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Dipolarizing flux bundles (DFBs) have been suggested to transport energy and momentum from regions of reconnection in the magnetotail to the high latitude ionosphere, where they can generate localized ionospheric currents that can produce large nighttime geomagnetic disturbances (GMDs). In this study we identified DFBs observed in the midnight sector from ~7 to ~10 RE by THEMIS A, D, and E during days in 2015-2017 whose northern hemisphere magnetic footpoints mapped to regions near Hudson Bay, Canada, and have compared them to isolated GMDs observed by ground magnetometers. We found 6 days during which one or more of these DFBs coincided to within ±3 min with ≥6 nT/s GMDs observed by latitudinally closely spaced ground-based magnetometers located near those footpoints. Spherical elementary current systems (SECS) maps and all-sky imager data provided further characterization of two events, showing short-lived localized intense upward currents, auroral intensifications and/or streamers, and vortical perturbations of a westward electrojet. On all but one of these days the coincident DFB--GMD pairs occurred during intervals of high-speed solar wind streams but low values of SYM/H. The observations reported here indicate that isolated DFBs generated under these conditions influence only limited spatial regions nearer Earth. In some events, in which the DFBs were observed closer to Earth and with lower Earthward velocities, the GMDs occurred slightly earlier than the DFBs, suggesting that braking had begun before the time of the DFB observation. [ABSTRACT FROM AUTHOR]

In this study, we investigate the effects caused by interplanetary (IP) shock impact angles on the subsequent ground dB/dt variations during substorms. IP shock impact angles have been revealed as a major factor controlling the subsequent geomagnetic activity, meaning that shocks with small inclinations with the Sun‐Earth line are more likely to trigger higher geomagnetic activity resulting from nearly symmetric magnetospheric compressions. Such field variations are linked to the generation of geomagnetically induced currents (GICs), which couple to artificial conductors on the ground leading to deleterious consequences. We use a sub‐set of a shock data base with 237 events observed in the solar wind at L1 upstream of the Earth, and large arrays of ground magnetometers at stations located in North America and Greenland. The spherical elementary current system methodology is applied to the geomagnetic field data, and field‐aligned‐like currents in the ionosphere are derived. Then, such currents are inverted back to the ground and dB/dt variations are computed. Geographic maps are built with these field variations as a function of shock impact angles. The main findings of this investigation are: (a) typical dB/dt variations (5–10 nT/s) are caused by shocks with moderate inclinations; (b) the more frontal the shock impact, the more intense and the more spatially defined the ionospheric current amplitudes; and (c) nearly frontal shocks trigger more intense dB/dt variations with larger equatorward latitudinal expansions. Therefore, the findings of this work provide new insights for GIC forecasting focusing on nearly frontal shock impacts on the magnetosphere. Plain Language Summary: Space weather effects caused by solar perturbations can increase the intensity of electric currents flowing in the Earth's upper atmosphere. These currents in turn induce geoelectric currents on the ground that couple with ground infrastructure, such as power transmission lines, oil/gas pipelines, and railways, which further cause deleterious consequences. Recent studies have shown that the angle with which solar perturbations impact the Earth closely controls intensifications of electric currents in the upper atmosphere and the subsequent ground magnetic field perturbations causing geoelectric currents. When these space weather drivers impact Earth more frontally, the resulting atmospheric current enhancements and the subsequent ground magnetic field perturbations are generally more intense. In this study, we use a large amount of magnetic field data recorded in North America and Greenland. We find that the most intense geomagnetic field disturbances are caused by the impacts of nearly frontal solar perturbations on the Earth and cover large geographic areas reaching southern Canada and northern United States. The results of this work can be used by space weather forecasters when predicting events with significant magnetic field variations leading to intense geoelectric currents, particularly when tracking space weather drivers that are forecasted to impact Earth nearly frontally. Key Points: Typical substorm‐time ground dB/dt variations (5–10 nT/s) are caused by interplanetary shocks with moderate inclinations (140°–160°)The more frontal the shock, the more intense and the broader the ionospheric current amplitudesThe more frontal the shock, the more intense, larger the area coverage, and the lower the equatorward latitudinal extent of dB/dt variations [ABSTRACT FROM AUTHOR]

Extreme (>20 nT/s) geomagnetic disturbances (GMDs, also denoted as MPEs—magnetic perturbation events)—impulsive nighttime disturbances with time scale ∼5–10 min, have sufficient amplitude to cause bursts of geomagnetically induced currents (GICs) that can damage technical infrastructure. In this study, we present occurrence statistics for extreme GMD events from five stations in the MACCS and AUTUMNX magnetometer arrays in Arctic Canada at magnetic latitudes ranging from 65° to 75°. We report all large (≥6 nT/s) and extreme GMDs from these stations from 2011 through 2022 to analyze variations of GMD activity over a full solar cycle and compare them to those found in three earlier studies. GMD activity between 2011 and 2022 did not closely follow the sunspot cycle, but instead was lowest during its rising phase and maximum (2011–2014) and highest during the early declining phase (2015–2017). Most of these GMDs, especially the most extreme, were associated with high‐speed solar wind streams (Vsw >600 km/s) and steady solar wind pressure. All extreme GMDs occurred within 80 min after substorm onsets, but few within 5 min. Multistation data often revealed a poleward progression of GMDs, consistent with a tailward retreat of the magnetotail reconnection region. These observations indicate that extreme GIC hazard conditions can occur for a variety of solar wind drivers and geomagnetic conditions, not only for fast‐coronal mass ejection driven storms. Plain Language Summary: Geomagnetically induced currents (GICs) can give rise to the most extreme space weather impact of disrupting electric power distribution. GICs can be driven by extreme geomagnetic disturbances (GMDs) that are observed as large amplitude (several hundred nT amplitude) and rapid (5–10 min period) changes in the geomagnetic field. We found that extreme high‐latitude GMDs occur throughout the solar cycle, but are preferentially observed during the declining phase of the solar cycle (the several year interval just past solar maximum) and in conjunction with high‐speed solar wind streams. They also all occur within 80 min of a geomagnetic substorm, but are not directly related to its explosive onset. These GMDs are most often observed to propagate poleward consistent with tailward retreat of magnetotail reconnection regions. These results show that extreme GMDs and hence large GICs can be driven by a variety of solar wind conditions and not just fast coronal mass ejections. Key Points: Most large (6 nT/s) and extreme (>20 nT/s) high latitude geomagnetic disturbances (GMDs) occurred during the declining phase of the sunspot cycleMost extreme GMDs occurred during high‐speed solar wind streams and often within 25 min after a substorm onset, but seldom within 5 minMany extreme GMDs showed a poleward progression, consistent with the tailward retreat of the magnetotail reconnection region [ABSTRACT FROM AUTHOR]

Space weather, a natural hazard, can adversely impact human technological assets. High‐voltage electric power transmission grids constitute one of the most critical technological systems vulnerable to space weather driven geomagnetically induced currents (GICs). One of the major challenges pertaining to the study of GICs over the continental United States has been the availability of GIC measurements, which are critical for validation of geoelectric field and power flow models, for example. In this study, we analyze GIC measurements collected at 17 Electrical Power Research Institute (EPRI) SUNBURST transformer locations across the United States for which a GIC value of 10 A or greater was recorded. This data set includes 52 individual geomagnetic storms with Kp index 6 and above during the period from 2010 to 2021. The analysis confirms that there is a good correlation between the number of geomagnetic storms per year and the number of recorded GIC events. Our results also show that about 76% of the top 17 GIC events are associated with the storm main phase, while only 24% are attributed to storm sudden commencements. In addition, it is shown, for the first time, that mid‐latitude positive bays can cause large GICs over the continental United States. Finally, this study shows that the largest measured GIC event in the data set was associated with a localized intense dB/dt structure, which could be attributed to substorm activity. Plain Language Summary: Space weather, a natural hazard, can adversely impact human technological assets. High‐voltage electric power transmission grids constitute one of the most critical technological systems vulnerable to induced currents produced by enhanced space weather conditions. One of the major challenges pertaining to the study of these induced currents over the continental United States has been the lack of measurements. In this study, we analyze induced current measurements collected at 17 high‐voltage power transformer locations across the United States for which a value of 10 A or greater was recorded during the period from 2010 to 2021. The analysis confirms a good correlation between the number of geomagnetic storms per year and the number of recorded induced current events. The results also show that about 76% of the top 17 induced current events are associated with the storm main phase, while only 24% are attributed to storm sudden commencements. In addition, it is shown for the first time that mid‐latitude positive bays can cause large induced currents over the continental United States. Finally, this study also shows that the largest measured GIC event in the data set was associated with a localized intense dB/dt structure, which could be attributed to substorm activity. Key Points: We analyze geomagnetically induced current (GIC) measurements collected under the EPRI SUNBURST project from across the United States and CanadaAbout 76% of the top 17 GIC events occur during main phase (MP) of geomagnetic storms, while 24% during sudden storm commencementFor the first time it is directly shown that mid‐latitude positive bays can cause large GICs at US locations [ABSTRACT FROM AUTHOR]

We present a comprehensive statistical analysis of high‐frequency transient‐large‐amplitude (TLA) magnetic perturbation events that occurred at 12 high‐latitude ground magnetometer stations throughout Solar Cycle 24 from 2009 to 2019. TLA signatures are defined as one or more second‐timescale dB/dt interval with magnitude ≥6 nT/s within an hour event window. This study characterizes high‐frequency TLA events based on their spatial and temporal behavior, relation to ring current activity, auroral substorms, and nighttime geomagnetic disturbance (GMD) events. We show that TLA events occur primarily at night, solely in the high‐latitude region above 60° geomagnetic latitude, and commonly within 30 min of substorm onsets. The largest TLA events occurred more often in the declining phase of the solar cycle when ring current activity was lower and solar wind velocity was higher, suggesting association to high‐speed streams caused by coronal holes and subsequent corotating interaction regions reaching Earth. TLA perturbations often occurred preceding or within the most extreme nighttime GMD events that have 5–10 min timescales, but the TLA intervals were often even more localized than the ∼300 km effective scale size of GMDs. We provide evidence that shows TLA‐related GMD events are associated with dipolarization fronts in the magnetotail and fast flows toward Earth and are closely temporally associated with poleward boundary intensifications (PBIs) and auroral streamers. The highly localized behavior and connection to the most extreme GMD events suggests that TLA intervals are a ground manifestation of features within rapid and complex ionospheric structures that can drive geomagnetically induced currents. Plain Language Summary: Large changes of the surface geomagnetic field can drive geomagnetically induced currents (GICs) that are a hazardous impact of space weather events. Extreme magnetic field changes with timescales of minutes and hours are the most dangerous because they are the most effective at inducing currents on Earth. Shorter‐period geomagnetic disturbances are less effective at current induction, but we show in this study that they are inherently related to and play an active role in space weather events that are capable of driving GICs. We analyze these high‐frequency events that occurred at stations in the high‐magnetic latitude region throughout Solar Cycle 24 and investigate their association to longer (5–10 min) geomagnetic disturbances that can cause GICs. We show that they are often closely correlated and that the associated events can be driven by bursty flows in the magnetotail and subsequent small‐scale auroral structures. Key Points: Transient‐large‐amplitude (TLA) magnetic field disturbances were most common during the declining phase of Solar Cycle 24A majority of extreme nighttime geomagnetic disturbances (GMDs) from 2015 to 2019 had TLA intervals that preceded or occurred within the GMDTLA‐related GMDs are related to inner magnetosphere dipolarizations, subsequent poleward boundary intensifications, and auroral streamers [ABSTRACT FROM AUTHOR]

We report the first observations of the association between equatorward extending streamers and overshielding using the THEMIS all‐sky imagers and ground magnetometers. Because auroral streamers indicate plasma sheet flow bursts, these observations uncover the effect of flow bursts on overshielding. Results show that, in general, bright equatorward extended streamers were associated with an increase in equatorial electrojet (EEJ) on the nightside and a decrease in the dayside EEJ, indicating a striking correspondence between auroral streamers and overshielding conditions. Thus, the driving of overshielding at equatorial latitudes can be identified via bright equatorward extended streamers, indicating that flow bursts are an alternate means to discern the earthward injections that increase the region 2 field aligned currents and associated overshielding electric fields. Repetitive auroral streamers were associated with repetitive overshielding, resulting in a stepwise development of the dayside and nightside EEJ. The stepwise intensifications were also observed in the midlatitude positive bay and Pi2 pulsations. Our results could explain the occurrence of overshielding conditions at equatorial latitudes during substorms and nonsubstorm times without a northward turning of IMF‐Bz. As seen through streamers, the localized current structures (wedgelets) associated with flow bursts giving injection that leads to overshielding is titled northeast‐to‐southwest. Our results add a new element to the understanding of high‐to‐low latitude electrodynamical coupling by demonstrating the association between bright equatorward extended auroral streamers and enhanced shielding electric fields caused by earthward injections associated with flow bursts. Key Points: The driving of overshielding seen at equatorial latitudes by injections can be identified via equatorward extending auroral streamersRepetitive streamers, thus repetitive injections, are associated with repetitive overshielding conditionsAs seen via streamers, localized current system with flow bursts giving injection driven overshielding is tilted northeast‐to‐southwest [ABSTRACT FROM AUTHOR]

A predictive model for the variation of ionospheric currents is of great scientific and practical importance to our modern industrial society. To study the response of ionospheric currents to external drivers including geomagnetic indices and solar radiation, we developed a feedforward neural network model trained on the Equivalent Ionospheric Current (EIC) data from 1st January 2007 to 31st December 2019. Due to the highly imbalanced nature of the ionospheric currents data, which means that the data of extreme events are much less than those of quiet times, we utilized different loss functions to improve the model performance. Our model demonstrates the potential to predict the active events of ionospheric currents reasonably well (e.g., EICs during substorms) within a timescale of a few minutes. Although the data used for training are measurements over the North American and Greenland sectors, our model is not only able to predict EICs within this region, but is also able to provide a promising out‐of‐sample prediction on a global scale. Plain Language Summary: Studying the ionospheric current system is important for our modern society, since its drastic change can impact the ground‐based facilities such as power grid. We used a neural network model to predict the influence of geomagnetic activities and upstream condition on the variation of ionospheric currents over the North American and Greenland regions during a whole solar cycle period. Because the data set of ionospheric currents is highly imbalanced, we used different loss function to optimize our model. We found that our model can reasonably well predict the ionospheric current events which are related to the magnetospheric activities. We also found that our model can provide predictions beyond the North American and Greenland regions. Key Points: A neural network is trained to predict the response of Equivalent Ionospheric Currents (EIC) to geomagnetic indicesA comparative study between the mean square error and focal loss has been made to address the EIC's imbalance regression problemOur model can spatially and temporally predict the eastward‐westward EIC component [ABSTRACT FROM AUTHOR]

Low‐altitude observations of magnetospheric particles provide a unique opportunity for remote probing of the magnetospheric and plasma states during active times. We present the first statistical analysis of a specific pattern in such observations, energetic electron flux dropouts in the low‐altitude projection of the plasma sheet. Using 3.5 years of data from the ELFIN CubeSats we report the occurrence distribution of 145 energetic electron flux dropout events and identify characteristics, including their prevalence in the dusk and premidnight sectors, their association with substorms and enhanced auroral activities, and their correlation with the region‐1 (R1) field‐aligned current region. We also investigate three representative dropout events which benefit from satellite conjunctions between ELFIN, GOES, and THEMIS, to better understand the magnetospheric drivers and magnetic field conditions that lead to such dropouts as viewed by ELFIN. One class of dropouts may be associated with magnetic field mapping distortions due to local enhancements and thinning of cross‐tail current sheets and amplification of R1 field‐aligned currents. The other class may be associated with the increase in perpendicular anisotropy of magnetospheric electrons due to magnetic field dipolarizations near premidnight. These plasma sheet flux dropouts at ELFIN provide a valuable tool for refining magnetospheric models, thereby improving the accuracy of field‐line mapping during substorms. Plain Language Summary: Low‐Earth Orbit (LEO) observations of magnetospheric particles can sometimes provide critical information on complex high‐altitude processes that are challenging to access. Here we introduce a novel type of LEO particle measurement: energetic electron flux dropouts in the low‐altitude projection of the plasma sheet. These particle observations may prove valuable for remotely probing magnetospheric configurations during geomagnetically disturbed times. Utilizing 3.5 years of data from the ELFIN CubeSats, we conduct the first statistical analysis of these flux dropout events. Our findings reveal that they predominantly occur in the dusk and premidnight magnetic local time sectors during substorms, and exhibit a well‐defined relationship with the large‐scale field‐aligned current region. We also examine three representative dropout events to gain a deeper understanding of the magnetospheric drivers and magnetic field conditions that lead to such dropouts at low altitudes. These plasma sheet flux dropouts at ELFIN provide a valuable tool for refining magnetospheric models, consequently enhancing the accuracy of field‐line mapping during disturbed times. Key Points: We report statistical observations of energetic electron flux dropouts at the low‐altitude extent of the plasma sheet by ELFINFlux dropouts occur preferentially at dusk and premidnight and are collocated with R1 FAC poleward of the electron isotropy boundaryTwo types of magnetospheric drivers of ELFIN flux dropouts are discussed by analyzing three conjunction events with GOES and THEMIS [ABSTRACT FROM AUTHOR]

Foreshock transient (FT) events are frequently observed phenomena that are generated by discontinuities in the solar wind. These transient events are known to trigger global‐scale magnetic field perturbations (e.g., ULF waves). We report a series of FT events observed by the Magnetospheric Multiscale mission in the upstream bow shock region under quiet solar wind conditions. During the event, ground magnetometers observed significant Pc1 wave activity as well as magnetic impulse events in both hemispheres. Ground Pc1 wave observations show ∼8 min time delay (with some time differences) from each FT event which is observed at the bow shock. We also find that the ground Pc1 waves are observed earlier in the northern hemisphere compared to the southern hemisphere. The observation time difference between the hemispheres implies that the source region of the wave is the off‐equatorial region. Key Points: Magnetospheric Multiscale observed a series of foreshock transients near the Earth's bow shockPc1 waves and magnetic impulse events are observed by ground magnetometers in both hemispheres following the foreshock transientsThe difference in observation times between hemispheres implies that Pc1 waves are generated in the off‐equatorial region [ABSTRACT FROM AUTHOR]

Our current knowledge of the geomagnetic poleward and equatorward boundary dynamics is limited, particularly, how deep those two latitudinal boundaries can extend into lower geomagnetic latitudes during magnetic storms. We want to understand the motion of the boundary because it is important in terms of the location and magnitude of the effects of geomagnetic disturbances associated with storms on the ground. In this study we derive spherical elementary ionospheric currents from ground magnetometer arrays covering North America and Greenland during six magnetic storms in 2015 and 2018. With two dimensional maps of the auroral region current, we select the equatorward boundary of the region 2 currents by‐eye and fit the boundary with an ellipse to derive the location of the equatorward boundary at magnetic midnight. We have obtained over 500 boundaries and find that the midnight boundary location varies between 45° and 66° magnetic latitude. We examine the influence of the interplanetary magnetic field (IMF), solar wind plasma, and geomagnetic indices on the location of the magnetic midnight equatorial boundary and find that the equatorial boundary location is best correlated with the IMF Bz, VBz, and the Sym‐H index. We demonstrate that as the Bz component becomes more negative, the magnitude of VBz increases, and the magnitude of the Sym‐H index increases, the magnetic midnight equatorial boundary shifts equatorward during periods of moderate to high geomagnetic activity. Plain Language Summary: Our current knowledge of the geomagnetic poleward and equatorward boundary dynamics is limited, particularly, how deep the equatorward boundary of the auroral current system can extend into lower magnetic latitudes during magnetic storms. In this study we use two dimension ionospheric current maps derived from ground magnetometer arrays covering North America and Greenland during magnetic storms in 2015 and 2018 to understand the variability of the equatorward boundary of the auroral currents at magnetic midnight. We examine the influence of the interplanetary magnetic field (IMF), solar wind plasma, and geomagnetic indices on the location of the magnetic midnight equatorial boundary and find that the magnetic midnight equatorial boundary location is best correlated with the Bz component of the interplanetary magnetic field, the solar wind electric field (VBz), and the Sym‐H geomagnetic index. We demonstrate that as the Bz becomes more negative, the magnitude of VBz increases, and as the magnitude of the Sym‐H increases the magnetic midnight equatorial boundary moves equatorward. Key Points: We show the temporal and spatial development of the region 2 current system equatorward boundary during magnetic stormsWe show that the equatorward boundary at magnetic midnight varies in magnetic latitude between 45° and 71° MLatWe demonstrate a correlation between the equatorward boundary location and IMF Bz southward, VBz, Sym‐H, and the ε parameter [ABSTRACT FROM AUTHOR]

We have examined the most intense external (magnetospheric and ionospheric) and internal (induced) |dH/dt| (amplitude of the 10 s time derivative of the horizontal geomagnetic field) events observed by the high-latitude International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers between 1994 and 2018. While the most intense external |dH/dt| events at adjacent stations typically occurred simultaneously, the most intense internal (and total) |dH/dt| events were more scattered in time, most likely due to the complexity of induction in the conducting ground. The most intense external |dH/dt| events occurred during geomagnetic storms, among which the Halloween storm in October 2003 featured prominently, and drove intense geomagnetically induced currents (GICs). Events in the prenoon local time sector were associated with sudden commencements (SCs) and pulsations, and the most intense |dH/dt| values were driven by abrupt changes in the eastward electrojet due to solar wind dynamic pressure increase or decrease. Events in the premidnight and dawn local time sectors were associated with substorm activity, and the most intense |dH/dt| values were driven by abrupt changes in the westward electrojet, such as weakening and poleward retreat (premidnight) or undulation (dawn). Despite being associated with various event types and occurring at different local time sectors, there were common features among the drivers of most intense external |dH/dt| values: preexisting intense ionospheric currents (SC events were an exception) that were abruptly modified by sudden changes in the magnetospheric magnetic field configuration. Our results contribute towards the ultimate goal of reliable forecasts of dH/dt and GICs. [ABSTRACT FROM AUTHOR]

During minor to moderate geomagnetic storms, caused by corotating interaction regions (CIRs) at the leading edge of high-speed streams (HSSs), solar wind Alfvén waves modulated the magnetic reconnection at the dayside magnetopause. The Resolute Bay Incoherent Scatter Radars (RISR-C and RISR-N), measuring plasma parameters in the cusp and polar cap, observed ionospheric signatures of flux transfer events (FTEs) that resulted in the formation of polar cap patches. The patches were observed as they moved over the RISR, and the Canadian High-Arctic Ionospheric Network (CHAIN) ionosondes and GPS receivers. The coupling process modulated the ionospheric convection and the intensity of ionospheric currents, including the auroral electrojets. The horizontal equivalent ionospheric currents (EICs) are estimated from ground-based magnetometer data using an inversion technique. Pulses of ionospheric currents that are a source of Joule heating in the lower thermosphere launched atmospheric gravity waves, causing traveling ionospheric disturbances (TIDs) that propagated equatorward. The TIDs were observed in the SuperDual Auroral Radar Network (SuperDARN) high-frequency (HF) radar ground scatter and the detrended total electron content (TEC) measured by globally distributed Global Navigation Satellite System (GNSS) receivers. [ABSTRACT FROM AUTHOR]

An interplanetary shock can abruptly compress the magnetosphere, excite magnetospheric waves and field‐aligned currents, and cause a ground magnetic response known as a sudden commencement (SC). However, the transient (<∼1 min) response of the ionosphere‐thermosphere system during an SC has been little studied due to limited temporal resolution in previous investigations. Here, we report observations of a global reversal of ionospheric vertical plasma motion during an SC on 24 October 2011 using ∼6 s resolution Super Dual Auroral Radar Network ground scatter data. The dayside ionosphere suddenly moved downward during the magnetospheric compression due to the SC, lasting for only ∼1 min before moving upward. By contrast, the post‐midnight ionosphere briefly moved upward then moved downward during the SC. Simulations with a coupled geospace model suggest that the reversed E⃗×B⃗ $\vec{E}\times \vec{B}$ vertical drift is caused by a global reversal of ionospheric zonal electric field induced by magnetospheric compression during the SC. Plain Language Summary: It is well‐known that a shock wave can suddenly compress objects they directly interact with. In this study, we report a special case in the geospace environment in which an interplanetary shock produced a concussion‐like response in the ionosphere that was tens of thousands of kilometers away from the location where the shock first impacted. The ionized part of the atmosphere, or the ionosphere, was remotely connected to the magnetosphere‐the region of geospace dominated by the Earth's magnetic field‐via electric currents. When the magnetosphere was abruptly compressed after the shock arrival, a pair of electric currents flowing along the geomagnetic field lines was generated in the dayside mid‐latitudes. The newly generated currents flipped the dayside ionospheric electric field from eastward to westward, leading to a downward motion of dayside ionospheric charged particles. Within 1 minute, the vertical motion and zonal electric field flipped again to the direction before the compression due to the generation of another pair of electric currents with an opposite sense to the first pair. This study depicts a global picture of the transient ionospheric response using multi‐point high‐resolution measurements and simulations with a state‐of‐the‐art fully coupled geospace model. Key Points: Dayside ionospheric plasma undergoes a transient motion from downward to upward during a sudden commencement (SC)Both observations and simulations show that the reversed vertical drift is a global response of the ionosphere to the SCThe transient response is caused by a reversal of induced zonal electric field during the SC [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We have examined the most intense external (magnetospheric and ionospheric) and internal (induced) | d H /dt | (amplitude of the 10 s time derivative of the horizontal geomagnetic field) events observed by the high-latitude International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers between 1994 and 2018. While the most intense external | d H /dt | events at adjacent stations typically occurred simultaneously, the most intense internal (and total) | d H /dt | events were more scattered in time, most likely due to the complexity of induction in the conducting ground. The most intense external | d H /dt | events occurred during geomagnetic storms, among which the Halloween storm in Oct 2003 featured prominently, and drove intense geomagnetically induced currents (GIC). Events in the prenoon local time sector were associated with sudden commencements (SC) and pulsations, and the most intense | d H /dt | values were driven by abrupt changes in the eastward electrojet due to solar wind dynamic pressure increase or decrease. Events in the premidnight and dawn local time sectors were associated with substorm activity, and the most intense | d H /dt | values were driven by abrupt changes in the westward electrojet, such as weakening and poleward retreat (premidnight) or undulation (dawn). Despite being associated with various event types and occurring at different local time sectors, there were common features among the drivers of most intense external | d H /dt | values: pre-existing intense ionospheric currents (SC events were an exception) that were abruptly modified by sudden changes in the magnetospheric magnetic field configuration. While proper description of the fast changes during SC events appears to require 1 s data, pulsations and substorms may be sufficiently described by 10 s | d H /dt |. 1 min data, however, significantly underestimates the | d H /dt | peaks. Our results contribute towards the ultimate goal of reliable forecasts of d H /dt and GIC. [ABSTRACT FROM AUTHOR]

Electron injections are critical processes associated with magnetospheric substorms, which deposit significant electron energy into the ionosphere. Although wave scattering of <10 keV electrons during injections has been well studied, the link between magnetotail electron injections and energetic (≥100 keV) electron precipitation remains elusive. Using conjugate observations between the Electron Loss and Fields Investigation (ELFIN) and Magnetospheric Multiscale (MMS) missions, we present evidence of tens to hundreds of keV electron precipitation to the ionosphere potentially driven by kinetic Alfvén waves (KAWs) associated with magnetotail electron injections and magnetic field gradients. Test particle simulations adapted to observations show that dipolarization‐front magnetic field gradients and associated ∇B drifts allow Doppler‐shifted Landau resonances between the injected electrons and KAWs, producing electron spatial scattering across the front which results in pitch‐angle decreases and subsequent precipitation. Test particle results show that such KAW‐driven precipitation can account for ELFIN observations below ∼300 keV. Plain Language Summary: Energetic electron precipitation from magnetospheric injections has a major impact on magnetosphere‐ionosphere coupling. This energy deposition is largely in the form of electron precipitation driven by wave‐particle interactions in the magnetotail. Although wave‐driven precipitation with energies less than approximately ∼10 keV has been studied extensively, the link between energetic electron precipitation (≥∼100 keV) and electron injections remains elusive. Combining observations and simulations, this paper provides evidence of such precipitation driven by kinetic Alfvén waves, which have been previously observed to be ubiquitously associated with magnetospheric electron injections but have not been considered as an important driver for precipitation of such electrons. Key Points: Conjugate Electron Loss and Fields Investigationand Magnetospheric Multiscale observations reveal evidence of tens to hundreds of keV electron precipitation associated with kinetic Alfvén wavesDipolarized magnetic field gradients and perpendicular magnetic drifts allow Landau resonance between waves and injected electronsAdiabatic transport and spatially varying E × B and grad‐B drifts induce perpendicular momentum scattering and electron losses [ABSTRACT FROM AUTHOR]

Wind spacecraft measurements are analyzed to obtain a current sheet (CS) normal width d cs distribution of 3374 confirmed magnetic reconnection exhausts in the ecliptic plane of the solar wind at 1 au. The d cs distribution displays a nearly exponential decay from a peak at d cs = 25 d i to a median at d cs = 85 d i and a 95th percentile at d cs = 905 d i with a maximum exhaust width at d cs = 8077 d i . A magnetic field θ -rotation angle distribution increases linearly from a relatively few high-shear events toward a broad peak at 35° < θ < 65°. The azimuthal Ď• angles of the CS normal directions of 430 thick d cs ≥ 500 d i exhausts are consistent with a dominant Parker-spiral magnetic field and a CS normal along the ortho-Parker direction. The CS normal orientations of 370 kinetic-scale d cs < 25 d i exhausts are isotropic in contrast, and likely associated with AlfvĂ©nic solar wind turbulence. We propose that the alignment of exhaust normal directions from narrow d cs ∼ 15â€"25 d i widths to well beyond d cs ∼ 500 d i with an ortho-Parker azimuthal direction of a large-scale heliospheric current sheet (HCS) is a consequence of CS bifurcation and turbulence within the HCS exhaust that may trigger reconnection of the adjacent pair of bifurcated CSs. The proposed HCS-avalanche scenario suggests that the underlying large-scale parent HCS closer to the Sun evolves with heliocentric distance to fracture into many, more or less aligned, secondary CSs due to reconnection. A few wide exhaust-associated HCS-like CSs could represent a population of HCSs that failed to reconnect as frequently between the Sun and 1 au as other HCSs. [ABSTRACT FROM AUTHOR]

The impact of interplanetary shocks on the magnetosphere can trigger magnetic substorms that intensify auroral electrojet currents. These currents enhance ground magnetic field perturbations (dB/dt), which in turn generate geomagnetically induced currents (GICs) that can be detrimental to power transmission infrastructure. We perform a comparative study of dB/dt variations in response to two similarly strong shocks, but with one being nearly frontal and the other highly inclined. Multi‐instrument analyses by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Los Alamos National Laboratory spacecraft show that nightside substorm‐time energetic particle injections are more intense and occur faster in the case of the nearly head‐on impact. The same trend is observed in dB/dt variations recorded by THEMIS ground magnetometers. THEMIS all‐sky imager data show a fast and clear poleward auroral expansion in the first case, which does not clearly occur in the second case. Strong field‐aligned currents computed with the spherical elementary current system (SECS) technique occur in both cases, but the current variations resulting from the inclined shock impact are weaker and slower compared to the nearly frontal case. SECS analyses also reveal that geographic areas with dB/dt surpassing the thresholds 1.5 and 5 nT/s, usually linked to high‐risk GICs, are larger and occur earlier due to the symmetric compression caused by the nearly head‐on impact. These results, with profound space weather implications, suggest that shock impact angles affect the geospace driving conditions and the location and intensity of the subsequent dB/dt variations during substorm activity. Plain Language Summary: Solar perturbations propagating in the interplanetary (IP) space can cause significant geomagnetic activity when they impact Earth. Such magnetic disturbances occur in the geospace and on the ground, manifested as, for example, satellite surface charging and undesirable geoelectric currents flowing in large‐scale power transmission lines. In this study, we compare the effects caused by two similarly strong solar perturbations that impacted Earth with two very distinct orientations: one nearly head‐on and the other highly inclined. We use an extensive list of data sets covering observations in the IP space, in the geospace, and on the ground. We find that magnetic field perturbations and auroral brightening are much more intense in the nearly frontal case, while these observations are considerably weaker in the second case. We attribute these effects to the highly symmetric conditions in which electromagnetic energy was released in the first case, which were quite asymmetric in the second case. These results are relevant to scientists whose goal is to predict and forecast geomagnetic activity that will follow solar perturbations detected in the IP space by solar wind monitors. Key Points: We perform a comparative study of geospace and ground multi‐instrument response to two similarly strong shocks with different orientationsEnergetic particle injections and dB/dt variations are faster and more intense in the nearly frontal case due to symmetric compressionAreas with intense dB/dt peaks are larger and occur earlier in the first case due to the symmetric compression by the head‐on shock [ABSTRACT FROM AUTHOR]

Large changes of the magnetic field associated with magnetic perturbation events (MPEs) with amplitudes |ΔB| of hundreds of nT and 5–10 min duration have been frequently observed within a few hours of midnight. This study compares the statistical location of nighttime MPEs with |dB/dt| ≥ 6 nT/s within the auroral current system observed during 2015 and 2017 at two stations, Cape Dorset and Kuujjuarapik, in Eastern Canada. Maps of the two dimensional nightside auroral current system were derived using the Spherical Elementary Current Systems (SECS) technique. Analyses were produced at each station for all events, and for premidnight and postmidnight subsets. We examine four MPE intervals in detail, two accompanied by auroral images, and show the varying associations between MPEs and overhead ionospheric current systems including electrojets and the field‐aligned like currents. We find 225 of 279 MPEs occurred within the westward electrojet and only 3 within the eastward electrojet. For the premidnight MPEs 100 of 230 events occurred within the Harang current system while many of the remainder occurred within either the downward region 1 current system or the upward region 2 current system. Many of the 49 postmidnight MPEs occurred in either the downward region 1 (11 events) or upward region 2 current system (27 events). These result suggest that the source of MPEs in the premidnight sector is somewhere between the inner to mid plasma sheet and the source for the MPEs in the postmidnight sector is somewhere between the inner magnetosphere and the inner plasma sheet. Key Points: Magnetic perturbation events most frequently occur within a westward electrojetPremidnight MPEs commonly occur within the Harang current system and Postmidnight MPEs commonly occur within the region 2 upward currentsMagnetic perturbation events with all sky image data display a sudden brightening and distortion of auroral arcs [ABSTRACT FROM AUTHOR]

Rapid changes of magnetic fields associated with nighttime magnetic perturbation events (MPEs) with amplitudes |ΔB| of hundreds of nT and 5–10 min duration can induce geomagnetically induced currents (GICs) that can harm technological systems. Here we present superposed epoch analyses of large nighttime MPEs (|dB/dt| ≥ 6 nT/s) observed during 2015 and 2017 at five stations in Arctic Canada ranging from 64.7° to 75.2° in corrected geomagnetic latitude (MLAT) as functions of the interplanetary magnetic field (IMF), solar wind dynamic pressure, density, and velocity, and the SML, SMU, and SYM/H geomagnetic activity indices. Analyses were produced for premidnight and postmidnight events and for three ranges of time after the most recent substorm onset: (a) 0–30 min, (b) 30–60 min, and (c) >60 min. Of the solar wind and IMF parameters studied, only the IMF Bz component showed any consistent temporal variations prior to MPEs: a 1–2 h wide 1–3 nT negative minimum at all stations beginning ∼30–80 min before premidnight MPEs, and minima that were less consistent but often deeper before postmidnight MPEs. Median, 25th, and 75th percentile SuperMAG auroral indices SML (SMU) showed drops (rises) before pre‐ and post‐midnight type A MPEs, but most of the MPEs in categories B and C did not coincide with large‐scale peaks in ionospheric electrojets. Median SYM/H indices were flat near −30 nT for premidnight events and showed no consistent temporal association with any MPE events. More disturbed values of IMF Bz, Psw, Nsw, SML, SMU, and SYM/H appeared postmidnight than premidnight. Key Points: Superposed epoch analyses of 2 years of >6 nT/s magnetic perturbation events (MPEs) from 5 high latitude Arctic stations are presentedOf the solar wind and interplanetary magnetic field (IMF) parameters studied, only IMF Bz showed any consistent pattern: a drop and rise prior to MPE occurrenceMost of the MPEs that occurred more than 30 min after a substorm onset did not coincide with peaks in the westward electrojet [ABSTRACT FROM AUTHOR]

In this study, we investigate ionospheric responses, including currents and aurorae, to solar wind dynamic pressure (SW Pdyn) sudden increases, which are critical for understanding solar windmagnetosphere-ionosphere coupling. We focus on two similar SW Pdyn pulse events that occurred on January 24, 2012 and November 12, 2010. In both cases, equivalent ionospheric currents (EIC) vortices were generated within about 10 min after the pressure pulse arrival, with a counter-clockwise rotating vortex (viewed from above) observed on the dusk side in the former case, and a clockwise vortex observed on the dawn side in the latter. Simultaneous ground-based All-Sky Imager (ASI) observations in the vicinity of the observed EIC vortex in each case showed that aurorae intensified on the dusk side and diminished on the dawn side. These observations provide direct evidence of the scenario proposed by Shi et al. (2014) that magnetospheric flow vortices generated by a solar wind pressure pulse carry fieldaligned currents into the ionosphere and thereby modulate auroral activity. The dawn/dusk asymmetry in the auroral intensification is a direct result of the opposite sense of vortex rotation on the dawn and dusk sides, which generate oppositely directed field-aligned currents into/out of the ionosphere [ABSTRACT FROM AUTHOR]

Reconnection-generated earthward flows, magnetic field dipolarizations, and auroral expansions are related to substorm current wedge (SCW) development. It has been suggested that fieldaligned currents (FACs) within the SCW can be generated by flow vortices, pressure gradients, or both. Observations related to these generation mechanisms differ from one event to another, due to their different locations relative to SCW's central meridian and timing relative to the SCW's evolutionary state. A pattern of in situ observations consistent with these generation mechanisms has yet to emerge. Obtaining such a pattern of in situ observations relies on the satellite locations relative to the FAC driver regions, which are hard to determine because coincident magnetotail observations are sparse. To solve this problem, an SCW inversion technique was used to model the FAC locations and determine the connections between magnetospheric and ionospheric phenomena. Using this technique, the magnetic flux, a parameter that is relatively insensitive to FAC locations, was analyzed during an isolated substorm on February 13, 2008. We compared the temporal variations of the accumulated flux that caused magnetic dipolarization in the SCW and the flux within the auroral poleward boundary. We found them to be in good agreement with the flux transported by earthward flows. This agreement suggests that the accumulation of the magnetic flux leads to the generation of the SCW, causing magnetic dipolarization and auroral poleward expansion. The amount of accumulated flux was found to be positively correlated with the amplitudes of these substorm-related phenomena. [ABSTRACT FROM AUTHOR]

Sudden changes of the solar wind dynamic pressure have significant impacts on the dynamics of the magnetosphere‐ionosphere system. On 18 February 2011, a sudden decrease in solar wind dynamic pressure was observed by the Wind satellite, which drove the entire magnetosphere‐ionosphere system as recorded in many ground‐based and space‐based measurements. In the magnetosphere, Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft observed a counterclockwise plasma flow vortex propagating tailward. Near the magnetic footprints of the THEMIS spacecraft, the ground magnetometers observed magnetic field variations that corresponded to a counterclockwise vortex in the equivalent ionospheric currents, which in turn indicated the presence of upward field‐aligned currents. The all‐sky imager at RANK station near the THEMIS footprints also observed a simultaneous enhancement of the auroral brightness. Therefore, this comprehensive case study demonstrates a causal chain that links the solar wind dynamic pressure variations to magnetospheric, ionospheric, and auroral activities. Plain Language Summary: In the near‐Earth space, the shape and dynamics of the magnetosphere and the ionosphere are strongly affected by solar wind variations. There have been numerous studies demonstrating the effects of magnetospheric compression caused by a sudden enhancement in the solar wind dynamic pressure. However, the opposite case with sudden decrease in the solar wind dynamic pressure has been rarely investigated. Here we perform a comprehensive, observational study to demonstrate the consequences of sudden decrease of solar wind dynamic pressure in the entire magnetosphere‐ionosphere system, which include the formation of magnetospheric flow vortices, generation of field‐aligned currents, perturbations of the geomagnetic field, and enhancement of auroral brightness near the magnetic footprints of the magnetospheric flow vortices. In other words, this case study provides a causal chain that shows the magnetospheric, ionospheric, and auroral responses to a sudden decrease of the solar wind dynamic pressure. Key Points: Small‐scale auroral brightening is observed after a sudden decrease in solar wind dynamic pressureSolar wind discontinuity produces a tailward‐moving magnetospheric flow vortex associated with field‐aligned currentsClear relationship between upward field‐aligned currents, ionospheric currents, and auroral activities are presented [ABSTRACT FROM AUTHOR]

Abstract: Interactions between the solar wind and the Earth's magnetosphere manifest many important space weather phenomena. In this paper, magnetosphere‐ionosphere drivers of intense dB/dt produced during geomagnetic storms that occurred on 9 March 2012 and 17 March 2015 are analyzed. A multi‐instrument approach combining Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission space‐borne and ground‐based observations was adopted to examine the magnetosphere‐ionosphere signatures associated with the dB/dt extremes during each storm. To complement the THEMIS measurements, ground‐based magnetometer recordings and All‐Sky Imager observations, equivalent ionospheric currents derived from magnetometer chains across North America and Greenland, and geosynchronous observations from the Los Alamos National Laboratory Synchronous Orbit Particle Analyzer are also examined. Our results show that the most extreme dB/dt variations are associated with marked perturbations in the THEMIS magnetospheric measurements, poleward expanding discrete aurora passing over the magnetometer sites (seen by the ground‐based THEMIS All‐Sky Imagers), intense Pc5 waves, rapid injection of energetic particles, and intense auroral westward currents. Substorms are considered as the major driver with a possible contribution from magnetospheric waves. The findings of this study strongly suggest that the localization of extreme dB/dt variations is most likely related to the mapping of magnetosphere currents to local ionospheric structures. [ABSTRACT FROM AUTHOR]

The presence of very few statistical studies on auroral omega bands motivated us to test-use a semi-automatic method for identifying large-scale undulations of the diffuse aurora boundary and to investigate their occurrence. Five identical all-sky cameras with overlapping fields of view provided data for 438 auroral omega-like structures over Fennoscandian Lapland from 1996 to 2007. The results from this set of omega band events agree remarkably well with previous observations of omega band occurrence in magnetic local time (MLT), lifetime, location between the region 1 and 2 field-aligned currents, as well as current density estimates. The average peak emission height of omega forms corresponds to the estimated precipitation energies of a few keV, which experienced no significant change during the events. Analysis of both local and global magnetic indices demonstrates that omega bands are observed during substorm expansion and recovery phases that are more intense than average substorm expansion and recovery phases in the same region. The omega occurrence with respect to the substorm expansion and recovery phases is in a very good agreement with an earlier observed distribution of fast earthward flows in the plasma sheet during expansion and recovery phases. These findings support the theory that omegas are produced by fast earthward flows and auroral streamers, despite the rarity of good conjugate observations. [ABSTRACT FROM AUTHOR]