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MLT distribution of CNA events classified into (left column) substorm-related, (middle column) ULF pulsation-related, and (right column) nonclassified types. The riometer stations are (first row) Ivalo, (second row) Sodankylä, (third row) Oulu, and (fourth row) Jyväskylä. Each line color corresponds to a day relative to zero epoch as indicated in the legend.
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The effect of solar wind high-speed streams (HSSs) on energetic particle precipitation at auroral and subauroral latitudes (L = 3.8–5.7) is studied by using cosmic noise absorption (CNA) data measured by the Finnish riometer chain during 95 HSS events occurring between 2006 and 2008. The data are divided into “long” and “short” HSS events, dependin...
Contexts in source publication
Context 1
... CNA events corresponding neither to SS type nor to ULF type are labeled as "nonclassified" (NC type). Figure 5 shows the MLT distributions of SS-type (left column), ULF-type (middle column), and NC-type (right column) CNA events from day D-2 until day D4 (see legend for color codes). These distributions are given for IVA (first row), SOD (second row), OUL (third row), and JYV (fourth row). ...
Context 2
... distributions are given for IVA (first row), SOD (second row), OUL (third row), and JYV (fourth row). The L values of these riometer stations are indicated in the top left-hand corner of Figure 5 (left column). ...
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... all cases, SS-type CNA events have the greatest absorption values in the night and morning sectors. While the number of events for SS-type CNA peaks at midnight for IVA and SOD (Figure 5), the largest absorption values take place as late as 4-8 MLT ( Figure 6). However, for ULF-type events, the number of events and absorption values have similar MLT distributions. ...
Context 4
... in mind this interpretation of each CNA type, let us discuss their MLT and magnetic latitude distributions. Our observations cover L values from 3.8 to 5.7 ( Figure 5). SS-type CNA events have their MLT distribution peaking around 1-2 MLT, and they occur mostly at auroral latitudes (L = 5.2-5.7). ...
Context 5
... very distinct feature for the dayside high-latitude (IVA and SOD) CNA events is that a majority of them are ULF type after the arrival of HSS ( Figure 5). This may indicate that compressional ULF waves indeed play an important role in modifying the growth rate of VLF whistler waves [Coroniti and Kennel, 1970]. ...
Context 6
... of CNA in the noon sector would correspond to the drift time for E ≈ 70 keV electrons with a small pitch angle (í µí»¼ = í µí¼∕12). Given that the MLT distribution of ULF-type CNA events at the auroral stations peaks between 9 and 12 MLT during days D1 and D2 ( Figure 5), even electrons with energies slightly lower than 70 keV could produce dayside CNA about 30 min after the substorm injection. Also, the 30 min delay found from the cross correlation of ULF-type CNA with the AE index is consistent with results from Beharrell et al. [2015], based on observations and modeling. ...
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This investigation shows that the significant electric field disturbances in the dip‐equatorial ionosphere during the geomagnetic storm of 6–8 September 2017 are due to the passage of two consecutive interplanetary coronal mass ejections (ICMEs). During the passage of the first ICME sheath, a long duration (∼10 hr) prompt penetration (PP) event is...
Citations
... HSSs interact with the preceding slow solar wind to form stream-, and subsequently, corotating interaction regions (SIRs/CIRs), which give rise to the creation of shocks, compression and rarefaction regions, as well as forward and reverse waves (Belcher & Davis 1971). These are widely recognized as the origin of recurrent geomagnetic activity at approximately 27 day intervals, affecting various conditions in the Earth's atmosphere, ionosphere, and magnetosphere (e.g., Alves et al. 2006;Grandin et al. 2015Grandin et al. , 2017Kilpua et al. 2017;Richardson 2018). From in situ measurements at 1 au, Schwenn (1990) defined HSSs as solar wind with speeds of approximately v p ≈ 400-800 km s −1 , low densities (n p ≈ 3 cm −3 ), and high temperatures (T p ≈ 2.5 × 10 5 K). ...
High-speed solar wind streams (HSSs) interact with the preceding ambient solar wind to form stream interaction regions (SIRs), which are a primary source of recurrent geomagnetic storms. However, HSSs may also encounter and subsequently interact with interplanetary coronal mass ejections (ICMEs). In particular, the impact of the interaction between slower ICMEs and faster HSSs represents an unexplored area that requires further in-depth investigation. This specific interaction can give rise to unexpected geomagnetic storm signatures, diverging from the conventional expectations of individual SIR events sharing similar HSS properties. Our study presents a comprehensive analysis of solar wind data spanning from 1996 to 2020, capturing 23 instances where such encounters led to geomagnetic storms ( SymH < −30 nT). We determined that interaction events between preceding slower ICMEs and faster HSSs possess the potential to induce substantial storm activity, statistically nearly doubling the geoeffective impact in comparison to SIR storm events. The increase in the amplitude of the SymH index appears to result from heightened dynamic pressure, often coupled with the concurrent amplification of the CMEs rearward ∣ B ∣ and/or B z components.
... Multi-point observations of CNAs are necessary to clarify the spatio-temporal development of high-energy particle precipitation (Berkey et al., 1974;Grandin et al., 2017;Kavanagh et al., 2007;Ranta et al., 1981;Rosenberg & Dudeney, 1986;Spanswick et al., 2007). Rosenberg and Dudeney (1986) reported the dependence of CNA on substorm, local time, and season statistically using a one-year data set obtained at the Siple and Halley stations located in Antarctica at L = 4. ...
... They found that the initial electron injection region must have a radial extent of less than 1 Earth radius and is capable of having an azimuthal extent of at least 1 hour of local time. Grandin et al. (2017) measured energetic particle precipitation from 95 solar wind high-speed streams (HSS) that occurred in 2006-2008 in a Finnish riometer chain deployed latitudinally at L = 3.8-5.7. Their results showed that CNAs occurred frequently and extended to subauroral latitudes when the HSS lasted more than one day. ...
Enhancements in electron density in the D‐region ionosphere attributed to the precipitation of high‐energy electrons, have previously been inferred from increases in cosmic radio noise absorption (CNA) using ground‐based riometers. However, there have been few studies of CNA observations at multi‐point stations distributed in longitudes. Thus, the spatio‐temporal development of the global distribution of CNA is not well understood. In this study, we investigated the longitudinal extent of CNA using simultaneous riometer observations at six stations at subauroral latitudes in Canada, Alaska, Russia, and Iceland. These stations are located encircling the earth at ∼60° north magnetic latitudes. We have conducted simultaneous observations of CNA at these stations since October 2017. Here we focus on seven substorms during a geomagnetic storm 25–28 August 2018 and study the spatio‐temporal development of the global distribution of CNA during these substorms. For all seven substorms, some stations observed CNA enhancements after the substorm onsets. In five cases, the CNA enhancements started around midnight and expanded eastward. The other two cases show westward and anti‐sunward development of CNA. The eastward expansion of CNA indicates the eastward drift of high‐energy electrons, which is the source of the CNA, due to gradient and curvature drift in the geomagnetic field. The westward expansion of CNA may correspond to westward expansion of the substorm injection region due to dawn‐to‐dusk electric fields. These results indicate that spatio‐temporal development of CNA at subauroral latitudes corresponds to high energy electron drift in the inner magnetosphere.
... It is well known that the 30-100 keV electrons are absorbed in the atmosphere typically between 70 and 100 km altitudes and they produce cosmic noise absorption (CNA). Measurements of CNA near the magnetic latitude of Tromsø show a distinct diurnal variation with a maximum in the pre-noon sector (09-10 MLT) and a deep minimum close to dusk (Grandin et al., 2017;Kavanagh et al., 2004). The MLT location of the CNA maximum is in accordance with Figure 12. Figure 6 shows that the energetic electron precipitation expands to pre-midnight hours during active geomagnetic conditions (Hpo ≥3). ...
... This is in agreement with earlier studies; for example, using NOAA POES data, Lam et al. (2010) showed that the number flux of >30 keV electrons significantly increases during active geomagnetic conditions and it is maximum in the dawn sector outside of the plasmapause. Furthermore, Grandin et al. (2017) found that CNA associated with substorm activity dominates in the 21-06 MLT sector. ...
We use the EISCAT incoherent scatter radar data measured in years 2001–2021 to study statistical characteristics of 1–100 keV electron precipitation at 66.7° MLAT over Tromsø. Peak energies, auroral powers and number fluxes of precipitating electrons are derived from electron density altitude profiles measured along the geomagnetic field line during periods of no photoionization. The method allows us to include energetic 30–100 keV electrons, which are poorly covered in earlier satellite‐based studies. Locations of the radar within the auroral oval are determined using a model with the 1‐hr Hpo geomagnetic index as input. The average peak energy of precipitating electrons increases almost monotonically from evening (18 MLT) to morning hours (09 MLT). The 30–50 keV electrons dominate the energetic electron precipitation before 06 MLT, after which the 50–100 keV precipitation becomes dominant. Large auroral powers (>60 mWm⁻²) are observed in the 18–02 MLT sector in the main auroral oval. We obtain occurrence rate of electron precipitation in Tromsø by calculating the fraction of data points with auroral power larger than 2 mWm⁻². The occurrence rate peaks during the declining phases of solar cycles (sc), in 2002–2004 for sc23 and in 2015–2017 for sc24, caused by variations in geomagnetic activity. In addition, the occurrence rate has maxima during March and September, minimum in December to January, and it increases monotonically from evening to morning hours, reaching maximum at 05–06 MLT.
... In particular, the impact of the so-called "sheath" periods of the ICMEs were found to be considerably lower at JYV than at the other stations. In a somewhat similar statistical manner, Grandin et al. (2017a) studied the impact of the solar wind high-speed streams on the high-latitude ionosphere. These events were divided into two categories, labelled "long" and "short", depending on the duration of the solar wind speed increase to a maximum value. ...
The Antarctic and Arctic regions are Earth's open windows to outer space. They provide unique opportunities for investigating the troposphere–thermosphere–ionosphere–plasmasphere system at high latitudes, which is not as well understood as the mid- and low-latitude regions mainly due to the paucity of experimental observations. In addition, different neutral and ionised atmospheric layers at high latitudes are much more variable compared to lower latitudes, and their variability is due to mechanisms not yet fully understood. Fortunately, in this new millennium the observing infrastructure in Antarctica and the Arctic has been growing, thus providing scientists with new opportunities to advance our knowledge on the polar atmosphere and geospace. This review shows that it is of paramount importance to perform integrated, multi-disciplinary research, making use of long-term multi-instrument observations combined with ad hoc measurement campaigns to improve our capability of investigating atmospheric dynamics in the polar regions from the troposphere up to the plasmasphere, as well as the coupling between atmospheric layers. Starting from the state of the art of understanding the polar atmosphere, our survey outlines the roadmap for enhancing scientific investigation of its physical mechanisms and dynamics through the full exploitation of the available infrastructures for radio-based environmental monitoring.
... However, it is yet unclear whether the obtained response functions and the approach itself may be effectively applied during disturbed conditions. Indeed, a number of studies claim that magnitudes of substorm-associated energetic electron precipitation and auroral absorption depend on the preceding activity ranging from hours up to several days [2,[14][15][16][17][18][19]. Particularly, some reports state that substorm clusters (also known under the names of recurrent substorms, compound substorms, etc.) show significantly larger flux increases than single substorm events do [2]. ...
... Our aim here is to derive empirical response functions for the auroral absorption and test the LPF approach during long active periods containing clustered substorms during non-storm times. Starting from Tsurutani and Gonzales [20] this class of disturbances attracted large attention, because of their appearance during high-speed solar wind streams (HSS) and stream interaction regions and remarkable association with the major radiation belt flux increases [21,22] and EEP enhancements [14,15,23]. According to [23] "On average, the flux of >30 keV electrons is enhanced by a factor of~10 during the passage of the high-speed stream at all geographic longitudes", which explains their importance for space weather and climate [24]. ...
... These observations raise a question concerning the role of 'long memory' effects on the EEP intensity which has been widely discussed in the recent literature [2,14,15,[17][18][19]. ...
Recently we proposed a spatio-temporal model of auroral absorption for isolated substorms based on linear prediction filter technique, which describes the precipitation effects as a sum of properly weighted and time-delayed contributions of short dipolarizations/injections inferred from magnetic MPB index. Here we apply a similar approach to a more general and practically important type of continuous activity including substorms clusters, which is accompanied by intense energetic electron precipitation lasting for many hours and may affect ozone concentration and climate. Unexpectedly, in spite of very different geophysical background, the derived precipitation response to unit-scale injection appeared almost identical to that obtained for isolated substorms. Significant part of absorption variance during active non-storm periods turns out to be the result of superposition of previous injections with short memory less than 4 h. Our results indicate that, while the injection efficiency is roughly the same, large difference in precipitated fluxes and ionospheric response between two different types of activity is mostly provided by a more frequent appearance and increased intensities of dipolarizations/injections during active periods. In both event types dipolarizations are the decisive factor which determines energetic electron precipitation.
... Global ULF waves, furthermore, displace electrons radially inward, bringing them closer to the Earth where equatorial geomagnetic field is stronger and thus the loss cone wider (Brito et al. 2015). As different wave modes in the inner magnetosphere have distinct MLT dependences, the precipitation signatures also show clear MLT dependence as demonstrated by Grandin et al. (2017), who also found that the substorm-related events peaked strongly close to midnight, while ULF-associated events peaked close to noon. ...
This open access book serves as textbook on the physics of the radiation belts surrounding the Earth. Discovered in 1958 the famous Van Allen Radiation belts were among the first scientific discoveries of the Space Age. Throughout the following decades the belts have been under intensive investigation motivated by the risks of radiation hazards they expose to electronics and humans on spacecraft in the Earth’s inner magnetosphere.
This textbook teaches the field from basic theory of particles and plasmas to observations which culminated in the highly successful Van Allen Probes Mission of NASA in 2012-2019. Using numerous data examples the authors explain the relevant concepts and theoretical background of the extremely complex radiation belt region, with the emphasis on giving a comprehensive and coherent understanding of physical processes affecting the dynamics of the belts. The target audience are doctoral students and young researchers who wish to learn about the physical processes underlying the acceleration, transport and loss of the radiation belt particles in the perspective of the state-of-the-art observations.
... We successfully tested this algorithm version using observations during a 5 days-long period of intense and complex magnetic activity. Good correlation between nonisolated CNA and AL variations have been also noticed by Grandin et al. (2017) during a disturbed period occurring during the high-speed solar wind stream. With the remark that, by construction, only the injection-related component of substorm precipitation is reproduced by LPF method, the reasonable scores suggest a potential usefulness of such prediction tool. ...
Previously, suggesting a scenario of “injected and drifting electron cloud” to describe the enhancements and precipitations of energetic electrons during substorms we used the linear prediction filter (LPF) method to build the empirical dynamical model of auroral absorption in the middle of auroral zone, driven by the midlatitude positive bay (MPB) index time series. In this paper, to understand better the relationship between magnetic dipolarization, injection, and energetic electron precipitation, we quantitatively explore correlations between dipolarization proxies (MPB and SML indices, substorm current wedge (SCW) intensity, location, and size) and precipitation‐related auroral absorption in the morning maximum region for 148 isolated substorms. We confirm good correlation of precipitation peak values with dipolarization proxies and show that the absorption amplitude is most strongly controlled by total SCW current and its azimuthal size, and is also influenced by solar wind‐dependent background energetic electron flux in the conjugate plasma sheet. This result confirms the adequacy of our starting assumption that there is a close relationship between the intensities and size of substorm dipolarizations and energetic particle injections. To extend the latitudinal coverage, we computed the LPF response functions for individual riometers of NORSTAR array. Finding similar shapes and amplitudes of their response at latitudes 63°–69° in the regions where maximal precipitation is statistically observed, we suggest, test, and approve that the auroral absorption in the belt 63°–69° may be predicted using a set of LPFs determined empirically in the center of auroral zone for various magnetic local times.
... Energetic electron precipitation is routinely monitored from the ground using riometers, which measure the cosmic noise absorption in the D region of the ionosphere associated with particle precipitation (e.g. Hargreaves, 1969;Rodger et al., 2013;Grandin et al., 2017a). Phase and amplitude perturbations to subionospheric man-made narrow-band transmitter signals propagating over long distances are also routinely used to identify energetic electron precipitation . ...
The lower-thermosphere–ionosphere (LTI) system consists of the upper atmosphere and the lower part of the ionosphere and as such comprises a complex system coupled to both the atmosphere below and space above. The atmospheric part of the LTI is dominated by laws of continuum fluid dynamics and chemistry, while the ionosphere is a plasma system controlled by electromagnetic forces driven by the magnetosphere, the solar wind, as well as the wind dynamo. The LTI is hence a domain controlled by many different physical processes. However, systematic in situ measurements within this region are severely lacking, although the LTI is located only 80 to 200 km above the surface of our planet. This paper reviews the current state of the art in measuring the LTI, either in situ or by several different remote-sensing methods. We begin by outlining the open questions within the LTI requiring high-quality in situ measurements, before reviewing directly observable parameters and their most important derivatives. The motivation for this review has arisen from the recent retention of the Daedalus mission as one among three competing mission candidates within the European Space Agency (ESA) Earth Explorer 10 Programme. However, this paper intends to cover the LTI parameters such that it can be used as a background scientific reference for any mission targeting in situ observations of the LTI.
... The CNA values during the substorms studied here peaked between 0.5 and 1.0 dB (Fig. 4), which is between the longterm average CNA including the quiet times (CNA mainly below 0.5 dB, Kavanagh et al., 2004) and the absorption values related to geomagnetic storms driven by coronal mass ejection (CME) sub-structures, sheaths, and ejecta (CNA values typically around 1 dB, Kilpua et al., 2020). An earlier study on HSS-driven substorms reported CNA levels of 1-2 dB (Grandin et al., 2017). Thus, CNA values tend to increase with increasing magnetic activity on average. ...
... As was further discussed by Kilpua et al. (2020), the direct substorm injection into the ionosphere results in less intense precipitation than that driven by waveparticle interaction, which is in effect when the injected electrons drift into the morning sector after the injection. Grandin et al. (2017) also concluded that strong CNA is more likely to occur during a substorm event with a longer duration. ...
A set of 24 isolated, 46 compound, and 36 multi-night substorm events from the years 2008–2013 have been analysed in this study. Isolated substorm events
are defined as single expansion–recovery phase pairs, compound substorms
consist of multiple phase pairs, and multi-night substorm events refer to
recurring substorm activity on consecutive nights. Approximately 200 nights of
substorm activity observed over Fennoscandian Lapland have been analysed for their magnetic disturbance magnitude and the level of cosmic radio noise
absorption. Substorm events were automatically detected from the local
electrojet index data and visually categorized.
We show that isolated substorms have limited lifetimes and spatial extents as compared to the other substorm types. The average intensity (both in
absorption and ground-magnetic deflection) of compound and multi-night
substorm events is similar. For multi-night substorm events, the first night
is rarely associated with the strongest absorption. Instead, the high-energy
electron population needed to cause the strongest absorption builds up over
1–2 additional nights of substorm activity. The non-linear relationship
between the absorption and the magnetic deflection at high- and low-activity conditions is also discussed. We further collect in situ particle spectra for expansion and recovery phases to construct median precipitation fluxes at
energies from 30 eV up to about 800 keV. In the expansion
phases the bulk of the spectra show a local maximum flux in the range of a few keV to 10 keV, while in the recovery phases higher fluxes are seen
in the range of tens of keV to hundreds of keV. These findings are discussed
in the light of earlier observations of substorm precipitation and their
atmospheric effects.
... Balloon experiments flying in the stratosphere can detect Bremsstrahlung emission produced by precipitating particles interacting with neutrals in the atmosphere, as is done during BARREL campaigns (Woodger et al., 2015). Energetic electron precipitation is routinely monitored from the ground using riometers, which measure the cosmic noise absorption in the D region of the ionosphere associated with particle precipitation (e.g., Hargreaves, 1969;Rodger et al., 2013;Grandin et al., 2017a). Phase and amplitude 405 perturbations to subionospheric man-made narrow-band transmitter signals propagating over long distances are also routinely used to identify energetic electron precipitation . ...
The lower-thermosphere–ionosphere (LTI) system consists of the upper atmosphere and the lower part of the ionosphere, and as such comprises a complex system coupled to both the atmosphere below and space above. The atmospheric part of the LTI is dominated by laws of continuum fluid dynamics and chemistry, while the ionosphere is a plasma system controlled by electromagnetic forces driven by the magnetosphere and solar wind. The LTI is hence a domain controlled by many different physical processes. However, systematic in situ measurements within this region are severely lacking, although the LTI is located only 80 to 200 km above the surface of our planet. This paper reviews the current state of the art in measuring the LTI, either directly or by several different remote-sensing methods. We begin by outlining the open questions within the LTI requiring high-quality in situ measurements, before reviewing directly observable parameters and their most important derivatives. The motivation for this review has arisen from the recent retention of the Daedalus mission as one among three competing mission candidates within the European Space Agency (ESA) Earth Explorer 10 Programme. However, this paper intends to cover the LTI parameters such that it can be used as a background scientific reference for any mission targeting in situ observations of the LTI.
