of its beams were surveyed successively with the integration time of 6 s providing time resolution of 18 s at each beam. In a few cases, data received in the nighttime ionosphere showed signatures of the drift compressional mode. In one of them, the wave exhibited clear dependence of frequency on azimuthal wave number m, which corresponds with the theory of drift compressional waves. In another case, merging of the drift compressional and Alfvén modes at some critical value of m was shown. A number of the observed waves’ frequencies were compared with the Alfvén ones inferred from the spacecraft data. The results showed that in a large part of cases waves had sub-Alfvénic frequencies. This finding complies with the assumption that at least a part of the waves observed with the radar should be identified with the drift compressional mode, whose frequency can be lower than frequency of field line resonance (FLR). Additionally, a case of simultaneous spacecraft and radar wave observation was presented. As the wave had poloidal structure, diamagnetic properties, and its frequency was considerably lower than the minimal frequency of the FLR, it was assumed that it was some kind of compressional mode, apparently the drift compressional wave.
Long‐lasting Pc5 ultralow frequency (ULF) waves spanning the dayside and extending from L ∼ 5.5 into the polar cap region were observed by conjugate ground magnetometers. Observations from MMS satellites in the magnetosphere and magnetometers on the ground confirmed that the ULF waves on closed field lines were due to fundamental toroidal standing Alfvén waves. Monochromatic waves at lower latitudes tended to maximize their power away from noon in both the morning and afternoon sectors, while more broadband waves at higher latitudes tended to have a wave power maximum near noon. The wave power distribution and MMS satellite observations during the magnetopause crossing indicate surface waves on a Kelvin‐Helmholtz (KH) unstable magnetopause coupled with standing Alfvén waves. The more turbulent ion foreshock during an extended period of radial interplanetary magnetic field (IMF) likely plays an important role in providing seed perturbations for the growth of the KH waves. These results indicate that the Pc5 waves observed on closed field lines and on the open field lines of the polar cap were from the same source.
An analytic model of ultra‐low‐frequency (ULF) waves and test particle simulations are used to investigate signatures of electron drift resonance observed by the Van Allen Probes during the passage of an interplanetary shock on 31 October 2012, 15:30–16:00 UT. Electron fluxes binned in energy and pitch angle to match the Magnetic Electron Ion Spectrometer (MagEIS) instrument reproduce observed time series, while fluxes binned at higher resolution decay more slowly. The slower rate of decay is because oscillations in particle fluxes produced by ULF waves are phase‐mixed across the finite energy width of MagEIS energy channels. Simulations of N = 0 drift resonance and N = −2 drift‐bounce resonance are presented to provide insight into ion wave–particle interactions that are a science target of the recently launched Japanese ARASE spacecraft.
This paper deals with Pc5 magnetospheric pulsations featuring positive azimuthal wave numbers registered with the mid-latitude coherent decameter radar located near Ekaterinburg (EKB). The azimuthal wave numbers are determined using adjacent high time resolution beams directed toward the magnetic pole. Approximately 13 % of all steady waves registered with the radar propagate eastward. We have examined ten cases of wave observations with both small and high positive wave numbers, which occurred between April 2014 and March 2015. We performed a wavelet analysis of the data sets, estimated wavelength in radial direction for four cases, and determined meridional phase propagation direction. In three cases, the results are consistent with field line resonance behavior. However, in the majority of the studied events wave frequencies are considerably lower than those of field line resonance, which were derived from satellite data on magnetic field and particle density. These waves may be classed with the drift-compressional mode.
This paper deals with Pc5 magnetospheric pulsations featuring positive azimuthal wave numbers registered with the mid-latitude coherent decameter radar located near Ekaterinburg (EKB). The azimuthal wave numbers are determined using adjacent high time resolution beams directed toward the magnetic pole. Approximately 13 % of all steady waves registered with the radar propagate eastward. We have examined ten cases of wave observations with both small and high positive wave numbers, which occurred between April 2014 and March 2015. We performed a wavelet analysis of the data sets, estimated wavelength in radial direction for four cases, and determined meridional phase propagation direction. In three cases, the results are consistent with field line resonance behavior. However, in the majority of the studied events wave frequencies are considerably lower than those of field line resonance, which were derived from satellite data on magnetic field and particle density. These waves may be classed with the drift-compressional mode.
Super Dual Auroral Radar Network (SuperDARN) observations show that ionospheric flow fluctuations of millihertz or lower-frequency range with horizontal velocities of a few hundred meters per second appeared in the subauroral to midlatitude region during a magnetic storm on 27 March 2017. A set of the radars have provided the first ever observations that the fluctuations propagate azimuthally both westward and eastward simultaneously, showing bifurcated phase propagation associated with substorm expansion. Concurrent observations near the conjugate site in the inner magnetosphere made by the Arase satellite provide evidence that multiple drifting clouds of electrons in the near-Earth equatorial plane were associated with the electric field fluctuations propagating eastward in the ionosphere. We interpret this event in terms of mesoscale pressure gradients carried by drifting ring current electrons that distort field lines one after another as they drift through the inner magnetosphere, causing eastward propagating ionospheric electric field fluctuations.
A statistical study of waves in the magnetosphere registered with the midlatitude coherent decameter radar located near Ekaterinburg (EKB), Russia is presented. The radar monitors ionospheric flow velocities whose small-scale variations are evoked by magnetosphere ultra low frequency (ULF) waves of Pc5-range. Data from 16 events observed during 7 months in 2014 and 2015 underwent wavelet analysis. Frequencies of the oscillating components were compared with Alfvén eigenfrequencies, which were inferred from THEMIS and Van Allen spacecraft data. The comparison showed that only a minor part of the oscillations registered with the radar in the nightside ionosphere could be attributed to the Alfvén mode. A majority of the waves have lower frequencies, which do not show dependence on Alfvén eigenfrequency of a field line.
As shown within the limits of gyrokinetics, drift-compression waves can propagate in the magnetosphere in the direction of energetic electron drift. Plasma is assumed to be composed of cold particles with hot additives such as protons with Maxwell distribution and electrons with inverse distribution. We have determined conditions of existence of such waves and their intensification due to resonance interaction with energetic electrons (drift instability). The results can be helpful for interpretation of observation of wave phenomena in the magnetosphere with frequencies in the range of geomagnetic pulsations Pc5 and below.
A new mechanism of generation of the high-m compressional ULF waves in the magnetosphere is considered. It is suggested that the wave can be generated by the non-resonant instability of coupled Alfvén and drift compressional modes in the energetic component of the magnetospheric plasma. A stability analysis of the of the coupled modes in the inhomogeneous finite-β plasma in the dipole-like field in gyrokinetics is performed. A quadratic equation was obtained that determines mode frequency and the growth rate. The frequencies of both modes depend on the azimuthal wave number, m. The branches are merged at some critical m value, forming a mode with both real and imaginary parts of the wave frequency. This mode is amplified due to the instability called the drift coupling instability. The instability criterion was found. Its growth rate is determined by the mode coupling.
The problems of large scale wave propagation and amplification in the outer magnetosphere are considered. KH instability growth rate of the magnetospheric waveguide eigenmodes is investigated as a function of a coordinate along the magnetopause. The problem of solar wind MHD wave penetration into the waveguide is investigated for a broad range near Pc3 and Pc5 geomagnetic pulsation frequencies and realistic models of the magnetospheric waveguide. The expression for the waveguide eigenmode energy flux is obtained. This expression includes the effects of external wave penetration and mode amplification due to the KH instability, as well as losses due to dissipation in the vicinity of the Alfven resonance which are incorporated into the growth rate coefficient together with the instability.
Poloidal mode ultra-low-frequency (ULF) waves with high azimuthal mode number (high-m) are common throughout Earth's magnetosphere. The electric fields in these waves are on the order of tens of millivolts per meter, and are large enough to energize ions and electrons via drift- and drift-bounce wave-particle resonance. In this article, we present full-orbit test-particle simulations of drift-bounce resonance using a numerical model of ULF waves. The model uses a dipole magnetic field and can accommodate a realistic ionosphere with height-resolved Pedersen and Hall conductivity. It is demonstrated that second-harmonic poloidal mode waves can energize ions to tens of keV depending on the wave amplitude. The test-particle simulations also predict ion flux oscillations that have energy dispersion similar to many satellite observations. It will be shown that features in the energy dispersion are in agreement with theoretical considerations.
Ionospheric radar systems have proved to be a powerful tool for the investigation of magnetospheric ULF waves. High-m poloidal waves become much more attenuated in ground magnetometer data than low-m toroidal waves. For this reason ionospheric radar systems have been effective in the study of high-m poloidal waves driven by energetic particle populations within the magnetosphere, and it is this class of ULF waves that is discussed in this chapter. More recent ionospheric observations of high-m ULF waves have taken advantage of the Super Dual Auroral Radar Network (SuperDARN). SuperDARN is a global array of high-frequency (HF) radars. In a Doppler sounder, use is made of the direct reflection of a radio wave from the ionosphere, rather than a scattering process. A number of alternative techniques are available for exploring the ionospheric signatures of such wave events, which are relatively underexploited, and have the potential to provide important new observations.
A case study of shortwave radar observations of magnetospheric Pc5 ULF waves (wave periods of 150–600 s) that occurred on 26 December 2014 in the nightside magnetosphere during substorm activity is presented. The radar study of waves in the magnetosphere is based on analysis of scattering from field-aligned irregularities of the ionospheric F layer. Variations of their ⃗ E × ⃗ B drift velocity at F layer heights are associated with the wave electric field. Analysis of the observations from the Ekaterinburg (EKB) radar shows that the frequency f of the observed wave depends on the azimuthal wave number m (positive correlation of about 0.90): an increase in frequency from 2.5 to 5 mHz corresponds to increased m number from 20 to 80. Of the known types of waves in the magnetosphere corresponding to the Pc5 range, only drift compressional waves have such azimuthal dispersion: the frequency of the drift compressional mode is directly proportional to the azimuthal wave number and the gradient-curvature drift velocity of energetic particles in the magnetic field. This wave has a kinetic nature and represents the most common kind of the compressional modes, demanding for its existence only finite pressure and plasma inhomogeneity across magnetic shells.
Theory and observations have linked equatorial VLF waves with pulsating aurora for decades, invoking the process of pitch-angle scattering of 10's keV electrons in the equatorial magnetosphere. Recently published satellite studies have strengthened this argument, by showing strong correlation between pulsating auroral patches and both lower-band chorus and 10's keV electron modulation in the vicinity of geosynchronous orbit. Additionally, a previous link has been made between Pc4-5 compressional pulsations and modulation of whistler-mode chorus using THEMIS. In the current study, we present simultaneous in-situ observations of structured chorus waves and an apparent field line resonance (in the Pc4-5 range) as a result of a substorm injection, observed by Van Allen Probes, along with ground-based observations of pulsating aurora. We demonstrate the likely scenario being one of substorm-driven Pc4-5 ULF pulsations modulating chorus waves, and thus providing the driver for pulsating particle precipitation into the Earth's atmosphere. Interestingly, the modulated chorus wave and ULF wave periods are well correlated, with chorus occurring at half the periodicity of the ULF waves. We also show, for the first time, a particular few-Hz modulation of individual chorus elements that coincides with the same modulation in a nearby pulsating aurora patch. Such modulation has been noticed as a high-frequency component in ground-based camera data of pulsating aurora for decades, and may be a result of nonlinear chorus wave interactions in the equatorial region.
The Hankasalmi SuperDARN radar in Finland, while operating in a high spatial and temporal resolution mode, has measured the ionospheric signature of a naturally occurring ULF wave in scatter artificially induced by the Tromsø Heater. The wave had a period of 100 s and exhibited curved phase fronts across the heated volume (about 180 km along a single radar beam). Spatial information provided by the radar has enabled an m-number for the wave of about 38 to be determined. It is demonstrated here that the curved phase fronts are a generic feature of nonstationary poloidal waves in a transverse resonator, caused by the common action of the field line curvature, the plasma pressure, and the equilibrium current. Some features of the observed event agree with the resonator in the vicinity of the ring current, where it is proposed that the wave is excited by a moving source in the form of a proton cloud drifting in the magnetosphere in the azimuthal direction.
One month's worth of Polar ultraviolet imager (UVI) data were subjected to a ``blob'' analysis to determine the statistical dynamics of substorm features observed in the Lyman-Birge-Hopfield long (LBHL) band (152-188 nm). Adapted from similar DoD analyses of target images, the analysis consists of finding, on a frame-by-frame basis beginning at substorm onset, the following aspects of an individual auroral feature: peak power (i.e., power of precipitating electrons), total power, centroid location (magnetic local time (MLT) and magnetic latitude (MLAT), and speed of centroid. Over 120 individual auroral features were successfully acquired at onset and tracked until dissipation during January 1997. The power in the peak pixel and total power were random in time but displayed transient spikes that lasted 5-10 min. Over the course of a substorm, the total energy of blobs averaged ~2.0×104GJ. A histogram of these energies suggests no preferred energy but that lower energies were more common than higher energies. Analysis of the blob positional dynamics generally supports a poleward and westward movement. During the course of a substorm, 90% of the blobs moved poleward, while over 60% moved westward. However, these movements were not steady and displayed random components. Furthermore, a sizable minority (~35%) of the blobs moved eastward, which does not agree with the conventional picture of auroral surges. Blob speeds varied from essentially zero up to several kilometers per second. However, during the January substorms the blobs did appear to have a preferred speed of 0.84+/-0.34kms-1.
This letter reports observations of large-scale ionospheric plasma flows
from the SuperDARN radar network and global auroral images from the
Polar ultraviolet imager during two large and rare substorm events. The
substorm events were characterized by predominantly westward expansion
for one and predominantly eastward expansion (or lack of westward
expansion) for the other. The westward expansion case was associated
with a positive y-component of interplanetary magnetic field (IMF),
whereas the eastward expansion case was associated with a negative
y-component of IMF. The concurrent plasma flow measurements from the
SuperDARN radar network suggest that the westward (eastward) expansion
is embedded in the large, round dusk (dawn) convection cell. A simple
explanation is offered for the observed preponderance of westward
expansions, namely, that the pre-midnight onset location favors the
westward plasma flow that is associated with the dusk cell. Eastward
expansion occurs only when a large IMF By negative condition
causes the midnight sector to be dominated by eastward plasma flows.
A self-consistent field line resonance problem in the model of an axisymmetric dipole-like magnetosphere is solved. The spatial structure not only of resonance Alfven waves but also of the magnetosonic wave exciting them was obtained in this paper. The field structure of the magnetosonic wave has been determined both inside the magnetospheric cavity and in the solar wind region near the magnetosphere. This made it possible to compare the amplitudes of magnetosonic waves incident from the solar wind on the magnetosphere and the resonance Alfven oscillations which they excite in the magnetosphere. It is shown that as magnetosonic oscillations penetrate deep into the magnetosphere, their am- plitude decreases several orders of magnitude. The amplitude of Alfven oscillations excited inside the magnetosphere is comparable with that of magnetosonic waves in the solar wind region. The amplitude of resonance Alfven oscillations on internal magnetic shells is significantly larger than that of resonance oscillations excited in the transition layer of the magnetosphere.
It is generally agreed that giant pulsations (Pgs) are the result of a particle instability that occurs inside the magnetosphere rather than the consequence of an external stimulus. Previous studies have suggested that protons with energies ~5-30 keV play a role in Pg excitation. It is shown that protons with energies ~5-30 keV, injected into the inner magnetosphere on the nightside, will only drift westward around the Earth on enclosed paths if the ExB drifts due to the magnetospheric convection and corotation electric fields are small. This is the case when the magnetosphere is quiet. If the ExB drifts are large, as is the case for more disturbed times, then their influence may overcome that of the gradient-curvature drift for these lower energy protons, detrapping them from their enclosed paths and allowing them to follow convective paths to the dayside magnetopause. At these times, the lower energy protons which may be an important factor in Pg generation will not reach the early morning sector where Pgs occur. This phenomena can explain the rarity and occurrence during quiet times of Pgs. It can also explain the quashing of Pg activity during substorms and the tendency for Pgs to occur on successive days, 24 hours apart. A similar reasoning can also explain why radially polarized waves with large azimuthal wave numbers, thought to be generated by the bounce resonance mechanism, are frequently observed in the afternoon/evening sector of the magnetosphere but occur infrequently in the morning sector.
This paper deals with the stability of coupled Alfvén and drift-mirror modes in a one dimensionally non-uniform plasma in a gyrokinetic framework. A dispersion relation for the mode is obtained and solved for different values of the coupling parameter, proportional to the square of the radial pressure gradient. Even a weak coupling substantially alters the modes' properties. The frequency of the drift-mirror mode is substantially different from that in the decoupled case the drift-mirror instability can develop for lower values of plasma anisotropy. The weak coupling also causes a decrease in the Alfvén mode frequency and leads to an instability whose growth rate is proportional to the coupling parameter. If the coupling is strong, the notions of the Alfvén and drift-mirror modes lose their meaning since their respective oscillation branches merge and further split at some anisotropy value. Another nomenclature is suggested, the unstable and stable Alfvén-mirror modes, the former being unstable at any anisotropy value, and the latter, in contrast, is always damped. Another effect of coupling is the transverse dispersion of the modes, that is, the dependence of the wave frequency on the wave vector transverse component. This effect can be responsible for the mode structure across the magnetic field and perpendicular energy transfer.
Compressional Pc5 pulsations in space and Ps6 pulsations on the ground are common features observed in the morning sector. Here we use a conjunction study of Equator-S, Geotail, and ground stations in Canada to show that Ps6 pulsations can be the ground counterpart of compressional Pc5 pulsations observed by satellites. Because strong Ps6 pulsations are associated with optical omega-band signatures, we also suggest that the omega-band counterparts in space might be compressional pulsations on the Pc5 scale. We also discuss the magnetic field configuration that makes all these observations consistent.
1] Over the first 2.5 years of operation, the FUV instrument on the IMAGE spacecraft observed more than 2400 substorm onsets in the Northern Hemisphere. The observations confirm earlier results of statistical studies in terms of a median substorm onset location at 2300 hours MLT and 66.4 degrees magnetic latitude. The purpose of this report is to publish the list to allow for further investigation. The list can easily be searched for onsets close to certain ground stations or at specific magnetic latitudes or local times. As one example of such use, we demonstrate how the probability of onset observation was determined for the ground-based automatic observatories of the THEMIS (Time History of Events and Macroscale Interactions during Substorms) project.
Direct imaging of the magnetosphere by the IMAGE spacecraft will be supplemented by observation of the global aurora, the footprint of magnetospheric regions. To assure the simultaneity of these observations and the measurement of the magnetospheric background neutral gas density, the IMAGE satellite instrument complement includes three Far Ultraviolet (FUV) instruments. In the wavelength region 120-190 nm, a downward-viewing auroral imager is only minimally contaminated by sunlight, scattered from clouds and ground, and radiance of the aurora observed in a nadir viewing geometry can be observed in the presence of the high-latitude dayglow. The Wideband Imaging Camera (WIC) will provide broad band ultraviolet images of the aurora for maximum spatial and temporal resolution by imaging the LBH N2 bands of the aurora. The Spectrographic Imager (SI), a monochromatic imager, will image different types of aurora, filtered by wavelength. By measuring the Doppler-shifted Ly-, the proton-induced component of the aurora will be imaged separately. Finally, the GEO instrument will observe the distribution of the geocoronal emission, which is a measure of the neutral background density source for charge exchange in the magnetosphere. The FUV instrument complement looks radially outward from the rotating IMAGE satellite and, therefore, it spends only a short time observing the aurora and the Earth during each spin. Detailed descriptions of the WIC, SI, GEO, and their individual performance validations are discussed in companion papers. This paper summarizes the system requirements and system design approach taken to satisfy the science requirements. One primary requirement is to maximize photon collection efficiency and use efficiently the short time available for exposures. The FUV auroral imagers WIC and SI both have wide fields of view and take data continuously as the auroral region proceeds through the field of view. To minimize data volume, multiple images are taken and electronically co-added by suitably shifting each image to compensate for the spacecraft rotation. In order to minimize resolution loss, the images have to be distortion-corrected in real time for both WIC and SI prior to co-adding. The distortion correction is accomplished using high speed look up tables that are pre-generated by least square fitting to polynomial functions by the on-orbit processor. The instruments were calibrated individually while on stationery platforms, mostly in vacuum chambers as described in the companion papers. Extensive ground-based testing was performed with visible and near UV simulators mounted on a rotating platform to estimate their on-orbit performance. The predicted instrument system performance is summarized and some of the preliminary data formats are shown.
The Far Ultraviolet Wideband Imaging Camera (WIC) complements the magnetospheric images taken by the IMAGE satellite instruments with simultaneous global maps of the terrestrial aurora. Thus, a primary requirement of WIC is to image the total intensity of the aurora in wavelength regions most representative of the auroral source and least contaminated by dayglow, have sufficient field of view to cover the entire polar region from spacecraft apogee and have resolution that is sufficient to resolve auroras on a scale of 1 to 2 latitude degrees. The instrument is sensitive in the spectral region from 140–190nm. The WIC is mounted on the rotating IMAGE spacecraft viewing radially outward and has a field of view of 17 in the direction parallel to the spacecraft spin axis. Its field of view is 30 in the direction perpendicular to the spin axis, although only a 1717 image of the Earth is recorded. The optics was an all-reflective, inverted Cassegrain Burch camera using concentric optics with a small convex primary and a large concave secondary mirror. The mirrors were coated by a special multi-layer coating, which has low reflectivity in the visible and near UV region. The detector consists of a MCP-intensified CCD. The MCP is curved to accommodate the focal surface of the concentric optics. The phosphor of the image intensifier is deposited on a concave fiberoptic window, which is then coupled to the CCD with a fiberoptic taper. The camera head operates in a fast frame transfer mode with the CCD being read approximately 30 full frames (512256pixel) per second with an exposure time of 0.033s. The image motion due to the satellite spin is minimal during such a short exposure. Each image is electronically distortion corrected using the look up table scheme. An offset is added to each memory address that is proportional to the image shift due to satellite rotation, and the charge signal is digitally summed in memory. On orbit, approximately 300frames will be added to produce one WIC image in memory. The advantage of the electronic motion compensation and distortion correction is that it is extremely flexible, permitting several kinds of corrections including motions parallel and perpendicular to the predicted axis of rotation. The instrument was calibrated by applying ultraviolet light through a vacuum monochromator and measuring the absolute responsivity of the instrument. To obtain the data for the distortion look up table, the camera was turned through various angles and the input angles corresponding to a pixel matrix were recorded. It was found that the spectral response peaked at 150nm and fell off in either direction. The equivalent aperture of the camera, including mirror reflectivities and effective photocathode quantum efficiency, is about 0.04cm2. Thus, a 100 Rayleigh aurora is expected to produce 23 equivalent counts per pixel per 10s exposure at the peak of instrument response.
The Super Dual Auroral Radar Network (SuperDARN) has been operating as an international co-operative organization for over 10 years. The network has now grown so that the fields of view of its 18 radars cover the majority of the northern and southern hemisphere polar ionospheres. SuperDARN has been successful in addressing a wide range of scientific questions concerning processes in the magnetosphere, ionosphere, thermosphere, and mesosphere, as well as general plasma physics questions. We commence this paper with a historical introduction to SuperDARN. Following this, we review the science performed by SuperDARN
over the last 10 years covering the areas of ionospheric convection, field-aligned currents, magnetic reconnection, substorms, MHD waves, the neutral atmosphere, and E-region ionospheric irregularities. In addition, we provide an up-to-date description of the current network, as well as the analysis techniques available for use with the data from the radars. We conclude the paper with a discussion of the future of SuperDARN, its expansion, and new science opportunities.
We consider in detail the intense Pc5 pulsations of the magnetic field, riometric absorption, and electron fluxes occurred
on the recovery phase of the strong magnetic storm on November 21, 2003. The global structure of these disturbances is studied
using the world network of magnetometers and riometers supplemented by the data of particle detectors onboard the LANL geosynchronous satellites. The local spatial structure is investigated according to data of the regional network of Finnish
vertical riometers and of stations of the IMAGE magnetic network. Though a certain similarity is observed in the frequency
composition and time evolution of the variations of magnetic field and riometric absorption, the local spatial structure of
these oscillations turns out to be different. It is suggested that these variations can be manifestations of oscillatory properties
of two weakly connected systems: the magnetospheric MHD waveguide/resonator and the system cyclotron noise + electrons. The
recorded Pc5 oscillations are, presumably, a result of excitation of the magnetospheric waveguide on the morning and evening
flanks of the magnetosphere. At high velocities of the solar wind this waveguide can appear in a metastable state. Not only
jumps in the solar wind density, but injection of electrons into the magnetosphere as well, can serve as a trigger for the
waveguide excitation.
The generation of an Alfvén wave by an azimuthally drifting cloud of high-energy particles injected in the Earth’s magnetosphere
is studied analytically. In contrast to the previous studies where the generation mechanisms associated with the resonant
wave-particle interaction were considered, a nonresonant mechanism is investigated in which the wave is excited by the alternating
current produced by drifting particles. It is shown that, at a point with a given azimuthal coordinate, a poloidally polarized
wave, in which the magnetic field lines oscillate predominantly in the radial direction, is excited immediately after the
passage of the particle cloud through this point. As the cloud moves away from that point, the wave polarization becomes toroidal
(the magnetic field lines oscillate predominantly in the azimuthal direction). The azimuthal wavenumber m is defined as the ratio of the wave eigenfrequency to the angular velocity of the cloud (the drift velocity of the particles).
It is shown that the amplitudes of the waves so generated are close to those obtained under realistic assumptions about the
density and energy of the particles.
A case study of SuperDARN observations of Pc5 Alfvén ULF wave activity
generated in the immediate aftermath of a modest-intensity substorm expansion
phase onset is presented. Observations from the Hankasalmi radar reveal that
the wave had a period of 580 s and was characterized by an intermediate
azimuthal wave number (m=13), with an eastwards phase propagation. It had a
significant poloidal component and a rapid equatorward phase propagation
(~62° per degree of latitude). The total equatorward phase
variation over the wave signatures visible in the radar field-of-view
exceeded the 180° associated with field line resonances. The wave
activity is interpreted as being stimulated by recently-injected energetic
particles. Specifically the wave is thought to arise from an eastward
drifting cloud of energetic electrons in a similar fashion to recent
theoretical suggestions (Mager and Klimushkin, 2008; Zolotukhina
et al., 2008; Mager et al., 2009). The
azimuthal wave number m is determined by the wave eigenfrequency and the
drift velocity of the source particle population. To create such an
intermediate-m wave, the injected particles must have rather high energies
for a given L-shell, in comparison to previous observations of wave
events with equatorward polarization. The wave period is somewhat longer than
previous observations of equatorward-propagating events. This may well be a
consequence of the wave occurring very shortly after the substorm expansion,
on stretched near-midnight field lines characterised by longer
eigenfrequencies than those involved in previous observations.
Using magnetic field and plasma observations from four of the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, a surface wave at the dawn flank of the magnetopause was identified on 25 April 2007. The wave had an amplitude of about 1 RE and propagated tailward with a velocity of about 190–240 km/s. Its azimuthal wavelength was in the range of 9–11 RE. Magnetosheath velocity values support the hypothesis that this surface wave was generated by the Kelvin-Helmholtz instability. Simultaneously, an ultralow-frequency (ULF) pulsation event was detected by the fifth THEMIS spacecraft deeper in the magnetosphere, at a distance of about 5–7 RE from the magnetopause. This ULF event showed all the signatures predicted for waves generated by the classical field line resonance process. Frequency and phases of the detected ULF oscillations were found to be in close agreement with the magnetopause surface periodic disturbances. We conclude that the observed ULF wave event was most likely directly generated by the magnetopause surface wave and thus represents one of the few known manifestations of the classical field line resonance process in space directly observed, a conclusion made possible due to the special configuration of the THEMIS mission and its five spacecraft.
An HF Doppler sounder, DOPE (DOppler Pulsation Experiment) with three azimuthally-separated propagation paths is used to provide the first statistical examination of small scale-sized, high m waves where a direct measurement of the azimuthal wavenumber m , is made in the ionosphere. The study presents 27 events, predominantly in the post-noon sector. The majority of events are Pc4 waves with azimuthal m numbers ranging from –100 to –200, representing some of the smallest scale waves ever observed in the ionosphere. 4 Pc5 waves are observed in the post-noon sector. The fact that measurements for the wave azimuthal m number and the wave angular frequency are available allows the drift-bounce resonance condition to be used to hypothesise potential particle populations which could drive the waves through either a drift or drift-bounce resonance interaction mechanism. These results are compared with the statistical study presented by Baddeley et al. (2004) which investigated the statistical likelihood of such driving particle populations occurring in the magnetospheric ring current. The combination of these two studies indicates that any wave which requires a possible drift resonance interaction with particles of energies >60 keV, is statistically unlikely to be generated by such a mechanism. The evidence presented in this paper therefore suggests that in the pre-noon sector the drift-bounce resonance mechanism is statistically more likely implying an anti-symmetric standing wave structure while in the post-noon sector both a drift or drift-bounce resonance interaction is statistically possible, indicating both symmetric and anti-symmetric standing mode structures. A case study is also presented investigating simultaneous observations of a ULF wave in ground magnetometer and DOPE data. The event is in the lower m range of the statistical study and displays giant pulsation (Pg) characteristics.
Keywords. Ionosphere (Ionosphere-magnetosphere interactions) – Magnetospheric physics (MHD waves and instabilities) – Space plasma physics (Wave-particle interactions)
HF radar backscatter which has been artificially-induced by a high power RF facility has been demonstrated to provide ionospheric electric field data of unprecedented temporal resolution and accuracy. Here such data, induced by the SPEAR high power radar on Svalbard, are used to investigate ULF wave processes observed by the CUTLASS HF radars. Observations are presented of both waves with a large-scale nature, driven externally to the magnetosphere and those with small azimuthal scale lengths, driven by wave-particle interactions. For ULF wave events with large azimuthal scale lengths an excellent agreement in the observed wave polarisation ellipse is found between the radar observations and ground-based magnetometer data. In contrast, for the small scale events, no ground-based magnetic counterpart is observed. Indeed the data from the two CUTLASS radars seem inconsistent, and each radar must be interpreted separately, as the spatial resolution of the radars is sufficient to resolve the wave characteristics along the radar beams, but insufficient to resolve the wave characteristics across the beams. A high azimuthal wave number (m) wave with a period of 300 s and m~−60 is observed to occur over Svalbard at ~14:00 magnetic local time. This confirms the existence of waves driven by wave-particle interactions with trapped particle populations in the outer magnetosphere. A comparison of the observed wave characteristics with previous, lower latitude, observations suggests that these high latitude waves have a similar azimuthal scale size to those generated in the inner magnetosphere; the azimuthal wave number of −60 observed in the present study is comparable to previous values of −20– −50, but suggests an increase of m with latitude. A similar energy source in drifting proton populations is also suggested, but with lower characteristic proton energies of 10 keV implicated at high latitude, compared to the 20–60 keV energies invoked for previous lower latitude observations.
We analyzed the spectral-polarized characteristics of Pc5 ULF waves observed on 17 September 2000 after the 03:20:25 UT substorm onset with the satellites GOES 8 and 10 located east and west of the onset location. In the course of the event, the wave polarization changed from mixed (between toroidal and poloidal) to poloidal, and then to mixed again. The hodogram of magnetic field oscillations rotated counterclockwise at GOES 8, and clockwise at GOES 10. It is suggested that the satellites detected the waves generated by the substorm injected clouds of the charged particles drifting in the magnetosphere in the opposite azimuthal directions: GOES 8 (located east of the substorm onset) detected the wave generated by an electron cloud, and GOES 10 (west of the onset) detected the wave generated by a positive ion cloud. This interpretation is confirmed by the energetic particles data recorded by LANL satellites.
A case study of a poloidal ULF pulsation near the dayside plasmapause is presented based on Cluster observations of magnetic and electric fields. The pulsation is detected close to the magnetic equatorial plane at L shells L=[4.4, 4.6] and oscillates with a frequency of f=23 mHz. Investigating the wave energy flux reveals the standing wave nature of the observed pulsation. An estimation of the azimuthal wave number exposes a narrow azimuthal structure of the wave field with m≈160. Spatial and temporal characteristics of the pulsation are analyzed in detail by representing data in a field line related coordinate system and a range-time-intensity representation. This allows an estimation of both the spatial extension of the wave field in the radial direction and its temporal decay rate. The analysis furthermore indicates that the same field lines are excited to a standing wave oscillation twice. Furthermore an accurate identification of a phase jump of the wave field across L shells is possible. Comparing the radial localization of the detected wave with theoretically expected field line eigenfrequencies reveals that the wave field is confined in the Alfvén resonator at the outer edge of the plasmapause.
The generation of a high- m Alfvén wave by substorm injected energetic particles in the magnetosphere is studied. The wave is supposed to be emitted by an alternating current created by the drifting particle cloud or ring current inhomogeneity. It is shown that the wave appears in some azimuthal location simultaneously with the particle cloud arrival at the same spot. The value of the azimuthal wave number is determined as m ~ω/ω <sub>d</sub> , where ω is the eigenfrequency of the standing Alfvén wave and ω <sub>d</sub> is the particle drift frequency. The wave propagates westward, in the direction of the proton drift. Under the reasonable assumption about the density of the energetic particles, the amplitude of the generated wave is close to the observed amplitudes of poloidal ULF pulsations.
Spatial localization and azimuthal wave numbers m of poloidal Alfvén waves generated by energetic particles in the magnetosphere are studied in the paper. There are two factors that cause the wave localization across magnetic shells. First, the instability growth rate is proportional to the distribution function of the energetic particles, hence waves must be predominantly generated on magnetic shells where the particles are located. Second, the frequency of the generated poloidal wave must coincide with the poloidal eigenfrequency, which is a function of the radial coordinate. The combined impact of these two factors also determines the azimuthal wave number of the generated oscillations. The beams with energies about 10 keV and 150 keV are considered. As a result, the waves are shown to be strongly localized across magnetic shells; for the most often observed second longitudinal harmonic of poloidal Alfvén wave (N=2), the localization region is about one Earth radius across the magnetic shells. It is shown that the drift-bounce resonance condition does not select the m value for this harmonic. For 10 keV particles (most often involved in the explanation of poloidal pulsations), the azimuthal wave number was shown to be determined with a rather low accuracy, -100mmN=1), the azimuthal wave number is determined with a better accuracy, but both of these numbers are too small (if the waves are generated by 150 keV particles), or the waves are generated on magnetic shells (in 10 keV case) which are too far away. The calculated values of γ/ω are not large enough to overcome the damping on the ionosphere. All these have cast some suspicion on the possibility of the drift-bounce instability to generate poloidal pulsations in the magnetosphere.
Many theories state that Ultra Low Frequency (ULF) waves with a high azimuthal wave number ( m ) have their energy source in wave-particle interactions, yet this assumption has been rarely tested numerically and thus many questions still remain as to the waves' exact generation mechanism. For the first time, this paper investigates the cause and effect relationship between the driving magnetospheric particle populations and the ULF wave signatures as observed in the conjugate ionosphere by quantitatively examining the energy exchange that occurs. Firstly, a Monte Carlo method is used to demonstrate statistically that the particle populations observed during conjugate ionospheric high m wave events have more free energy available than populations extracted at random. Secondly, this paper quantifies the energy transferred on a case study basis, for two classes of high m waves, by examining magnetospheric Ion Distribution Functions, (IDFs) and directly comparing these with the calculated wave energy dissipated into the conjugate ionosphere. Estimates of the wave energy at the source and the sink are in excellent agreement, with both being of the order of 10<sup>10</sup>J for a typical high m wave. Ten times more energy (10<sup>11</sup>J) is transferred from the magnetospheric particle population and dissipated in the ionosphere when considering a subset of high m waves known as giant pulsations (Pgs). Previous work has demonstrated that 10<sup>10</sup>J is frequently available from non - Maxwellian IDFs at L=6, whereas 10<sup>11</sup>J is not. The combination of these studies thus provides an explanation for both the rarity of Pgs and the ubiquity of other high m waves in this region.
HF radar backscatter which
has been artificially-induced by a high power RF facility such as the EISCAT
heater at Tromsø has been demonstrated to provide ionospheric electric field
data of unprecedented temporal resolution and accuracy. Here such data are used
to investigate ULF wave processes observed by the CUTLASS HF radars. Within a
short period of time during a single four hour experiment three distinct wave
types are observed with differing periods, and latitudinal and longitudinal
phase evolution. Combining information from the three waves allows them to be
divided into those with a large-scale nature, driven externally to the
magnetosphere, and those with small azimuthal scale lengths, driven by
wave-particle interactions. Furthermore, the nature of the wave-particle
interactions for two distinct small-scale waves is revealed, with one wave
interpreted as being driven by a drift resonance process and the other by a
drift-bounce resonance interaction. Both of these mechanisms with m ≈ -35 and proton energies of 35–45 keV appear to be viable wave energy
sources in the postnoon sector.Key words. Ionosphere (active experiments;
wave-particle interactions) – Magnetospheric physics (MHD waves and
in-stabilities).
We have determined a characteristic of Pc 5 pulsation in the morning sector by use of ground magnetometer and riometer (cosmic noise absorption) data, in conjunction with data acquired with satellites which include the magnetic fields above the ionosphere and electron fluxes at geosynchronous orbit. We have found that the onset of a flux increase in energetic electrons of 30 keV to 200 keV at geosynchronous orbit almost coincides with the onset of Pc 5 pulsation activity and riometer absorption on the ground. We have ascertained that when the Pc 5 pulsation occurs on the ground, the large-scale Birkeland current system observed at ionospheric altitude splits into a number of small-scale Birkeland current pairs. We could infer that the electron flux enhancement, presumably supplied from the tail plasma sheet associated with the substorm onset, provides stress to cause the background large-scale plasma vortex to split into the small-scale vortices. We suggest that the field-aligned currents in the small-scale vortices propagate along the field lines and sustain the standing Alfvenic oscillations at several different, but neighboring shells of the field lines
The Dual Auroral Radar Network (DARN) is a global-scale network of HF
and VHF radars capable of sensing backscatter from ionospheric
irregularities in the E and F-regions of the high-latitude ionosphere.
Currently, the network consists of the STARE VHF radar system in
northern Scandinavia, a northern-hemisphere, longitudinal chain of HF
radars that is funded to extend from Saskatoon, Canada to central
Finland, and a southern-hemisphere chain that is funded to include
Halley Station, SANAE and Syowa Station in Antarctica. When all of the
HF radars have been completed they will operate in pairs with common
viewing areas so that the Doppler information contained in the
backscattered signals may be combined to yield maps of high-latitude
plasma convection and the convection electric field. In this paper, the
evolution of DARN and particularly the development of its SuperDARN HF
radar element is discussed. The DARN/SupperDARN network is particularly
suited to studies of large-scale dynamical processes in the
magnetosphere-ionosphere system, such as the evolution of the global
configuration of the convection electric field under changing IMF
conditions and the development and global extent of large-scale MHD
waves in the magnetosphere-ionosphere cavity. A description of the HF
radars within SuperDARN is given along with an overview of their
existing and intended locations, intended start of operations, Principal
Investigators, and sponsoring agencies. Finally, the operation of the
DARN experiment within ISTP/GGS, the availability of data, and the form
and availability of the Key Parameter files is discussed.
Ultra-low frequency (ULF) field line resonances (FLRs) cause
oscillations in F-region plasma flows and can be detected in SuperDARN
measured line-of-sight (l-o-s) velocities. In this paper, we
characterize a ULF wave event with coordinated use of SuperDARN HF
radars, optical instrumentation, ground based and space based
magnetometers. On December 26, 2000 from 00:00-04:00 UT, the
SuperDARN Pykkvibaer radar observes first and second harmonic FLR
signatures at 0.8 mHz, while the Kodiak and Hankasalmi radars
simultaneously observe the driver wave on open field lines at exactly
the same 0.8 mHz frequency. These observations show that SuperDARN can
provide a diagnostic of MHD wave propagation on open field lines and
potentially be used to monitor MHD wave transmission across the
magnetopause, and through the outer regions of the magnetosphere. MHD
waves in the outer magnetosphere, which couple to FLRs, are seldom
observed, and as far as we know this is the first report of a ground
based observation of the driver wave. The observation of first and
second harmonic FLRs in SuperDARN data is a unique and useful
observation in the sense that it supports the theoretical body of work
on expected behavior of FLRs, and their potential use in estimating
magnetospheric properties such as density and magnetic topology. During
the time interval of interest, Geotail is in the solar wind just outside
the dawn flank region, and observes clear oscillations in the IMF Bz
component at 0.8 mHz. High coherence is shown between the Geotail Bz
oscillations and the radar Doppler velocities at 0.8 mHz, confirming
that the 0.8 mHz FLR harmonic and the driver wave on open field lines is
directly driven by the 0.8 mHz oscillation in the solar wind. Discrete
ULF oscillations in the solar wind as direct drivers of ULF waves in the
magnetosphere is a controversial topic, and the results reported here
add to a growing body of evidence in support of direct solar wind
drivers of ULF waves.
The Kelvin-Helmholtz instability is investigated analytically by using a one-dimensional nonuniform model of the Earth’s magnetosphere and the adjacent solar wind region. Its properties are shown to be essentially governed by the presence of an MHD cavity that arises in the magnetosphere because of the non-uniformity of the latter and also because of the jump in the parameters of the medium at the magnetopause (the outer boundary of the magnetosphere). System oscillations constitute a discrete spectrum of eigenmodes, which are determined by the wave vector k
t
along the tangential discontinuity and also by the mode number n = 0, 1, 2, …, playing the role of the wavenumber along a coordinate normal to the magnetopause. Analytic expressions are obtained for the frequency and instability growth rate of each eigenmode and for the functions describing its spatial structure. All these quantities depend parametrically on the solar wind velocity V
W
, or more precisely, on the Doppler frequency shift ωW
= k
t
· V
W
. For each eigenmode, there is a lower instability threshold depending on the parameter ωW
and a sharp maximum in the growth rate at the eigenfrequency of the magnetospheric cavity. For ωW
values below the threshold, the properties of an eigenmode are highly sensitive to the type of solar wind nonuniformity. Three cases are considered: a uniform solar wind and solar winds in which the speed of sound increases or decreases away from the magnetopause.
Results are presented of an analysis of five pulsation events observed by the Bistatic Auroral Radar System radars (McNamara et al., 1983). It is found that one of these events is similar to the high-m storm-time Pc 5 pulsations reported by Allan et al. (1982) and Walker et al. (1982). The other four also have large values of m, yet differ significantly from the first pulsation event, resembling rather the equatorward traveling band events observed by the Sweden and Britain Auroral Radar Experiment system (Tian et al., 1991; Yeoman et al., 1992), except for smaller m values of the latter events.
On 17 August 2002 the Cluster spacecraft moved through the dayside magnetosphere. Between 16:00 and 18:30 UT clear monochromatic oscillations are seen in both electric field and magnetometer data. The frequency is 4.2 mHz in the spacecraft frame of reference. The oscillations have a clear spatial localisation. The magnetic field oscillations are radially polarised in the plane perpendicular to the background magnetic field, indicating that the wave is in the poloidal mode. From the difference in phase between the satellites we estimate the azimuthal wave number, m, to be about 130, consistent with the magnetic field polarisation. The frequency is stable for different L-values as well as over time. From the value of m, the Doppler shift due to satellite motion is estimated to 0.5 mHz. By looking at the phase of the electric and the magnetic field close to the equator we conclude that the oscillations are in a mode with an odd number of half wavelengths between the two ionospheres.
Properties of compressional Pc 5 waves as deduced from multiple-satellite observations at geosynchronous orbit are presented. The occurrence characteristics of the waves are determined, and the relation between variations in particle fluxes and magnetic field is examined. The spatiotemporal structure of the waves is considered, including the propagation perpendicular to the ambient magnetic field and the relation of the frequency characteristics to harmonic waves. It is demonstrated that the waves have large azimuthal wave numbers from 40 to 120, westward propagation at a typical velocity of 10 km/s, frequency roughly 25 percent of the second harmonic of the poloidal wave, and phase lag of 180 deg between the parallel and radial components of the wave magnetic field and + or -90 deg between the parallel and azimuthal components. These features are discussed in the light of existing theories of instabilities in the ring current plasma.
The Doppler Pulsation Experiment (DOPE) is an HF Doppler sounder deployed at Tromsø, northern Norway, for the investigation of the ionospheric signatures of ULF wave phenomena of magnetospheric origin. This high-latitude Doppler sounder has been demonstrated to be a highly sensitive instrument for the investigation of ULF waves of large azimuthal wavenumber m. Such waves are assumed to be driven by magnetospheric wave-particle interactions and are difficult to study by other ground-based techniques owing to their strong spatial integration between the ionosphere and the ground. A new population of high-m waves has been identified in the ground-based data from DOPE, which could represent a significant new sink for ring current energy. These waves have m numbers of order 100 and are observed primarily in the morning sector. Their characteristics are described and compared to previous statistical observations from spacecraft. The DOPE sounder now offers routine measurements of the occurrence and characteristics of such high-m wave populations.
The Johns Hopkins University/Applied Physics Laboratory HF radar at Goose Bay often sees F-region drifts or electric fields which are associated with field line resonances in the Earth's magnetosphere. These resonances are seen in the interval from local midnight to morning, and have discrete, latitude-dependent frequencies at approximately 1.3, 1.9, 2.6–2.7, and 3.2–3.4 mHz. We show that these frequencies are compatible with MHD waveguide modes, with antisunward propagation and reflection at the magnetopause and at turning points on dipolar field lines.
Geosynchronous orbit data from charged particle analyzer (30- to 300-keV electron flux) and magnetometer instruments on board the 1984-129, GOES 6, GOES 5, and 1982-019 satellites distributed in the midnight-to-dawn sector are examined during a particle injection event that occurred at 0820 UT on May 21, 1986. The injection was detected by 1984-129 in the midnight sector as a dispersionless and short-lived (duration was 9 min) event. The injected electron cloud drifted eastward and was detected by 1982-019 in the dawn sector. Meanwhile, the GOES 6 and 5 satellites were in the postmidnight sector between the satellites which measured the electron flux. They observed transient field line oscillations during the periods when the azimuthal plasma pressure arising from the electron cloud was in the postmidnight sector. It is suggested that field-aligned currents are likely to be built up preferentially by a divergence of diamagnetic currents in the region where the azimuthal plasma pressure gradient dominates. It is argued that these currents excite an azimuthally polarized transient field line oscillation.
Numerous field line resonance events have been observed with three HF
radars (Saskatoon, Kapuskasing, and Goose Bay) of the Super Dual Auroral
Radar Network (SuperDARN). The field line resonances cause oscillations
in the F region plasma flows which are detected in the measured line of
sight Doppler velocities. After analysis, it was found that the
resonances were of two types: those with low azimuthal wave number,
low-m, and those with high azimuthal wave number, high-m. The high-m
events showed many similarities with high-m pulsations of previous
reports including local time of most occurrences (noon-dusk), pulsation
frequencies, westward propagation, increase in phase with latitude, and
north-south polarization. The low-m events exhibited typical field line
resonance characteristics and were found near dusk and dawn with
anti-Sunward propagation. The most notable result was the fact that the
high- and low-m events shared many common features. They both were found
to occur at the same discrete and stable frequencies. The most common
frequencies were 1.3, 1.9, and 2.5-2.6 mHz, which have previously been
associated with magnetospheric waveguide modes. They also occurred at
other less common frequencies, such as 1.5-1.6 mHz. Both types of events
were localized in latitude with an inverse relation between frequency
and latitude. Both were characterized by a wave packet structure with a
duration of approximately 1 hour. The numerous features shared by the
high- and low-m resonances strongly suggest that they are caused by the
same source mechanism. A dispersive waveguide model as a source for the
field line resonances is discussed.
The ULF magnetic field oscillations observed at geostationary orbit of predominantly compressional nature that are accompanied by strong modulations of the energetic ion fluxes in antiphase to the magnetic field oscillations are studied in detail. The events are classified as either diamagnetic or nondiamagnetic. In the case of diamagnetic events, the oscillations commenced after a change in the magnetic field/hot plasma configuration initialized by the arrival of drifting ion bunches. The nondiamagnetic type of oscillations were encountered by the satellite without any preceding change in the plasma/magnetic field configuration.
The behavior of ULF activity in the Pc5–6/Pi3 band (1–7mHz) on a global scale during the main phase of the intense magnetic storm on May 15, 1997, is studied using data from stations in the INTERMAGNET, Greenland Coast Array, IMAGE, and CPMN (210 MM) networks. Two regions of long-period broad-band ULF intensification are observed during the magnetic storm: one is in the early morning hours, the other is near dusk. The first one is, probably, related to an energetic electron injection, whereas the other one is due to the injection of ring current protons. It is suggested that ULF waves may be an intermediate agent transferring energy from the ring current protons/electrons to the relativistic electrons. The azimuthal scale of ULF disturbances in the dusk sector as determined from the IMAGE array was about 500km, which is an order of magnitude less than had been often assumed in modeling of resonant electron acceleration by MHD waves. The features of the observed broad-band ULF oscillations impose some constraints on models of relativistic electron acceleration by MHD waves during magnetic storms.
A unified theory of low frequency instabilities in a two component magnetospheric plasma is suggested. It is shown that the low frequency oscillations comprise compressional Alfven and drift mirror mode. No significant coupling between them is found in the long-wave approximation. Instabilities due to spontaneous excitation of these oscillations are considered. It is found that the temperature anisotropy significantly influences the instability growth rate at low frequency. Instability due to the temperature anisotropy and density gradient appears when the frequency of compressional Alfven waves is close to the drift mirror mode frequency. The theoretical predictions are compared with a Pc5 event of observed simultaneously by GEOS 2 and the STARE radar. It is shown that the experimental results can be interpretated in terms of a compressional Alfven wave driven by the drift anisotropy instability.
The trejectories of mirroring protons and electrons are obtained by tracing in a relistic model of the magnetosphere. Day-night asymmetry is found in the particle trajectories, which is caused by the asymmetry in gradient and curvature drifts. It is also found that the high-energy protons injected from the nightside escape from the duskside magnetopause. This may be important for the explanation of partial ring-current closure.
The asymptotic temporal behavior of hydromagnetic waves in a model of the inner magnetosphere is shown to be characterized by guided modes. The basic hydromagnetic wave equation is derived and applied to a cylindrical model of the inner magnetosphere. This model offers a good representation of the spatially dependent field line frequencies present in a dipole field. The initial value problem for the symmetric toroidal mode is solved, and its singularities are treated by Fourier superposition. Singularities also are present in the asymmetric poloidal mode wave equation and are shown to be logarithmic. Fourier superposition leads to the solutions for the electric and magnetic fields, which are asymptotic in time. The results indicate decay of the poloidal modes and domination by the toroidal modes, i.e., field line control of the propagation. At the latitude at which the maximum amplitude of a particular frequency occurs, the wave becomes linearly polarized. The asymptotic micropulsation periods depend on the characteristic field line periods and are usually longer at higher latitudes. Undamped guided waves lead to some terms, such as change density and parallel current density, that increase linearly with time. The inclusion of loss mechanisms in the wave equation, e.g., conductivity, limits the guided modes and prevents such nonphysical effects from occurring. Geomagnetic micropulsations with periods longer than about 30 s show systematic latitudinal variations in amplitude and frequency and similarities of period and form at c.onjugate points (see, e.g., the reviews of Troitskaya (1967], Saito [1969], Jacobs [1970], and Orr [1973]). The most familiar of these effects is that longer-period events occur at higher latitudes with the period being approximately proportional to the L value of the corresponding field line. These observations seem to indicate field line control of the hydromagnetic propagation not only by guiding the energy along the magnetic field but also by actually determining the spectra through resonances of the field lines themselves. However, an adequate theoretical justification for such guided wave phenomena to occur in the magnetosphere has not been available.
It has often been suggested that energetic protons are involved in the generation of giant pulsations (Pgs). A Pg event occurring in the early morning of 29 December 1987 has been observed by magnetometers in Scandinavia, the Faroe Islands and Iceland. The Pg has also been observed by the SABRE coherent radar system. The azimuthal drift velocity of the Pg disturbance region across the available longitudinal extent has been measured using the magnetometer data ans the radar data have been used to estimate the ionospheric convection electric field. The energies of protons that would drift at this velocity in the region of the magnetosphere where the Pg occurred have been deduced to be ~10-20 keV, depending on the pitch angle of the protons. Using azimuthal wavenumbers calculated on the EISCAT magnetometer cross and the SABRE radar for this event, the energies of protons that would satisfy the bounce resonance condition for this Pg have been calculated to be ~5-18 keV. The Pg is thought to be driven by protons associated with a substorm particle injection that occurred on 28 December 1987. The energies of protons that would drift westward from this injection region and reach the Pg disturbance region have been calculated to be ~11-24 keV. This study therefore presents good evidence to suggest that the bounce resonance interaction with protons is the mechanism responsible for the Pg and that these protons originated from a substorm particle injection.
Conditions for the realization of ULF waves with different polarization are formulated and verified with the use of observational
data from the spacecrafts which performed measurements in the earth’s magnetosphere. It is shown that the conditions formulated
are in good agreement with observed wave parameters.
The large-scale behavior of quasi-steady magnetic activity and energetic particle precipitation and their variations in the nominal Pc5-6/Pi3 band (1.7–) during the main phase of the intense magnetic storm on May 15, 1997, is studied using data from an array of magnetic and riometer stations in the Northern Hemisphere. The global azimuthal structure of magnetic and cosmic noise absorption disturbances at ∼64° geomagnetic latitude is revealed with the help of MLT-UT diagrams. The ionospheric westward electrojet intensifies in the morning sector and the eastward electrojet intensifies in the evening sector. Intense storm-related ULF magnetic activity is evident in two regions: in the early morning hours and in the evening sector. In the dawn sector, the westward electrojet intensification and ULF magnetic pulsations are accompanied by concurrent variations of cosmic noise absorption. In the dusk sector, where observations at two magnetic stations with small separations (<10°) are available, the data indicate small-scale structure of storm-related Pi3 pulsations with longitudinally sunward propagation, as might be expected according to the injected proton drift. The small-scale meridional pattern of ULF-modulated electron precipitation during the substorm interval as determined from the IRIS riometer in Scandinavia indicate poleward propagation in the dawn sector. The results suggest that the physical mechanism of long-period magnetic disturbances observed during the main phase of a magnetic storm is different from common dawn Pc5 pulsations and is related to the injection of energetic electrons and protons into the early morning and dusk sectors of magnetosphere, correspondingly. This difference is important for the modeling of relativistic electron energization by ULF waves.
The exchange of energy between a given wave and the energetic particle population is investigated, with emphasis on waves with a rapid East-West variation of phase. Reasons are given why these are of particular interest, but the field disturbance has yet to be computed in detail and here a physical account is presented leading to rough estimates for the damping or amplification. The techniques applied to gyroresonance are followed as closely as possible, but bounce resonance is more complicated, and great simplifications are made, particularly in neglecting harmonics. Transformation to the rotating frame of the wave gives a simple way of calculating the change in energy of a single particle for a given change in L. The resonance condition including the Doppler shift due to the East-West particle drift is a quadratic for the particle energy. Typically there is a low energy for which the bounce frequency almost equals the wave frequency and a high energy for which the Doppler shifted frequency is much higher than the wave frequency. The high energy resonance is more favourable for wave amplification by the kind of particle distribution known to exist. Typically the high energy electrons are relativistic and here attention is given to the high energy proton resonance. For westward travelling waves, protons which move out lose energy and the outer edge of a proton belt could supply energy to such a wave via bounce resonance.
We describe the observation of a magnetic pulsation with a period of 55 s, recorded at geostationary orbit by three satellites (ATS 6, SMS 1 and SMS 2) in the local time sector 2100–2400. We use magnetic data from all three spacecraft and also plasma data from ATS 6. The pulsation had a large compressional magnetic component which appeared to be balanced by pressure fluctuations in the hot ring current plasma which were in antiphase with the magnetic variations. This allows the wave to be guided along a field line. From the plasma data we are also able to obtain estimates of the field line displacement and hence the electric field, which enables us to conclude that this is a second harmonic field line resonance. We find that the wave has a very short East-West (E-W) wavelength (m≅100) and a westward azimuthal group velocity of about 30 km s−1. The most probable source for this wave is a bounce resonant interaction with ring current protons. The characteristics of this wave are in many ways similar to those of giant pulsations observed on the ground. ATS 6 was near the inner edge of the ring current electrons and as the wave converted the 10 keV electron Alfvén layer back and forth across ATS 6, we were able to estimate the Alfvén layer energy gradient and obtain a value of 1 keV in 1000 km. This gradient is considerably steeper than that predicted by a steady uniform convection electric field.
Detailed properties of a Pc5 pulsation with large azimuthal wavenumber observed using the STARE radar have recently been reported. A further four examples of this type of pulsation are presented, and it is shown that their properties are generally similar to those of the first example. However, there are some differences, the most important being that the variation of azimuthal phase velocity with latitude is significantly different for different time intervals during individual events, so that a mean phase velocity for a given latitude cannot be defined.When mapped to the equatorial plane in a dipole geomagnetic field, the variation of azimuthal phase velocity with L resembles the gradient-curvature drift of energetic protons in only a few time intervals within the events. The results are interpreted in terms of current theories of drift and bounce resonance of energetic particles with hydromagnetic waves. It is found that no single theory explains all aspects of the observations.
An often observed and still unexplained feature of the high-m Alfvén waves in the terrestrial magnetosphere is their equatorward phase motion, in contrast with low-m waves. We suggest an explanation of this fact in terms of a model of wave excitation by an azimuthally drifting particle inhomogeneity injected during substorm activity. The azimuthal direction of the phase velocity coincides with that of the cloud. If the drift velocity increases with the radial coordinate, the particle cloud is stretched into spiral in the equatorial plane which leads to a radial component of the phase velocity directed toward Earth, that is, an equatorward phase propagation.
A working model of simultaneous auroral activity over the entire polar region is presented in terms of the auroral substorm. The substorm has two characteristic phases, an expansive phase and a recovery phase. Each phase is divided into three stages, and characteristic auroral displays over the entire polar region during each stage are described in detail. Further, all the major features seen at a single station are combined into a consistent picture of large-scale auroral activity.РефератRaбoчaя мoдeль oднoвreмeннoйaктивнocти пoляrныч cияний нaд вceй пoляrнoй зoнoй нrcдcтaвляeтcя в ycлoвияч вызвaннoй пoляrным cияниeм cyббyrи. y cyб-бyrи имeютcя двe чaraктerныe фaзы—фaзaпacшиreния и фaзa reгeнeпaции. Кaздaя фaзa racпaдaeтcя нa тrи cтaдии и дaeтcя дeтaльнoe oпиcaниe тиЦичныч нoляrныч cияний нaд вceм нoляrным raйoнoм вo вreмя кaздoй cтaдии. Дaлee, вce глaвныe чaraктerиcтики, нaблюдaeмыe нa oдинarнoй cтaнции, coвмeщeны в coглacoвaннoм изoбraзeнии шиroкoй aктивнocти пoляrныч cияний.
Equatorward propagating bands of auroral radar backscatter of period 150–600 s are a commonly observed phenomenon on the SABRE VHF coherent radar, but until very recently were rarely observed on other coherent radar systems located at different latitudes. A recent statistical study (Mao Tian et al., 1991, Planet. Space Sci.39, 1239) provided strong evidence that such propagating bands on SABRE were geomagnetic pulsations with resonant structures on the plasmapause. Here a detailed analysis of such pulsations observed by SABRE, together with the SAMNET and EISCAT cross magnetometer arrays, is presented. The pulsations have rapid westward azimuthal phase propagation independent of local time and a latitudinal phase change in both geographic and geomagnetic coordinate systems which, on average, exceeds the 180° phase change predicted by field line resonance theory. Furthermore, the events exhibit a strong attenuation between their ionospheric signature and their ground magnetometer signature. These results are consistent with an excitation mechanism involving wave-particle interactions, similar to that postulated for storm-time Pc5 pulsation events. The azimuthal phase propagation of the pulsations, if used to calculate a drift velocity mapped out to the equatorial plane, is consistent with that of protons of energy 35–70 keV undergoing gradient-curvature drift. It is suggested that these particles drift westwards and interact with the dusk plasmapause region, producing a resonant wave which is detected by the coherent radar as an equatorward propagating pulsation event.