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Average NH FAC densities in NH winter as a function of geomagnetic latitude for By > 0 and By < 0 at (a) 6 MLT and (b) 18 MLT. (c, d) Same as panels (a, b), but for By > 5 nT and By < −5 nT. FAC, field‐aligned current; NH, Northern Hemisphere.
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Geomagnetic activity is mainly driven by the southward (Bz) component of the interplanetary magnetic field (IMF), which dominates all solar wind coupling functions. Coupling functions also depend on the absolute value of the dawn‐dusk (By) component of the IMF, but not on its sign. However, recent studies have shown that for a fixed level of solar...
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We present a statistical analysis of the occurrence of bifurcations of the Region 2 (R2) Field-Aligned Current (FAC) region, observed by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). Previously, these have been shown to occur as the polar cap contracts after substorm onset, the beginning of the growth phase. D...
Citations
... However, the B y dependence of auroral (1-30 keV) electron precipitation has not been yet studied directly. Nevertheless, statistical studies of field-aligned currents (FACs) have provided some indirect evidence (Anderson et al., 2008;Green et al., 2009;Laundal et al., 2018;Holappa et al., 2021;Workayehu et al., 2021), because the upward FACs are known to be related to electron precipitation (for example, Korth et al., 2014). ...
... The B y dependence of these FACs has been studied in the past (Weimer & Edwards, 2021;Anderson et al., 2008;Tenfjord et al., 2015;Laundal et al., 2018;Workayehu et al., 2021). The duskside R1 FAC is known to shift and expand into higher latitudes for positive B y in the NH summer and for negative B y in the SH summer (Green et al., 2009;Holappa et al., 2021). This is similar to the B y -effect in the low-energy part (<6.5 keV) of duskside precipitation in Figures 3 and 5. ...
Auroral particle precipitation is usually assumed to be equally strong for both signs of the By component of the interplanetary magnetic field (IMF). This is also the case in most currently used precipitation models, which parameterize solar wind driving by empirical coupling functions. However, recent studies have showed that geomagnetic activity is significantly modulated by the signs and amplitudes of IMF By and the Earth's dipole tilt angle Ψ. This so called explicit By dependence is not yet included in any current precipitation models. In this paper, we quantify this By dependence for auroral electron precipitation for the first time. We use precipitation measurements of the Defense Meteorological Satellite Program (DMSP) Special Sensor J instruments from years 1995–2022. We show that the dawnside electron precipitation at energies 13.9–30 keV is greater at auroral latitudes for opposite signs of By and Ψ in both hemispheres, while the dusk sector is mostly unaffected by By and Ψ. For energies below 6.5 keV the By dependence is strong poleward of the auroral oval in the summer hemisphere, also exhibiting a strong dawn‐dusk asymmetry. We also show that By dependence of precipitation modulates ionospheric conductance.
... We also notice that the duration of expansion and recovery phase is unaffected by the dawn-dusk IMF component. While some authors have suggested seasonal variation of the B Y effect (Holappa et al., 2021;M. V. Kubyshkina et al., 2023), our data set does not allow separation of the events by season due to the annual rotation of the satellite orbits. ...
In this work, we present a statistical study of substorms covering a five‐year period 2016–2020. Substorm phases were identified from time series of the SuperMAG AL (SML) index using a list of 5,077 previously identified substorm onsets, the SML peak value marking transition from expansion to recovery phase, and the recovery identified as return to activity less than −100 nT in the SML index. Magnetic field observations from THEMIS, RBSP, and MMS missions were used to study the magnetotail characteristics during the substorm evolution. A superposed epoch analysis indicates that the substorm onset occurs almost simultaneously with a few minutes of uncertainty throughout the magnetotail, ranging from geostationary orbit to 20 RE. The onset in the transition region precedes the ground onset by a few minutes. The peak SML time coincides with the peak of the outer transition region ΔBZ, which suggests that the field‐aligned currents driving the SML activity arise from the outer transition region. Analysis of 2D maps of the tail magnetic field shows that the magnetotail current changes are limited to the center of the tail within |Y| < 10RE. The substorm recovery is fastest in the inner transition region and lasts longer when moving further out. We did not find major asymmetries in the substorm signatures associated with IMF BY or BZ.
... Indications that the ionosphere is influenced not only by IMF B y but a combination of IMF B y and the Earth's dipole tilt has been reported long ago (e.g., Friis-Christensen and Wilhjelm, 1975;Crooker, 1992;Ruohoniemi and Greenwald, 2005). Recent statistical studies by Reistad et al. (2020), Holappa et al. (2021), Ohma et al. (2021) and Laitinen et al. (2024) indicate that the influence by both IMF B y and the dipole tilt angle appears, e.g., in the ionospheric fieldaligned current pattern, geomagnetic activity, substorm occurrence frequency, Hall conductance and the strength and width of the dawnside auroral electron precipitation region. This implies that the different ionospheric response during positive and negative IMF B y in combination with different dipole tilt angles may be coupled to tail dynamics as well. ...
... Additionally, Holappa et al. (2020) observed a higher electron precipitation flux at dawn and midnight for IMF B y > 0 in the NH winter, compared to that for IMF B y < 0; the reversed IMF B y effect was observed during the SH winter. Furthermore, Holappa et al. (2021) reported that the IMF B y effect on upward FACs and auroral electrojets was more pronounced in the dawn sector and negligible at dusk, with noon and nighttime not covered in their work. It is generally agreed that upward FACs are linked to electron precipitation (e.g., Knight, 1973;Korth et al., 2014), and a positive correlation exists between upward/downward FACs and particle precipitation (e.g., Robinson et al., 2020;Wang & Zou, 2022). ...
... Consequently, the ionospheric current system might be stronger for IMF B y > 0 during the December solstice (DeceS) and for IMF B y < 0 during the June solstice (JuneS) in both the summer and winter hemispheres, responding to increased global magnetospheric convection. In contrast to this perspective, an IMF B y effect on the auroral westward electrojet, represented by the auroral AL index, was reported to be most prominent during NH winter (Friis-Christensen et al., 2017;Holappa et al., 2021;Holappa & Mursula, 2018;Laundal et al., 2018;Smith et al., 2017). Workayehu et al. (2021) determined that the most pronounced IMF B y effect on hemispheric integrated ionospheric currents occurred during the local winter seasons. ...
... Our work shows that strongly positive IMF B y seems to enhance the dawn FACs, which is consistent with the previous work of Holappa et al. (2021) based on the constellation of polar-orbiting commercial Iridium satellites in 2010-2017. They observed that FACs in the dawn sector are significantly stronger for IMF B y > 0 than for IMF B y < 0 in the NH winter, and for IMF B y < 0 than for IMF B y > 0 in the SH winter. ...
In this work, IMF By effects on field‐aligned currents (FACs) are examined in different local time sectors, seasons, and hemispheres. At dusk and 09–14 MLT, when the eastward polar electrojet (PEJ) prevails, the northern FACp (poleward side FACs) are stronger when IMF By < 0 than when IMF By > 0. Conversely, at dawn, 21–02 MLT, and 09–14 MLT with westward PEJ, the northern FACp are stronger with IMF By > 0 compared to IMF By < 0. The southern FACp shows a reversed relationship with IMF By direction. The dependence of FACe (equatorward side FACs) on IMF By is weaker, except for the midday FACe, which shows opposite variations with respect to IMF By when compared to FACp. Stronger IMF By effect is observed in local summer in most of local times. The northern FACs are located at higher latitude for IMF By > 0 than for IMF By < 0 in local times with eastward PEJ, while the opposite trend is observed in other local times and in the Southern Hemisphere. The hemispheric difference in the peak latitude of FACs demonstrates an inverse relationship with its intensity, with stronger FACs located at lower latitudes. Overall, the local time and hemispheric differences in FACs strength and latitude are discussed in the context of interhemispheric field‐aligned currents linked to IMF By.
... Previous works on PEJs were based on the geomagnetic AL index (e.g., Holappa & Mursula, 2018), equivalent currents from ground-based magnetometers (e.g., Laundal et al., 2018), and auroral electrodynamic model outputs (e.g., Holappa et al., 2021). However, the accuracy of the estimations of PEJ intensity from the auroral dynamics model requires further verification using observations (e.g., Wang et al., 2008). ...
In this study, the impact of the interplanetary magnetic field (IMF) By component on the peak strength and position of polar electrojets (PEJs) is analyzed, based on 16 years of joint observations from Challenging Minisatellite Payload (CHAMP; 2000–2010) and Swarm (2014–2020) satellites, while focusing on the local time and hemispherical differences. With a more positive IMF By, the duskside eastward PEJ enhances significantly at June solstice in the Northern Hemisphere. Around midnight, the impact of IMF By on the PEJ is more significant in the winter hemisphere; the westward PEJ is stronger in the Northern (Southern) Hemisphere for a more positive (negative) IMF By. In 03–08 MLT, the impact of IMF By is similar in both the winter and summer hemispheres: for IMF By > 0 and By < 0, the PEJ in the dawn sector is stronger in the December and June solstices, respectively. Around noon, a more duskward (dawnward) IMF By component leads to a higher eastward (westward) PEJ in the northern (southern) summer hemisphere. The northern westward (eastward) PEJ peaks at higher latitudes than the southern PEJ for a negative (positive) IMF By.
... As seen from the two plots, the potential remains the same for the dawn and dusk sectors in both the hemispheres. The slight asymmetry with one of the potential lobes being larger than the other is due to the finite B y component of the incoming solar wind, and the pattern is consistent with the statistical pattern of the potential for positive IMF B y (Chisham et al., 2007;Holappa et al., 2021). ...
This study presents a recently developed two‐way coupled magnetosphere‐ionosphere model named “MagPIE” that enables the investigation of the impact of flux transfer events (FTEs) on the ionosphere. Our findings highlight the prominent role of cusp‐FTE reconnection in influencing the ionosphere. The typical morphology of an FTE signal, represented by field‐aligned currents (FACs) on the ionosphere, is shown to exhibit a distinct pattern characterized by an “I”‐shaped patch surrounded by a “U”‐shaped patch. Furthermore, we demonstrate that the effects of FACs resulting from FTEs may extend well into the region of closed field lines on the ionosphere. These FACs are seen to exhibit a remarkable resemblance to discrete dayside auroral arcs, providing further evidence that FTEs can be considered as a probable cause of such phenomena. Additionally, FTEs generate vortex‐like patterns of ionospheric flow, which can manifest as either twin vortices or a combination of multiple vortices, depending on the characteristics of the FACs producing them. Furthermore, we present compelling evidence of morphological similarities between the simulated ionospheric signatures obtained from the MagPIE model and an observation made by the SWARM satellites. The agreement between our model and observational data further strengthens the credibility of our model and opens up new avenues to theoretically explore the complex ionospheric effects caused by FTEs.
... (Friis-Christensen et al., 2017;Holappa et al., 2021;Smith et al., 2017) have shown that the westward auroral electrojet, routinely measured by the AL index (Davis & Sugiura, 1966) is stronger for B y > 0 during negative dipole tilt angle (ψ), which is the angle between the plane perpendicular to the Sun-Earth line and Earth's magnetic dipole axis. During positive ψ the AL index is stronger for negative B y . ...
The coupling of the interplanetary magnetic field (IMF) with the Earth's magnetic field can result in spectacular auroral displays, including substorms. An important feature of substorms is the westward electrojet that is routinely monitored by the AL index, based on ground magnetometer measurements. Recent research shows that, during periods of significant dipole tilt, the value of the AL index is strongly modulated by the dawn‐dusk (or y) component of the IMF (By): the AL index is stronger for By > 0 than By < 0 during negative dipole tilt (Northern Hemisphere winter). Kubyshkina et al. (2023, https://doi.org/10.1029/2022ja031275) argued that this By dependence of the AL index is not really caused by IMF By but by the component along the Sun‐Earth line (x axis)—namely IMF Bx, which is strongly anticorrelated with By. Here we provide strong evidence that the By dependence of the AL index is indeed much stronger than the suggested Bx dependence. In fact, the analysis of Kubyshkina et al. (2023, https://doi.org/10.1029/2022ja031275) provided similar evidence. We also note that the physical mechanism proposed by Kubyshkina et al. (2023, https://doi.org/10.1029/2022ja031275) to explain the suggested Bx dependence is contradicted by observations. However, we cannot completely rule out its existence.
... Similarly, Ohma et al. (2021) showed that substorms occur more frequently when By and the dipole tilt Ψ have opposite signs, as compared to the case of both having the same sign (note that the By is in the dawn-dusk direction, while the angle Ψ is in the noon-midnight meridian plane). In a series of recent works by Holappa and Mursula (2018), Holappa et al. (2019), Holappa et al. (2020), and Holappa et al. (2021), these authors show a stronger magnetospheric response to the solar wind driving in the Northern hemisphere for By > 0 and Ψ < 0. The effect was demonstrated to manifest itself in the auroral indices (AL), precipitating energetic electron flux, and ionospheric currents. However, those authors emphasized that the IMF Bx was of only minor importance. ...
... This study was motivated by a series of papers by Holappa et al. (2020Holappa et al. ( , 2019Holappa et al. ( , 2021 and Holappa and Mursula (2018), in which different response of the geomagnetic activity to otherwise the same interplanetary conditions was attributed to the IMF By. We begin with exploring the AL-index dependence on IMF Bx and By during periods with different dipole tilt angles, using the coupling function by Newell et al. (2007) as the interplanetary driver: ...
... Though we analyzed only the AL-index behavior, our findings may be transferred to other manifestations of geomagnetic activity, such as those addressed in studies that followed the work by Holappa and Mursula (2018), for example, Holappa et al. (2019), Holappa et al. (2020), Reistad et al. (2020), Ohma et al. (2021), and Holappa et al. (2021). In those papers, statistical dependence on the By polarity was revealed for a wide range of activity parameters: the potential drop across the polar cap, electron precipitating flux, the size of auroral oval, substorm probability, etc. ...
Plain Language Summary
Interaction of the solar wind with the Earth's magnetic field drives the magnetospheric activity and space weather. Magnetosphere's response to the external driving can be forecasted, once the incoming solar wind state and its dynamics are known. Of great importance for space weather forecasting is the interplanetary magnetic field (IMF). When its North‐South component (Bz) turns southward, the magnetosphere energy accumulates and the geomagnetic activity increases. The other two IMF components that lie in the equatorial plane also affect the magnetosphere, but there exist different opinions on their role. In this paper we show that both Sun‐Earth (Bx) and East‐West (By) components are equally important. The latter component mostly affects the magnetic field dynamics on the dayside, such that its increase (regardless of orientation) enhances the activity. IMF Bx component affects the magnetosphere in a more complex way: its impact is small at equinoxes but increases at solstices. Due to the global spiral‐shaped IMF geometry, Bx and By are strongly correlated and, hence, act simultaneously. Therefore, to evaluate the IMF impact on the magnetosphere, one should properly take into account the magnitude/orientation of all its components, combined with inclination of the Earth's dipole axis to the terminator plane.
... The control of GMD amplitude by IMF By reported here is consistent with the results of several earlier studies. Holappa et al. (2021b) noted that many studies using ground magnetometers, beginning with Friis-Christensen and Wilhjelm (1975) and using polar-orbiting satellites (Friis-Christensen et al., 2017;Smith et al., 2017), have shown that auroral electrojets in the northern hemisphere winter are stronger in both hemispheres for By > 0 than Figure 10. Histograms of the north-south time delay between GMD events observed at magnetically conjugate station pairs GDH-PG3, STF-PG4, and SKT-PG5, using events in all three categories of interplanetary magnetic field (IMF) By from (a) Artemis/Themis data and (b) OMNI data. ...
... for By < 0, and that in NH summer the dependence on the By sign is reversed. Holappa et al. (2021b) noted that this By sign dependence is very strong in the winter hemisphere, but it is weak in the summer hemisphere, and is much stronger in the westward electrojet than in the eastward electrojet. Holappa and Buzulokova (2022) noted that the physical mechanisms of IMF By effects, which apply not only to auroral zone electron precipitation and ionospheric conductance but also to the fluxes of energetic magnetosphere protons and the growth rate of the ring current, are still not fully understood. ...
Nearly all studies of impulsive geomagnetic disturbances (GMDs, also known as magnetic perturbation events MPEs) that can produce dangerous geomagnetically induced currents (GICs) have used data from the northern hemisphere. In this study, we investigated GMD occurrences during the first 6 months of 2016 at four magnetically conjugate high latitude station pairs using data from the Greenland West Coast magnetometer chain and from Antarctic stations in the conjugate AAL‐PIP magnetometer chain. Events for statistical analysis and four case studies were selected from Greenland/AAL‐PIP data by detecting the presence of >6 nT/s derivatives of any component of the magnetic field at any of the station pairs. For case studies, these chains were supplemented by data from the BAS‐LPM chain in Antarctica as well as Pangnirtung and South Pole in order to extend longitudinal coverage to the west. Amplitude comparisons between hemispheres showed (a) a seasonal dependence (larger in the winter hemisphere), and (b) a dependence on the sign of the By component of the interplanetary magnetic field (IMF): GMDs were larger in the north (south) when IMF By was >0 (<0). A majority of events occurred nearly simultaneously (to within ±3 min) independent of the sign of By as long as |By| ≤ 2 |Bz|. As has been found in earlier studies, IMF Bz was <0 prior to most events. When IMF data from Geotail, Themis B, and/or Themis C in the near‐Earth solar wind were used to supplement the time‐shifted OMNI IMF data, the consistency of these IMF orientations was improved.
... Holappa and Mursula (2018) quantified the difference in electrojet strength to be about 50 percent during winter conditions. During summer conditions, the IMF B y dependence reverses as the westward electrojet is stronger for negative compared to positive IMF B y , but the effect on the westward electrojet is minor compared to the difference during winter conditions (Holappa and Mursula, 2018;Holappa et al., 2021b). ...
... Based on the new analysis presented in section 2, together with recent advances in describing the geospace response during these conditions (Holappa and Mursula, 2018;Holappa et al., 2020;Reistad et al., 2020;Holappa et al., 2021b;Ohma et al., 2021), we conclude that the global dayside reconnection rate is likely to be enhanced when IMF B y and the dipole tilt have opposite signs (± and −/+), compared to when they have the same signs (−/− and +/+). This is referred to as a type A mechanism. ...
In the recent years, significant attention has been given to the combined effect of Interplanetary Magnetic Field (IMF) duskward component (By) and dipole tilt on the global magnetosphere-ionosphere system response. Numerous studies have pointed out that when the Earth's magnetic dipole is tilted away from the Sun (negative dipole tilt during northern winter), and IMF has a positive By component, the effects on ionospheric currents, particle precipitation, ionospheric convection, and average size of the auroral oval, is significantly more enhanced, compared to when IMF By is negative. Furthermore, this IMF By polarity effect reverses when Earth's dipole is tilted in the opposite direction. The underlying cause has remained unclear. Our analysis shows that substorms tend to be stronger during the same IMF By and dipole tilt polarity combination. Taken together with earlier results showing also more frequent substorms during the same conditions, our observations suggests that when IMF By and dipole tilt have opposite signs, there is a more efficient global dayside reconnection rate. We also show analysis of the occurrence frequency of periods of Steady Magnetospheric Convection, substorm onset latitude, and the isotropic boundary of proton precipitation, that are all consistent with our conclusion that the combination of IMF By and dipole tilt polarity affect the global dayside reconnection rate.
