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From the global simulation, we reproduce the solar wind‐magnetosphere‐ionosphere (S‐M‐I) interaction under the northward interplanetary magnetic field (IMF) with negative By. Reconnection structures, the plasma sheet, and lobes are formed in magnetospheric convection, while lobe/round‐merging/reciprocal/nightside cells appear in the ionosphere. Associated with the S‐M interaction, northern open field is generated at the evening open‐closed (O/C) boundary, due to successive cusp and interchange reconnections (in round‐merging cell) or by Dungey‐type reconnection (in nightside cell). Corresponding interchange and Dungey‐type reconnections occur at the southern null. Dungey‐type reconnection at the same time generates southern open field on the outer‐most magnetopause. Open field injected into the northern polar cap/void/lobe constructs the open field part of the round‐merging and nightside cells. After open‐field convection in the lobe, reclosures occur by again successive cusp and interchange reconnections on the dayside separator, or separator reconnection on the nightside separator. Former closed field line proceeds toward the evening O/C boundary through the dayside closed‐field convection in the round‐merging cell, while latter closed field line through the nightside closed‐field convection in the nightside cell. Shear that causes the large‐scale sun‐aligned arc is generated by the process of injecting open magnetic field into the void and the conjugate of process of connecting return flux from the plasma sheet to the nightside cell in counter hemisphere. Key Points: Under the northward IMF, the S‐M interaction has in front and off‐front components exciting the round‐merging cell and the nightside cellIn the nightside cell, closed flux is formed at nightside separator and transported along the nightside cell to the flank where reopenedThe HCA is explained by shear structure presented in this paper, without adopting evening‐morning symmetrical convection as in the DLR [ABSTRACT FROM AUTHOR]

Based on Global Coupled Ionosphere‐Thermosphere‐Electrodynamics Model, the solution of the 3‐dimensional current in the ionospheric region, the equivalent sheet current and filed‐aligned current are examined. The simulation study enables a comprehensive analysis of the effect of the geomagnetic field configuration, especially the non‐dipole component and tilt angle, on the ionospheric electrodynamics phenomena. Different geomagnetic field configurations are specified in the present work, including realistic geomagnetic field (RGF), tilted dipole geomagnetic field (TDGF) and zero‐declination dipole geomagnetic field (ZDGF). Our simulation focuses on the seasonal variation of Sq current, primarily governed by annual and semi‐annual variation. The modulation of the tilt angle of the geomagnetic field is globally distributed, whereas the modulation of the geomagnetic anomaly is localized. At mid‐latitudes, the annual mean and semi‐annual amplitudes of the Sq current are negatively correlated with the magnetic field strength, especially shown in geomagnetic anomaly area, while there is the opposite effect in the geomagnetic conjugate regions of the opposite hemispheres. The annual variation of Sq current system is more affected by the offset of the geomagnetic latitudes and geographical latitudes. The seasonal variation of the total Sq current is also modulated by the geomagnetic field. The annual mean, the annual and semi‐annual components of the total Sq current are negatively correlated with the magnetic field strength, while the annual variation is also controlled by the tilt angle of the geomagnetic field. The solar radiation affects the semi‐annual variation of the current more strongly than the annual variation. Plain Language Summary: Ionospheric currents and magnetic fields exert significant influences on energy and material transport within the ionosphere, playing a pivotal role in Earth's electromagnetic environment, which is the primary focus of ionospheric research. To investigate the impact of the geomagnetic field on the ionospheric current system, we employ a theoretical model (GCITEM) for simulation work. In this simulation, we consider both the realistic geomagnetic field configuration and two hypothetical idealized configurations of the geomagnetic field: one excluding non‐dipole terms and the other further aligning the geomagnetic axis with Earth's spin axis. The analysis of the longitude and the seasonal variation of the simulated currents under realistic magnetic field conditions has been validated through previous observation results. In comparison to simulation results obtained under hypothetical configurations, we discern and analyze the modulation and mechanism of non‐dipole terms and the tilt angle of geomagnetic field on the variability of the ionospheric current system, which are discussed in detail within this paper. This study facilitates the analysis and prediction of long‐term changes in the geomagnetic field's effects on ionospheric current generation and the impact on space's electromagnetic environment. Key Points: The geomagnetic field intensity exerts a significant influence on the Sq currents and exhibits a negative correlation with Sq currentThe annual mean and annual variation amplitude of Sq current are positively correlated with the geomagnetic anomaly at conjugate pointsThe tilt angle of the geomagnetic field affects the annual variation of Sq current globally, but hardly effects the semiannual variation [ABSTRACT FROM AUTHOR]

The recalibrated FAST/TEAMS data is used to study the response of O+ and H+ outflow to energy inputs in the nightside aurora during the 24–25 September 1998 geomagnetic storm, the same storm studied by Strangeway et al. (2005), https://doi.org/10.1029/2004JA010829. In contrast to the cusp, the Poynting flux and electron precipitation energy input are not as well correlated on the nightside, so their effects on outflow can be differentiated. The O+ outflow shows a strong correlation with both the Alfvénic Poynting flux (r = 0.71) and the soft electron precipitation (r = 0.69), while the H+ outflow only correlates well with the electron number flux (r = 0.74). This indicates that the auroral H+ outflow is close to its limiting flux without additional wave acceleration, while the outflow for the heavier O+ ion is increased by additional wave acceleration. Plain Language Summary: Geomagnetic activity can cause electrons to precipitate into the ionosphere in the nightside auroral region. It can also deliver electromagnetic wave energy to the same region. The electron precipitation can both heat and further ionize the ionosphere, leading to ions moving up along the field line. The wave energy can further accelerate the ions. If the ions are accelerated enough by these processes, they will flow out along the magnetic field, escaping the ionosphere. This paper finds that the H+ outflow increases with increased precipitating electrons in the nightside aurora. The O+ outflow increases with both electron precipitation and wave acceleration. Key Points: Electron precipitation and Poynting flux are less correlated on the nightside than the cusp, so their effects on outflow can be distinguishedO+ outflow is correlated with both Poynting flux and electron precipitation, while H+ is only correlated with electron precipitationParameterization of the outflow dependence is consistent between the dayside cusp and the nightside auroral regions [ABSTRACT FROM AUTHOR]

Geomagnetic disturbances caused by solar storms have the potential to create large‐scale geomagnetically induced currents in long conductors at the Earth's surface. These may disrupt the operation of electric power grids and cause blackouts. Ocean‐continent boundary regions are of particular concern because of the sharp contrast between the higher ocean electrical conductivity compared to the lower continental conductivity. This contrast may generate high‐amplitude geoelectric fields and cause power grids in coastal regions to be more vulnerable to space weather hazards. Previously, analytical calculations were used to estimate geomagnetically induced currents at ocean‐continent boundaries. However, for the analytical equations to be solvable, the physics and geometries of the problem were simplified. As a result of these simplifications and due to a lack of published measurements examining this issue in coastal regions, it is difficult to know for sure whether there are unique hazards to electric power grids at ocean‐continent boundaries. In this paper, a grid‐based, time domain modeling approach is used to solve the complete Maxwell's equations, which permits accommodation of (1) the complete physics of the propagating electromagnetic fields from disturbed ionospheric currents through the air and into the lithosphere and even into the ocean via the skin effect and (2) more realistic coastal geometries. Using this more robust approach, in the variety of scenarios studied in this paper, only a local enhancement of the electric fields was observed, which are expected to only pose potential hazards to power grids only a local scale near ocean‐continent boundaries. Key Points: Electromagnetic fields across ocean‐continent boundaries are solved via FDTD while accounting for a finite‐depth ocean and topographyAnalytical approaches are found to provide a close approximation to the more realistic FDTD results for the range of studied scenariosSurface electric field enhancements are highly localized along the coast, even for low frequencies and shallow oceans [ABSTRACT FROM AUTHOR]

We present a new model of the near‐Earth magnetospheric field produced by electric currents in the inner magnetosphere and the associated induced magnetic field. The model is designed to track hourly variations of these fields and accounts for their local time asymmetries. It is built by applying spherical harmonic analysis to vector measurements from the ground observatory network at low and mid‐latitudes. The primary and induced fields are separated with an approach in the time domain that uses a a priori radially‐symmetric electric conductivity model of the Earth. The model coefficients are computed at one‐hour time steps between 1997 and 2022. This model is shown to be consistent to within a few nT with previously developed indices which track the magnetospheric ring current. It is also validated against data from the Swarm, CHAMP and Øersted satellites. The fit to satellite data is comparable to that of the CHAOS‐7.15 model for geomagnetically quiet times, and improved by up to 20% on some components for geomagnetically moderate and active times. We attribute these differences mostly to a better representation of local time asymmetries, both on average and during individual geomagnetic storms. This model can be used in various applications, such as investigating the properties of the magnetospheric field and its sources and separating the magnetospheric field from the fields of other sources in geomagnetic field modeling. Plain Language Summary: Geomagnetic field modeling aims at building data‐based mathematical representations, or models, of the various contributions to the total Earth's magnetic field measured at or near the Earth's surface. One of such contributions is the magnetic field generated by electric currents in the inner magnetosphere, including the so‐called ring current. In this paper, we present a new model of the near‐Earth magnetic field generated in the inner magnetosphere based on data collected in ground magnetic observatories. The model covers the 1997–2022 time span, includes improved representations of the local time asymmetries of the field and of the effect of electrical induction in the Earth's mantle, and is validated against data collected in low Earth orbits by the Swarm, CHAMP and Oersted satellites. We find that the model provides an improved representation of the magnetic field generated in the inner magnetosphere during periods of moderate and high geomagnetic activity, including magnetic storms. Key Points: A new ground‐data based model of hourly variations of the primary inner near‐Earth magnetospheric and associated induced field is presentedComparison with satellite data shows up to 20% performance improvement compared to the CHAOS‐7 model for moderate and active timesThe model accounts for variations of the field with local time and can be used to study geomagnetic storms [ABSTRACT FROM AUTHOR]

This paper studies extremely low frequency (ELF) whistler‐mode waves' behavior within small‐scale magnetic field irregularities in the Earth's magnetosphere, known as magnetic ducts. Based on the magnetic fields' magnitude inside and outside these ducts, they are categorized as high‐magnetic ducts and low‐magnetic ducts (LBD). Using the whistler‐mode dispersion relation analysis, our primary focus is to show that LBDs are prone to leak electromagnetic energy outside the duct. We further investigate the hypothesis that whistlers can propagate within LBDs without any signal loss when the width of the duct corresponds to an integer multiple of the perpendicular wavelengths of the waves inside it. This condition offers a straightforward and effective method for identifying non‐leaking eigenmodes of LBDs. Our analysis of this non‐leaking condition reveals that every LBD possesses a finite number of non‐leaking eigenmodes directly proportional to the duct's width and the magnitude of the ambient magnetic field within it. The analytical results are then validated using two‐dimensional, time‐dependent simulations of the electron‐Magnetohydrodynamics model. Also, we model the non‐leaking propagation of an ELF whistler‐mode wave observed inside the LBD by the NASA Magnetospheric Multiscale mission satellites. Key Points: Magnetospheric Multiscale mission satellites observe whistler‐mode waves inside the low‐magnetic ducts (LBD)We provide an analytical explanation for the occurrence of leaking ducts and show that the LBD are prone to leakSimulations confirm our analytical prediction of the most trapped extremely low frequency wave in the LBD, provided by least degree of leakage [ABSTRACT FROM AUTHOR]

This study presents a number of previously unreported auroral features that form shortly after the IMF Bz ${\mathrm{B}}_{\mathrm{z}}$–component turns sharply northward. The auroral images were captured by the Boreal Aurora Camera Constellation (BACC) All‐sky camera at the Kjell Henriksen Observatory, located in Longyearbyen, Svalbard. These observations are different from those previously reported. There are several differences: (a) The Eastward‐Moving Auroral Forms (EMAFs) do not originate from type 1 or type 2 aurora. The EMAFs originate from a diffuse vertical auroral form in the far west part of the ASC image. Vertical refers to the auroral form oriented in the north‐south direction in the frame of the ASC image. (b) The EMAFs form crescent shapes after they begin to drift eastward. (c) The EMAFs are discrete auroral forms; previously reported ionospheric signatures of high‐latitude reconnection are described as patches of brightness that form within a pre‐existing auroral band. (d) The EMAFs are observed to first move eastward and then slightly poleward during their journey. The movements of previously reported Ionospheric signatures of high‐latitude reconnection are described as expanding patches of brightness within a pre‐existing auroral band. (e) Some of the EMAFs brighten again after fading, during their northeastward drift across the field of view. (f) The EMAFs begin their eastward motion aligned in the north‐south direction. During their northeast drift, they begin to tilt toward the east. Some of the EMAFs keep tilting until they are aligned in the east‐west direction. Plain Language Summary: Magnetic reconnection is a process which combines antiparallel magnetic field lines from different sources. Since magnetic field lines permeate our universe, there are many places where magnetic reconnection can occur. The solar‐terrestrial environment provides a great laboratory to study magnetic reconnection. Different orientations of the interplanetary magnetic field (IMF) allow it to connect to different regions of the Earth's geomagnetic field. Information from a magnetic reconnection event is transmitted to the ionosphere via the terrestrial magnetic field and is manifested in dazzling auroral displays. This paper presents ionospheric signatures of magnetic reconnection at high‐latitudes which are extremely different than past studies: (a) the origin of the observations in this study are different, (b) the ionospheric signatures form crescent shapes after they begin to drift eastward, (c) the ionospheric signatures are discrete auroral forms, (d) The ionospheric signatures are observed to first move eastward and then slightly poleward during their journey, (e) some of the ionospheric signatures brighten again after fading, during their northeastward drift across the sky, (f) during their northeast drift, they begin to tilt toward the east. Some of the EMAFs keep tilting until they are aligned in the east‐west direction. Key Points: Discrete auroral forms for northward IMF are aligned in the north‐south direction, propagating eastwardThese discrete auroral forms rotate clockwise during their eastward drift, some becoming aligned in the east west directionHigh‐latitude reconnection is a possible explanation for these discrete dayside auroral forms since the IMF clock angle is less than 90° [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive study of geomagnetic storm time currents using magnetic field recorded by multispacecraft polar‐orbiting mission, Swarm. During geomagnetic storm period, the magnetic field variations obtained after removing the internal geomagnetic field and quiet time contributions can be considered as a proxy for storm time currents and are found to follow the temporal profile of Dst index very closely. These variations at the equatorial crossings recorded by multiple‐spacecraft are used to estimate the Dst values and are found to have a good match with the ground‐based Dst index. The average deviation between these two is around 4–13%. We have estimated the asymmetry by taking the difference between the magnetic field variations at two local time sectors separated by 12 hr. The estimated asymmetry shows a good match with the AsyH‐index, especially when satellite traverses in the dawn‐dusk sector. In general, the magnetic field variations are stronger in the night‐to‐dusk sector than day‐to‐dawn sector, which could be due to the larger pressure‐gradients near night‐to‐dusk caused by ion movements. The important advantage of Swarm mission is that it provides an opportunity to investigate the longitudinal gradients in the storm time magnetic fields. It is observed that in general, the gradients are stronger during the main phase of the storm, centered near the equator with a latitudinal width of ~20–30° in both the hemispheres, and are supportive to the scenario of particle‐injection from the magnetotail. The stronger gradients are observed at higher latitudes (~40°) during the episodes of substorms and might be associated with the ionospheric/field‐aligned currents. Plain Language Summary: The space weather events such as geomagnetic storms can have a huge impact on the terrestrial magnetic and plasma environment, producing large currents, which in turn, impact spaceborne communication, navigation systems, aviation and satellite operations, and electric power grids, etc. The paper presents a new approach to estimate geomagnetic indices equivalent to Dst and AsyH during geomagnetic storms, making use of the increased observations from space‐based instruments in recent times, and demonstrates their utility in quantifying the geomagnetic environment. The important advantage of multispacecraft Swarm mission is that it provides an opportunity to investigate the spatial gradients in the storm time magnetic fields. The paper reports that strong azimuthal gradients in the magnetic field, near the equatorial sector, are associated with particle injection from the magnetotail. The paper demonstrates the utility of space‐based measurements in understanding the geomagnetic environment. Key Points: Geomagnetic storm time magnetic field variations are stronger in the dusk‐night than the dawn‐day sectorLongitudinal gradients in the storm time magnetic field are stronger near the equator and support the idea of particle injection from tailDuring episodes of substorm, stronger longitudinal gradients are observed at higher latitudes (~40°), which might be associated with FACs [ABSTRACT FROM AUTHOR]

Within the inner magnetosphere, the occurrence of plasma waves and/or magnetic field structures plays an important role in the dynamics of the particle populations, accelerating some particles to higher energies while scattering others. The 2013 Cluster Inner Magnetosphere Campaign was designed to investigate this process based on observations made at small separation scales. In addition, it provides an ideal opportunity for the study of small‐scale structures. This paper presents observations of previously unreported small‐scale magnetic field structures and investigates their characteristics and spatial properties. These structures, seen in the STAFF‐SC data set, are characterized by a rotation in the field direction and are observed to have spatial sizes of a few tens of kilometers, with a central core region around 10 km and are probably filamentary current structures. Using a variance analysis, it was determined that the maximum variance direction was close to being perpendicular to the external magnetic field. Based on the size of the central core region the current density is of the order of 10–40 nAm−2. While they may affect the profile of these structures, data calibration effects were ruled out as their origin. Key Points: Cluster STAFF‐SC magnetometer observations of previously unreported nonlinear structures in the inner magnetosphere are presentedThe small separation of two of the Cluster satellites enables their spatial and temporal properties to be investigatedWe argue that these structures can be generated by thin current filaments whose size is a few tens of kilometers [ABSTRACT FROM AUTHOR]

A geomagnetic storm is a phenomenon of weakening of Earth's magnetic field from a few hours to several days, which directly or indirectly affects navigation, transportation, communication, power grids, satellite electronics, etc. In general, the duration of the recovery phase is larger. Thus, its cumulative effect is greater than the main phase. Therefore, it is important to understand the physical processes involved during the recovery phase. In this paper, we have studied the recovery phase of 31 extreme geomagnetic storms that occurred from 1990 to 2020. Each storm demonstrates two distinct features of the recovery phase that is, initial fast and later slow recovery phase. During the fast recovery phase, the rate of recovery either linearly or non‐linearly depends on SYM−H, which has been characterized by an exponential or hyperbolic decay function in various reported studies. In our study, we have noted that the hyperbolic function explains the complete recovery phase of only 11 extreme events. Furthermore, both decay functions fail to explain the late recovery phase of storms. Interestingly, we have found that the rate of recovery during the slow phase shows steady variation (independent to SYM−H), that is, d(SYM−H)dt=constant. Our analysis suggests that the magnitude of the recovery rate during the late phase is proportional to the magnitude of the storm. Key Points: The exponential or hyperbolic decay function can only explain fast recovery phase of a stormsWe observe a constant recovery rate during the late (slow) recovery phase of extreme storms that is, d(SYM−H)dt=constantWe also find that the recovery rate during the late phase is directly proportional to the amplitude of the storm [ABSTRACT FROM AUTHOR]

The interaction of the solar wind with the Earth's magnetic field produces geomagnetic activity, which is critically dependent on the orientation of the interplanetary magnetic field (IMF). Most solar wind coupling functions quantify this dependence on the IMF orientation with the so‐called IMF clock angle in a way, which is symmetric with respect to the sign of the By component. However, recent studies have suggested that the sign of By is an additional, independent driver of high‐latitude geomagnetic activity, leading to higher (weaker) geomagnetic activity in Northern Hemisphere (NH) winter for By > 0 (By<0). In this paper we quantify the size of this explicit By effect with respect to the solar wind coupling function, both for northern and southern high‐latitude geomagnetic activity. We show that high‐latitude geomagnetic activity is significantly (by about 40%–50%) suppressed for By < 0 in NH winter and for By > 0 in Southern Hemisphere winter. When averaged over all months, high‐latitude geomagnetic activity in NH is about 12% weaker for By < 0 than for By > 0. The By effect affects the westward electrojet strongly, but hardly at all the eastward electrojet. We also show that the suppression of the westward electrojet in NH during By < 0 maximizes when the Earth's dipole axis points toward the night sector, that is, when the auroral region is maximally in darkness. Key Points: IMF By is an explicit driver of high‐latitude geomagnetic activityHigh‐latitude geomagnetic activity is suppressed in local winter for By < 0 in Northern Hemisphere and for By > 0 in Southern HemisphereExplicit By effect maximizes when the Earth's dipole axis points toward night [ABSTRACT FROM AUTHOR]

A dense network of ground global navigation satellite system receivers detected ionospheric total electron content (TEC) changes starting ~40 min before the 2011 Tohoku‐oki (Mw9.0) earthquake around the ruptured fault, together with the long‐lasting postseismic TEC drop. In this paper, we robustly estimate three‐dimensional (3‐D) distribution of both preseismic and postseismic ionospheric anomalies of the 2011 Tohoku‐oki earthquake by tomographic inversions of electron density anomalies. We set up >6,000 blocks, as large as 1.0° (east‐west) × 0.9° (north‐south) × 60 km (vertical), over the Japanese Islands, the Sea of Japan, and the Korean Peninsula, up to 870 km altitude. By using TEC anomalies of pairs exceeding ~1,300 stations and eight satellites obtained using reference curves, we estimated electron density anomalies within individual blocks. The results showed that the preseismic and postseismic anomalies do not overlap in space. The preseismic anomalies are composed of low (~300 km height) positive and high (~600 km height) negative anomalies. They occurred above the land of NE Japan without extending offshore, suggesting its origin related to surface electric charges. On the other hand, the postseismic electron depletion occurred offshore above the region where large coseismic uplift took place. These results demonstrate that the preseismic and postseismic ionospheric anomalies are independent not only temporarily but also spatially and certainly in underlying physical mechanisms. We propose a simple model to explain how surface charges redistribute ionospheric electrons to make the observed preseismic electron density anomalies. Plain Language Summary: A dense network of GNSS/GPS receivers found that redistribution of ionospheric electrons started ~40 min before the 2011 Tohoku‐oki earthquake around the fault. It was also found that a long‐lasting electron depletion occurred after the earthquake. We studied the three‐dimensional structures of the electron density anomalies immediately before and after the 2011 earthquake. We found that the preseismic anomaly occurred above land, but the postseismic anomaly occurred offshore. The preseismic change is characterized by the simultaneous growth of positive and negative electron density anomalies, while the postseismic change is dominated by an electron decrease. These differences reflect the different physical origins of the preseismic and postseismic anomalies. Key Points: We robustly analyzed the 3‐D structure of ionospheric anomalies immediately before and after the 2011 Tohoku‐oki (Mw9.0) earthquakeThe preseismic anomalies consist of positive (~300 km height) and negative (~600 km height) anomalies over the land of NE JapanThe postseismic electron depletion occurred offshore above the ruptured fault [ABSTRACT FROM AUTHOR]

We simulated the impact of long‐term changes in the geomagnetic field on the spatial pattern of the Weddell Sea Anomaly (WSA). The Weddell Sea Anomaly, belonging to the region near the tip of the Antarctic Peninsula, Falklands, and surrounding seas, is the best‐known example of ionospheric reversed diurnal cycle. This paper investigates whether and to what extent long‐term changes in the geomagnetic field influence spatial morphology of the WSA. The major concept centers on a normalized density difference index, serving as a measure of difference between dayside and nightside ionospheric density. Through combined analysis of in situ ionospheric electron density data from past mission CHAMP, ongoing ESA mission Swarm, and simulations based on the NCAR TIEGCM model, we examine changes in the spatial pattern of the WSA‐like anomalies, which could be linked to long‐term changes in the geomagnetic field. A series of simulations provides theoretical analysis of changes in the morphology of the WSA on time scales longer than several decades. Numerical analysis shows that from the time when the WSA was discovered (around 1960) till present, maximum of the WSA has migrated by approximately 7° in longitude towards the Pacific Ocean, showing clear westward drift, consistent with temporal evolution of the geomagnetic dipole component. Key Points: Numerical reconstruction of the WSA is derived with the TIEGCM modelTwo LEO satellite missions are used for determining and tracking maximum of the WSAThe WSA exhibits westward migration towards the Pacific Ocean [ABSTRACT FROM AUTHOR]

Usual depletions in the Northern Hemisphere form a tree fork junction feature near the south (i.e., toward the equator) in all‐sky airglow images, and its branches surge toward the north (i.e., poleward). We report in this paper unusual airglow depletions in OI 630.0‐nm imaging over India that surged equatorward and formed an inverted tree fork junction feature on 3 January 2011. Airglow images showed faint signatures of medium‐scale traveling ionospheric disturbances in the beginning that were east‐west aligned and propagated toward equator. Within 06 to 18 min of its passing, turbulent structures were noted in the field of view, and two depleted patches appeared out of low airglow background over the off‐equatorial edge. The apex height of the associated geomagnetic flux tubes varied from ~ 1,400 to 1,600 km. Later, these dark regions intensified and surged equatorward while drifting slowly toward west. While remaining almost steady, one of them further intensified and continued to surge equatorward. The second dark patch got linked up with another isolated depletion to form an elongated depleted feature. Next, the southern end of this attached feature surged equatorward. When two depletions were well formed, an inverted tree fork junction was noticeable. During this time, the equatorward motion of the equatorial ionization anomaly structure is also seen. To the best of our knowledge, this is the first imaging observation of an inverted tree fork junction feature and brings out the unknown facets of ionospheric irregularities. Plain Language Summary: Usual depletions form a tree fork junction feature near the equatorial edge in all‐sky airglow images; its branches surge poleward. On 3 January 2011, unusual depletions were noted over India that surged equatorward and formed an inverted tree fork junction. We present an independent usual depletion event. Such an inverted feature has been not reported earlier to the best of our knowledge and brings out the unknown facets of ionospheric irregularities. Key Points: This is the first report of off‐equatorial edge initiating and equatorward surging airglow depletionsUnlike usual depletions in the Northern Hemisphere, observed depletions appeared like an inverted tree fork junction (somewhat similar to those reported from the Southern Hemisphere) [ABSTRACT FROM AUTHOR]

Under northward interplanetary magnetic field conditions, magnetic reconnection at the Earth's magnetopause is usually thought to operate through the merging of magnetosheath magnetic field lines and open magnetic field lines from the magnetospheric lobe. However, reconnection also occurs between magnetosheath field lines and closed magnetic field lines in the magnetosphere. Under certain conditions, this nonlobe field line reconnection has distinct plasma and magnetic field signatures that distinguish it from reconnection of lobe field lines. A survey of these conditions suggests that nonlobe reconnection at the Earth's magnetopause may be common even for relatively strong northward IMF. Plain Language Summary: Magnetic reconnection interconnects magnetic field lines across a current sheet like that at the Earth's magnetopause or like that in the solar corona. This paper describes a type of interconnection of magnetic field lines at the magnetopause that was first reported for the solar corona. It appears that this type of reconnection is fairly common at the magnetopause. The different topology of this reconnection suggests a different type of interaction between the magnetic field lines in the solar wind and the field lines of the Earth. Key Points: Different magnetic field topologies at the magnetopause can be distinguished with plasma and magnetic field dataLobe and nonlobe reconnection occur at the magnetopause for northward IMFNonlobe reconnection appears to be common at the Earth's magnetopause for northward IMF [ABSTRACT FROM AUTHOR]

In this paper, we present our investigation of geomagnetic crochets observed following the X‐class solar flare events on 9 August 2011 (X6.9) and 24 September 2011 (X1.4). Both events occurred during geomagnetically quiet times (Kp ≤ 3). The analysis of the geomagnetic crochets near the magnetic equator revealed very different responses in the ionospheric currents between these two events. During the 9 August 2011 event, a reduction of the eastward equatorial electrojet was observed in all daytime stations studied, while the daytime eastward equatorial electrojet was observed to enhance during the 24 September 2011 event. A spherical harmonic analysis was applied to the observations from nearly 160 ground stations at middle and low latitudes to derive the equivalent ionospheric current system for the geomagnetic crochets as well as background solar‐quiet (Sq) variations for both events. The equivalent current system of the geomagnetic crochets for the 24 September 2011 event showed a similar pattern with the background Sq current system and thus can be attributed to the enhancement of Sq currents. However, the crochet current system for the 9 August 2011 event revealed a different pattern from the background Sq current system in both Northern and Southern Hemispheres. It is suggested that unusual geomagnetic crochets during the 9 August 2011 event may be dominated by the ionospheric currents at D‐region altitudes. Key Points: Following the X6.9 solar flare event of 9 August 2011, negative crochets were observed at dayside equatorial stationsGlobal ionospheric current system due to the solar flare that cannot be explained by an enhancement of the background Sq current systemContribution from the D‐region currents is suspected for the changes of the global ionospheric system [ABSTRACT FROM AUTHOR]

This paper reports the statistical analysis of zonal drift velocity of equatorial plasma bubbles (EPBs) inferred from advanced optical technique (all‐sky imager) with OI 630.0‐nm airglow emission from January to April of 2011 to 2013. Over 143 nights of observations have been carried out using all‐sky imager over low‐latitude station Kolhapur (16.8°N, 74.2°E; 10.6°N dip latitude), out of which 58 nights showed signatures of EPBs. We study the hourly, monthly, and seasonal variation in zonal drift velocity of EPBs. Also, the magnetically disturbed nights (Ap > 18) are separately studied. It is observed that (a) the daily peak zonal drift velocity is seen to be positively correlated to corresponding daily averaged 10.7‐cm solar flux. (b) The zonal drift velocity is found to be larger in the equinox months than that of winter months. (c) During the disturbed nights the zonal drift velocity gets slower and one of the disturbed nights showed the latitudinal variation in zonal drift velocity during the development phase of EPBs before midnight hours. We suggest that this variation might be occurred due to the disturbance dynamo electric fields at low latitude, which reduces the EPBs' eastward velocity. (d) The zonal drift velocity of EPBs presented in this paper is in good agreement with the previous studies as well as the HWM‐07 model values. Key Points: Positive correlation has been observed between solar 10.7‐cm flux and zonal drift velocity of EPBs over Indian low‐latitude regionIntense enhancement in OI 630.0‐nm emission is occurred during disturbed night causing reduction in zonal drift velocity of EPBsLatitudinal variation in zonal drift velocity of EPBs on disturbed night affected the morphology of EPBs [ABSTRACT FROM AUTHOR]

The ultraviolet imager (UVI) of the Polar spacecraft and an all‐sky camera at Longyearbyen contemporaneously detected an auroral vortex structure (so‐called "auroral spiral") on 10 January 1997. From space, the auroral spiral was observed as a "small spot" (one of an azimuthally aligned chain of similar spots) in the poleward region of the main auroral oval from 18 to 24 hr magnetic local time. These auroral spots were formed while the substorm‐associated auroral bulge was subsiding and several poleward‐elongated auroral streak‐like structures appeared during the late substorm recovery phase. During the spiral interval, the geomagnetically north‐south and east‐west components of the geomagnetic field, which were observed at several ground magnetic stations around Svalbard island, showed significant negative and positive bays caused by the field‐aligned currents related with the aurora spiral appearance. The negative bays were reflected in the variations of local geomagnetic activity index (SML) which was provided from the SuperMAG magnetometer network at high latitudes. To pursue the spiral source region in the magnetotail, we trace each UVI image along field lines to the magnetic equatorial plane of the nightside magnetosphere using an empirical magnetic field model. Interestingly, the magnetotail region corresponding to the auroral spiral covered a broad region from Xgsm ∼ −40 to −70 RE at Ygsm ∼ 8 to 12 RE. The appearance of this auroral spiral suggests that extensive areas of the magnetotail (but local regions in the ionosphere) remain active even when the substorm almost ceases, and geomagnetic conditions are almost stable. Plain Language Summary: Auroras that locally exhibit a vortex structure are referred to as auroral spirals. The fundamental features of auroral spirals, such as their generation process and source region in the magnetotail, are poorly understood. Based on the images obtained from the Polar ultraviolet imager (UVI) instrument and an all‐sky camera at Longyearbyen, we examined the morphological changes before and during the lifetime of an auroral spiral, which occurred during a comparatively quiet‐time of magnetotail recovery following a main substorm phase on 10 January 1997. The auroral spiral was formed after substorm‐associated auroral features subsided and the disappearance of several poleward‐elongated auroral streak‐like structures. During the spiral, the geomagnetic fields observed around Svalbard island and the geomagnetic activity index derived from the high‐latitude magnetometers showed significant negative/positive variations related with the aurora spiral appearance. According to the field‐aligned auroral spiral UVI image projections onto the nightside magnetic equatorial plane using an empirical geomagnetic field model, the source region of the auroral spiral spanned a broad region over 30 RE in the magnetotail direction with a dawn‐dusk width of ∼4 RE. These results suggest that extensive areas of the magnetotail are active enough to cause auroral spirals even during the late substorm recovery phase. Key Points: Auroral spiral was formed while the substorm‐associated auroral bulge was subsiding and poleward‐grown auroral streak structures appearedThe source region of the auroral spiral in the magnetotail is widely distributed over 30 RE, from Xgsm ∼ −40 to −70 REExtensive areas of the magnetotail are sufficiently active to cause auroral spirals even during the late substorm recovery phase [ABSTRACT FROM AUTHOR]

We present a new empirical model of quiet‐time F‐region ionospheric currents and associated magnetic fields. This model is designed to accurately represent these currents and fields at low and mid latitudes. For each individual Swarm satellite, the preprocessed data is represented as a non‐potential toroidal magnetic field using the Mie representation in a thin‐shell and spherical harmonic expansions. This approach allows to fully separate spatial and climatological variations as well as to assess the robustness of the model with respect to both measurement errors and data sampling with local time. The obtained model describes the toroidal magnetic fields and the associated radial poloidal electric currents at two distinct altitudes in the ionosphere F region. Clear signatures of low‐ and mid‐latitude interhemispheric field‐aligned currents (IHFACs) are identified. The model reproduces well‐known characteristics of the climatology of IHFACs and provides new insights, for example, on the average daily variations of IHFACs during winter in the Northern Hemisphere. It also well recovers the variations of IHFACs with longitude. The potential driving mechanisms of these variations, such as longitudinal variations of the main field and modulation by upward propagating atmospheric tides, are discussed. The new model can be used to analyze the relationship between atmospheric tides and IHFACs. It can also be used to investigate the connection between the magnetic fields and electric currents from the ionospheric E and F regions in order to improve the separation of these fields as well as our understanding of the overall ionospheric electric current system. Key Points: A new Swarm‐based model of low‐ and mid‐latitude ionospheric F‐region magnetic fields and electric currents is presentedThe model provides a detailed picture of the climatology of low‐ and mid‐latitude interhemispheric field‐aligned currentsThe model provides constraints on the relationship between interhemispheric field‐aligned currents and upward propagating atmospheric tides [ABSTRACT FROM AUTHOR]

We present a comparison between properties of quasiperiodic (QP) extra low frequency/very low frequency emissions observed by the low-altitude DEMETER spacecraft and ultra-low frequency (ULF) geomagnetic field pulsations measured on the ground by the Canadian Array for Realtime Investigations of Magnetic Activity system of flux-gate magnetometers and by the Sodankylä Geophysical Observatory magnetometer. Altogether, we have analyzed 398 QP events observed at the times when DEMETER was close to the ground-based magnetometers. The modulation periods of the analyzed QP events were larger than 10 s and their frequency bandwidths were larger than 200 Hz. For a part of QP emissions with modulation periods about 30 s, there was a good agreement between the modulation periods and peak frequencies of ULF magnetic field pulsations measured on the ground. These QP emissions appear to be closely associated with coincident geomagnetic pulsations (QP1 type), and they represent ~18% of the total number of analyzed QP events. No corresponding geomagnetic pulsations were identified in the remaining 82% of QP events (QP2 type). The intensity of QP1 events does not seem to correlate with the intensity of geomagnetic field pulsations, while the intensity of QP2 events increases with the integral intensity of geomagnetic field pulsations. Based on the observed association between QP emissions and geomagnetic field pulsations, we estimate the radial distance of the generation region of QP1 emissions to L ~ 7. [ABSTRACT FROM AUTHOR]

Plasma in the inner magnetosphere is affected by various processes, such as substorms. In this study, we have statistically investigated the ring current properties based on the observations of Van Allen Probes from 2012 to 2019 to examine the substorm effects on the plasma pressure and current system in the inner magnetosphere. The results show that the plasma pressure increases significantly, leading to a ∼3 nT/hr geomagnetic depression during intense substorms. The contribution of <100 keV H+ ions to the plasma pressure increases during substorms, which is more significant at lower L‐shells. Opposite to the H+ ions, the proportion of >10 keV O+ ions contributing to the plasma pressure increases as substorm intensity increases. In addition, the rise of plasma pressure is mainly distributed from dusk to midnight, resulting in the enhancement of asymmetric ring current and the region II field‐aligned currents connecting to the ionosphere. Our results provide a comprehensive view of the variation of plasma pressure and current system in the inner magnetosphere during quiet periods and intense substorms. Key Points: The pressure increase caused by intense substorms can lead to 3 nT/hr magnetic field depressionThe pressure contribution of both <100 keV H+ and >10 keV O+ increase during intense substormsThe ring current densities from dusk to midnight and region II FACs increase dramatically during intense substorms [ABSTRACT FROM AUTHOR]

The strong quasi 2‐day wave (Q2DW) event that occurred during January 2015 was studied by employing medium frequency and meteor radar observations of winds from three different locations at low latitudes with considerable longitudinal separation. The large‐amplitude wave activity that occurred in mid‐January is the focus of this study. Using the advantage offered by the longitudinal separation among the stations, the zonal structure of the Q2DW has been examined. The presence of a dominant westward propagating zonal wavenumber 3 (W3) mode was revealed in the analysis. The availability of geomagnetic field data from a number of stations at low latitudes enabled us to observe the wave event at ionospheric dynamo region heights. The phase‐longitude cross‐section of the quasi 2‐day oscillation in the Y component of the geomagnetic field reiterates the presence of the dominant W3 mode observed at lower altitudes. Similar zonal characteristics and periods of the wave signatures found at mesospheric altitudes and in the manifestation of quiet‐time ionospheric currents in geomagnetic field are in conformity with the notion of the wave coupling of the atmosphere‐ionosphere system. Plain Language Summary: This study examines the strong quasi 2‐day wave event that occurred during January 2015 at low latitudes using medium frequency and meteor radar observations. The analysis reveals the presence of a dominant westward propagating zonal wavenumber 3 mode. Geomagnetic field data from multiple stations at low latitudes show similar zonal characteristics and periods of the wave signatures, which suggests wave coupling of the atmosphere‐ionosphere system. Key Points: Strong quasi 2‐day wave in the mesosphere during January 2015Simultaneous presence of quasi 2‐day oscillation in magnetic field with similar zonal structureEvidence for vertical coupling by quasi 2‐day wave [ABSTRACT FROM AUTHOR]

In situ magnetic field measurements are critical to our understanding of a variety of space physics phenomena including field‐aligned currents and plasma waves. Unfortunately, high‐fidelity magnetometer measurements are often degraded by stray magnetic fields from the host spacecraft, its subsystems, and other instruments. One dominant source of magnetic interference on many missions are reaction wheels—spinning platters of varying rates used to control spacecraft attitude. This manuscript presents a novel approach to the mitigation of reaction wheel interference on magnetometer measurements aboard spacecraft where multiple magnetometer sensors are deployed. Specifically, multichannel singular spectrum analysis is employed to decompose multiple time series simultaneously. A technique for automatic component selection is proposed that classifies the decomposed signals into common geophysical signals and disparate locally generated signals enabling the robust estimation and removal of the local interference without requiring any assumptions about its characteristics or source. The utility of this proposed method is demonstrated empirically using in situ data from the CASSIOPE/Swarm‐Echo mission, and a data interval with near‐constant background field was shown to have its local reaction wheel interference reduced from 1.90 nT RMS, for the uncorrected outboard sensor, to 0.21 nT RMS (an 89.0% reduction). This technique can be generalized to arrays of more than two sensors, and should apply to additional types of magnetic interference. Plain Language Summary: In order to observe and study the magnetic fields near Earth, high‐quality magnetic field measurements are necessary. However, the spacecraft carrying magnetic field sensors are often magnetically noisy—they produce stray magnetic fields that contaminate the measurements. One dominant noise source on many spacecrafts are the systems that control the orientation of the spacecraft. This manuscript presents a novel approach to the suppression of magnetic interference caused by these systems, improving the quality of acquired data. Specifically, a technique for the simultaneous separation of multiple measurements into physically meaningful components is combined with an automated component selection technique. This allows for a high‐quality estimate of the spacecraft noise to be generated and subsequently removed from magnetic field measurements, greatly improving the data quality. For example, an interval of data captured by the CASSIOPE/Swarm‐Echo satellite was shown to have its local interference reduced by an average of 89%. Key Points: In situ magnetic field measurements are often degraded by stray magnetic fields from the host spacecraft, such as from reaction wheelsDecomposing magnetic field data from multiple sensors simultaneously using multichannel singular spectrum analysis allows separation of geophysical fields and local noiseAutomated component selection procedure allows for robust estimation and subtraction of local magnetic noise, improving data quality [ABSTRACT FROM AUTHOR]

Reconnection in the magnetotail occurs along so‐called X‐lines, where magnetic field lines tear and detach from plasma on microscopic spatial scales (comparable to particle gyroradii). In 2017–2020, the Magnetospheric MultiScale (MMS) mission detected X‐lines in the magnetotail enabling their investigation on local scales. However, the global structure and evolution of these X‐lines, critical for understanding their formation and total energy conversion mechanisms, remained virtually unknown because of the intrinsically local nature of observations and the extreme sparsity of concurrent data. Here, we show that mining a multi‐mission archive of space magnetometer data collected over the last 26 yr and then fitting a magnetic field representation modeled using flexible basis‐functions faithfully reconstructs the global pattern of X‐lines; 24 of the 26 modeled X‐lines match (Bz = 0 isocontours are within ∼2 Earth radii or RE) or nearly match (Bz = 2 nT isocontours are within ∼2RE) the locations of the MMS encountered reconnection sites. The obtained global reconnection picture is considered in the context of substorm activity, including conventional substorms and more complex events. Plain Language Summary: Magnetic reconnection is a fundamental process in plasmas which couples microscopic scales (∼electron to proton gyroradii) to explosive macroscopic phenomena many orders of magnitude larger, such as solar flares and geomagnetic storms/substorms. Reconnection forms along "X‐lines", rifts where oppositely directed magnetic field lines are forced together. In the Earth's magnetosphere, reconnection has been observed by satellites at isolated locations; however, the large‐scale structure of X‐lines and their time evolution remains unknown because of the rarity and local nature of observations. Here, ground based measurements of geomagnetic activity and solar wind measurements are used to data‐mine 26 yr of magnetometer data from 22 Earth‐orbiting satellites, which are then utilized to reconstruct the global magnetic field associated with X‐lines in Earth's magnetosphere. We show that these reconstructions pinpoint the reconnection locations by verifying their consistency with direct spacecraft observations. Key Points: Global structure of magnetotail reconnection inferred from data mining matches its locations revealed by in situ observationsReconstructed magnetotail reconnection structures include X‐ and O‐lines and magnetic nullsReconstructed multiscale current sheet structure is consistent with its formation mechanism by quasi‐adiabatic ion motions [ABSTRACT FROM AUTHOR]

We report the results from a statistical study of a new type of naturally occurring, but differently structured very low frequency (VLF) radio waves called "VLF bursty‐patches." Their main characteristic is that they are detected at frequencies above the electron gyrofrequency (fce) in the equatorial plane for the L‐shell of observation. Here we use the data from the station of Kannuslehto, Finland (KAN, MLAT = 64.4°N, L = 5.5) with fce ∼ 5–6 kHz where VLF bursty‐patches are commonly detected on at least 43% of its campaign days. We analyzed all available data for 2017 (∼8 months) separating the observations by spectral type, periodicity, number of bursts, observed frequency, and accompanying usual VLF emissions at lower frequencies (f < 6 kHz). We found that the overall highest occurrence rate is in the morning sector (05–12 MLT) correlating with general VLF occurrence rate at KAN (07–13 MLT, MLT = UT + 1.5 hr). Grouping usual VLF emissions and bursty‐patches showing similar spectral features, suggests that both groups are being generated by the same mechanisms (e.g., temperature anisotropy magnetospheric plasma distribution). However, unlike the situation for usual VLF emissions, the daily occurrence of bursty‐patches did not show a clear relationship with AE index. Superposed epoch analysis of magnetic components from the Sodankylä magnetometer (∼45 km from KAN) shows that VLF bursty‐patches are detected when the horizontal component of the magnetic field shows minimum variations. This suggests that propagation to the ground of bursty‐patches is conditioned to localized low variations of magnetic activity. Key Points: Bursty‐patches are related to usual natural very low frequency (VLF) waves at high latitudes, probably generated by same mechanism but different propagationThese VLF bursty‐patches have their highest occurrence rate in the local morning (5–12 MLT)Occurrence shows no correlation with auroral substorm activity, but appears associated with low variations of the local geomagnetic field [ABSTRACT FROM AUTHOR]

Magnetic field draping occurs when the magnetic field lines frozen in a plasma flow wrap around a body or plasma environment. The draping of the interplanetary magnetic field (IMF) around the Earth's magnetosphere has been confirmed in the early days of space exploration. However, its global and three‐dimensional structure is known from modeling only, mostly numerical. Here, this structure in the dayside of the Earth's magnetosheath is determined as a function of the upstream IMF orientation purely from in‐situ spacecraft observations. We show the draping structure can be organized in three regimes depending on how radial the upstream IMF is. Quantitative analysis demonstrates how the draping pattern results from the magnetic field being frozen in the magnetosheath flow, deflected around the magnetopause. The role of the flow is emphasized by a comparison of the draping structure to that predicted to a magnetostatic draping. Key Points: Global 3D reconstruction of the magnetic field draping around the Earth magnetosphere purely from in situ observationsDraping pattern is shown as a function of the interplanetary magnetic field cone angleThe role of the magnetosheath flow to which the magnetic field is frozen is emphasized by a comparison to a vacuum magnetostatic draping [ABSTRACT FROM AUTHOR]

Flux Transfer Events (FTEs) are transient magnetic flux ropes typically found at the Earth's magnetopause on the dayside. While it is known that FTEs are generated by magnetic reconnection, it remains unclear how the details of magnetic reconnection controls their properties. A recent study showed that the helicity sign of FTEs positively correlates with the east‐west (By) component of the Interplanetary Magnetic Field (IMF). With data from the Cluster and Magnetospheric Multiscale missions, we performed a statistical study of 166 quasi force‐free FTEs. We focus on their helicity sign and possible association with upstream solar wind conditions and local magnetic reconnection properties. Using both in situ data and magnetic shear modeling, we find that FTEs whose helicity sign corresponds to the IMF By are associated with moderate magnetic shears while those that do not correspond to the IMF By are associated with higher magnetic shears. While uncertainty in IMF propagation to the magnetopause may lead to randomness in the determination of the flux rope core field and helicity, we rather propose that for small IMF By, which corresponds to high shear and low guide field, the Hall pattern of magnetic reconnection determines the FTE core field and helicity sign. In that context we explain how the temporal sequence of multiple X‐line formation and the reconnection rate are important in determining the flux rope helicity sign. This work highlights a fundamental connection between kinetic processes at work in magnetic reconnection and the macroscale structure of FTEs. Plain Language Summary: In the vicinity of the Earth's magnetosphere outer boundary, the magnetopause, twisted magnetic field structures known as "Flux Transfer Events" (FTEs) are often detected by spacecraft in‐situ. They temporarily connect the solar wind to the Earth's ionosphere, allowing the transfer of solar wind flux into the magnetosphere. It is known that FTEs are produced as a consequence of magnetic reconnection, a process that rearranges the topology of sheared magnetic fields, between the shocked solar wind and the geomagnetic field. However, our understanding of how the microphysics of magnetic reconnection can lead to the macroscopic structures of FTEs is still limited. We revisit the in‐situ observations of FTEs made by the Cluster and Magnetospheric Multiscale missions. We focus on the twist feature of FTEs as characterized by their helicity and investigate its relationship to solar wind conditions and possible link to magnetic reconnection properties. By investigating local magnetic shear conditions around FTE locations, we found that the FTE helicity is determined by a kinetic feature of magnetic reconnection known as the "Hall magnetic field". Our study highlights a close connection between a kinetic process of magnetic reconnection and the global structure of FTEs, constituting a cross‐scale coupling effect in solar‐terrestrial interaction. Key Points: We study the helicity sign of Flux Transfer Events and investigate upstream solar wind conditions and local magnetic shear around themThe helicity sign is found to be unassociated to the Interplanetary Magnetic Field (By) component when the local magnetic shear is highThe FTEs' helicity sign in such cases may relate to the Hall field of magnetic reconnection in the absence of a guide field [ABSTRACT FROM AUTHOR]

Embedded Region 1 and 2 field‐aligned currents (FACs), intense FAC layers of mesoscale latitudinal width near the interface between large‐scale Region 1 and Region 2 FACs, are related to dramatic phenomena in the ionosphere such as discrete arcs, inverted‐V precipitation, and dawnside auroral polarization streams. These relationships suggest that the embedded FACs are potentially important for understanding ionospheric heating and magnetosphere‐ionosphere (M‐I) coupling and instabilities. Previous case studies of embedded FACs have led to the speculation that they may result from enhanced M‐I convection during active times. To explore this idea further, we investigate statistically their occurrence rates under a variety of geomagnetic conditions with a large event list constructed from 17 years of Defense Meteorological Satellite Program observations. The identification procedure is fully automated and explicit. The statistical results indicate that embedded Region 1 and 2 FACs are common, and that they have a higher chance to occur when the level of geomagnetic activity is higher (given by various indices), supporting the idea that they result from enhanced M‐I convection. Plain Language Summary: Earth's magnetosphere is coupled to its ionosphere by electric currents of different scales flowing along magnetic fields. Sometimes a special type of medium‐scale currents may significantly impact the ionosphere. From this statistical study, we found that these medium‐scale currents appear frequently and more often when the magnetosphere and ionosphere are characterized by a higher level of activity. These findings provide important information on how these medium‐scale systems are generated. Key Points: We designed an automated, explicit selection algorithm to build a large database of embedded Region 1 and 2 currentsEmbedded Region 1 and 2 (ER1 and ER2) currents appear frequently and preferentially under geomagnetically active conditionsThe latitudinal widths of ER1 and ER2 currents increase with geomagnetic activity but not as much as those of R1 and R2 currents do [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 perform a geomagnetic event simulation using a newly developed magnetohydrodynamic with adaptively embedded particle‐in‐cell (MHD‐AEPIC) model. We have developed effective criteria to identify reconnection sites in the magnetotail and cover them with the PIC model. The MHD‐AEPIC simulation results are compared with Hall MHD and ideal MHD simulations to study the impacts of kinetic reconnection at multiple physical scales. At the global scale, the three models produce very similar SYM‐H and SuperMag Electrojet indexes, which indicates that the global magnetic field configurations from the three models are very close to each other. We also compare the ionospheric solver results and all three models generate similar polar cap potentials and field‐aligned currents. At the mesoscale, we compare the simulations with in situ Geotail observations in the tail. All three models produce reasonable agreement with the Geotail observations. At the kinetic scales, the MHD‐AEPIC simulation can produce a crescent shape distribution of the electron velocity space at the electron diffusion region, which agrees very well with MMS observations near a tail reconnection site. These electron scale kinetic features are not available in either the Hall MHD or ideal MHD models. Overall, the MHD‐AEPIC model compares well with observations at all scales, it works robustly, and the computational cost is acceptable due to the adaptive adjustment of the PIC domain. It remains to be determined whether kinetic physics can play a more significant role in other types of events, including but not limited to substorms. Key Points: We perform a global simulation of a geomagnetic storm event with kinetic modeling of the magnetotail reconnectionThe kinetic region is adaptively embedded to the magnetohydrodynamic model and the reconnection sites are identified by physical criteria during the runtimeThe global scale, mesoscale, and electron scale features are observed simultaneously in one simulation [ABSTRACT FROM AUTHOR]

We report on the solar and interplanetary (IP) causes of the third largest geomagnetic storm (26 August 2018) in solar cycle 24. The underlying coronal mass ejection (CME) originating from a quiescent filament region becomes a 440 km/s magnetic cloud (MC) at 1 au after ∼5 days. The prolonged CME acceleration (for ∼24 hr) coincides with the time profiles of the post‐eruption arcade intensity and reconnected flux. Chen et al. (2019, https://doi.org/10.3847/1538-4357/ab3f36) obtain a lower speed since they assumed that the CME does not accelerate after ∼12 hr. The presence of multiple coronal holes near the filament channel and the high‐speed wind from them seem to have the combined effect of producing complex rotation in the corona and IP medium resulting in a high‐inclination MC. The Dst time profile in the main phase steepens significantly (rapid increase in storm intensity) coincident with the density increase (prominence material) in the second half of the MC. Simulations using the Comprehensive Inner Magnetosphere‐Ionosphere model show that a higher ring current energy results from larger dynamic pressure (density) in MCs. Furthermore, the Dst index is highly correlated with the main‐phase time integral of the ring current injection that includes density, consistent with the simulations. A complex temporal structure develops in the storm main phase if the underlying MC has a complex density structure during intervals of southward IP magnetic field. We conclude that the high intensity of the storm results from the prolonged CME acceleration, complex rotation of the CME flux rope, and the high density in the 1‐au MC. Plain Language Summary: Powerful coronal mass ejections (CMEs) are responsible for very intense geomagnetic storms due to the out of the ecliptic component of the magnetic field in the CME flux rope or in the sheath if shock‐driving (Gosling, 1993, https://doi.org/10.1029/93ja01896). The 26 August 2018 storm was very intense, but the CME was inconspicuous and weak near the Sun. However, over an extended period of time the CME accelerated slowly and picked up adequate speed to cause an intense storm. Due to the presence of coronal holes near the eruption region, the CME rotated in such a way that the CME magnetic field and Earth's magnetic field can efficiently couple to transfer energy into the magnetosphere to cause the geomagnetic storm. The energy transfer is expedited by the presence of dense material deep inside the CME. Key Points: A coronal mass ejection characterized by prolonged acceleration, rotation, and high‐density content results in the intense geomagnetic stormSimulations with a kinetic code confirm that the ring current injection involves both solar wind dynamic pressure and electric fieldEmpirical formulas for predicting Dst based on solar wind electric field work when the magnetic cloud has no high‐density enhancement [ABSTRACT FROM AUTHOR]

The nightside magnetic field at the middle and low latitude observatory is usually disturbed, decreasing in its north component (noted as Bx) during the periods of enhanced space weather activity. As indicated by the SYM‐H index, the decreasing field mainly originated from the magnetospheric westward ring current and the cross‐tail currents. In the meridian direction, the maximum of the disturbed field has been observed not always located at the geomagnetic dipole equator. One possible explanation is that the latitudinal position of the magnetotail current systems is affected by the orientation of solar wind to the Earth's geomagnetic dipole axis (noted as tilt angle). For a non‐zero tilt angle, the current deviates from the equatorial position (e.g., Malin & Mete Işikara, 1976; 10.1111/j.1365-246x.1976.tb07096.x). In this study, the mid‐low latitudinal distribution of the nightside disturbed field is systematically investigated every ∼1.5 hr, based on the magnetic surveys of the Swarm satellite constellation at polar orbits of ∼450 km altitude. The meridian distribution of Bx regularly appears like an upward‐opening parabolic shape with its vertex representing the magnetic minimum. In summer, the minimum is found generally located in the southern hemisphere, while in winter the pattern is reversed. The overall distribution demonstrates north‐south asymmetry. When the disturbed level is low, the deviation of the magnetic minimum from the dipole equator usually gets larger. The north‐south asymmetric distribution during storm times is further analyzed. In the initial phase, the magnetic minimum was shifted prominently (about 10°–30°) from the equator, then moved back (with the offset less than 10°) in the main phase and recovery phase in most cases. Moreover, the latitudinal deviation undergoes a diurnal variation, which is conspicuous in spring and autumn. It seems the asymmetric distribution is anti‐correlated to the contribution of the ring current, indicated by the SYM‐H index. The warping structure of the cross‐tail current sheet probably plays a predominant role in generating the north‐south asymmetry of the magnetic field disturbances for non‐zero dipole tilt angles. Monitoring the external magnetic configuration via the low‐Earth‐orbit satellites will provide fundamental evidence to identify the relative position of the ring current and the cross‐tail current, as well as the dynamic processes in the near magnetotail. Key Points: The nightside magnetic disturbed maximum at mid‐low latitudes annually shifts in meridian direction, which is notable when SYM‐H is quietThe disturbed maximum generally stayed close to the dipole equator in the main phase and the recovery phase of the geomagnetic stormThe cross‐tail current largely contributes to the near‐Earth disturbed fields, dominating the magnetic north‐south asymmetric distribution [ABSTRACT FROM AUTHOR]

We present a statistical study of magnetic field vector residual between Swarm observations and two Earth magnetic field models: the 13th generation International Geomagnetic Reference Field (IGRF) model and the CHAOS‐7 model. Statistics of these residuals are important for estimating potential errors for satellite operations when using Earth magnetic models as a reference, as well as for magnetosphere‐ionosphere‐thermosphere studies examining energy input into the system. Magnetic field residuals are calculated as vector differences between observations and model estimation at Swarm satellite positions from 2014 to 2020. Magnetic field residuals for both models increase as geomagnetic activity level increases, and the largest magnitude of vector difference can be around 1800 nT with relatively small angle differences. The CHAOS‐7 model shows lower magnetic field residuals compared to IGRF‐13. North‐south hemispheric asymmetries are seen in magnetic field residuals for high Kp values larger than 6 with the southern hemisphere (SH) having more frequent occurrence of magnetic field residuals larger than 300 nT, especially during SH summer. Most large residual values appear in the high‐latitude region with SH seeing additional large residuals around the South Atlantic Anomaly region. Midnight and noon sectors show the strongest interhemispheric asymmetries. The northern hemisphere shows more frequent occurrence of large residuals above 75° magnetic latitude throughout all local times compared to the SH. Identifying asymmetries in large magnetic residuals under high geomagnetic activity levels is helpful for studying the difference in response to ionospheric disturbances in the two hemispheres. Key Points: Comparison between Swarm magnetic field vector observations and Earth magnetic field modelsStatistical magnetic field residual maps show largest differences in high latitude region and around the South Atlantic AnomalyMore frequent occurrence of large magnetic field residual under high geomagnetic activity levels in southern hemisphere [ABSTRACT FROM AUTHOR]

The Super Dual Auroral Radar Network (SuperDARN) was built to study ionospheric convection and has in recent years been expanded geographically. Alongside software developments, this has resulted in many different versions of the convection maps data set being available. Using data from 2012 to 2018, we produce five different versions of the widely used convection maps, using limited backscatter ranges, background models and the exclusion/inclusion of data from specific radar groups such as the StormDARN radars. This enables us to simulate how much information was missing from older SuperDARN research. We study changes in the Heppner‐Maynard boundary (HMB), the cross polar cap potential (CPCP), the number of backscatter echoes (n) and the χ2/n statistic which is a measure of the global agreement between the measured and fitted velocities. We find that the CPCP is reduced when the PolarDARN radars are introduced, but then increases again when the StormDARN radars are added. When the background model is changed from the RG96 model, to the most recent TS18 model, the CPCP tends to decrease for lower values, but tends to increase for higher values. When comparing to geomagnetic indices, we find that there is on average a linear relationship between the HMB and the geomagnetic indices, as well as n, which breaks when the HMB is located at latitudes below ∼50° due to the low observational density. Whilst n is important in constraining the maps (maps with n > 400 data points are unlikely to differ), it is insufficient as the sole measure of quality. Plain Language Summary: The ionosphere, where space begins and the atmosphere ends, moves as a result of the Earth's magnetic field coupling with the Sun. The Super Dual Auroral Radar Network (SuperDARN) was built around the Earth's magnetic poles to study this phenomenon, known as ionospheric convection. Combining many line‐of‐sight convection measurements, we are able to build global maps of ionospheric convection from SuperDARN data. This encapsulates dynamics which are central to space weather phenomena. SuperDARN, which has been gathering data for decades, has over time undergone numerous transformations, including the development of new processing software and more radars being added to the network. Using data from the years 2012–2018, we perform a statistical analysis on processed SuperDARN convection maps for the entire data set and assess systematically how the data set has changed over the years. We consider how the addition of more data and differences to the convection mapping procedures can affect scientific studies in the context of this large database. Key Points: We identify changes in derived convection maps when PolarDARN and StormDARN are added, and show the impact of different processingDerived convection parameters are highly susceptible to processing variables and which radars are includedWe show how the number of backscatter echoes per map is critical to the integrity of the maps, and discuss how this impacts map quality [ABSTRACT FROM AUTHOR]

We examine the drift‐resonant particle dynamics for toroidal ultralow frequency (ULF) waves in a pure dipole background geomagnetic field. We confirm that the resonant condition originally believed to apply only for poloidal ULF waves, mωd=ω, also applies for compressional toroidal waves. The predicted phase relationships have been confirmed from Van Allen Probes observations. Their good agreement provides the first observational evidence for the drift resonance condition controlled by the compressional toroidal ULF wave. Moreover, we extend the drift resonance theory into a nonlinear regime. The resulting particle motion can be described by a modified pendulum equation with solutions depending on the wave number m. For high‐m toroidal waves, the resonant islands become asymmetric to perturb the particle trajectories within each potential well and consequently increase the trapping widths in both energy and L‐shell. We further carry out test‐particle simulations to show the evolution of electron distribution functions when they interact with either toroidal or poloidal waves. These findings demonstrate that toroidal ULF waves, like their poloidal counterparts, play an important role in magnetospheric particle dynamics. Plain Language Summary: In the magnetosphere, ultralow frequency (ULF) electromagnetic waves in the mHz range are usually categorized into poloidal and toroidal modes. It has been widely accepted that poloidal ULF waves, characterized by electric field oscillations in the azimuthal direction, play a key role in accelerating and transporting charged particles in the inner magnetosphere. On the other hand, toroidal waves have long been considered incapable of accelerating particles since their radial electric field is perpendicular to the azimuthal drift velocity of inner magnetospheric particles unless their drift paths are significantly distorted by dayside compression of the magnetosphere. Here we show that even in a pure dipole field, the toroidal waves can still resonate with energetic particles because of the nonzero curl of the wave electric field in association with the compressional magnetic field oscillations, and the resonant condition remains the same as its poloidal counterpart. The predictions of compressional toroidal wave‐particle drift resonance show good agreement with spacecraft observations. Further analysis shows that the particle nonlinear behavior in the toroidal wave field can be described by a modified pendulum equation. Therefore, it is important to consider not only the poloidal but also the toroidal modes in the study of ULF wave‐particle interactions. Key Points: Compressional toroidal ultra low frequency (ULF) waves have the same drift resonance condition as poloidal waves in a pure dipole modelDrift resonance between electrons and compressional‐toroidal ULF waves is identified from Van Allen Probes observationsA modified pendulum equation is derived to describe the nonlinear drift resonance between toroidal waves and charged particles [ABSTRACT FROM AUTHOR]

Electromagnetic waves observed in the inner magnetosphere at frequencies between about 0.5 and 4 kHz sometimes exhibit a quasiperiodic (QP) time modulation of the wave intensity with modulation periods from tens of seconds up to a few minutes. Such waves are typically termed "QP emissions" and their origin is still not fully understood. We use a large set of more than 2,000 of these events identified in the low‐altitude DEMETER spacecraft data to check for energetic electron flux variations matching the individual QP wave elements. Altogether, seven such events are identified and their detailed analysis is performed. Energetic electron fluxes are found to be modulated primarily at energies lower than about 250 keV. While the waves may propagate unducted across L‐shells, the energetic particles follow magnetic field lines from the interaction region down to the observation point. This is used to estimate the locations of anticipated generation regions to L‐shells between about 4 and 6, and the respective source radial dimensions to about 0.6–1.2 Earth radii. The frequencies of the events are confined below half of the equatorial electron gyrofrequency in the determined source regions. Finally, it is shown that individual QP elements exhibit a fine inner structure corresponding to the wave bouncing between the hemispheres. Key Points: Energetic electron precipitation bursts corresponding to quasiperiodic emission peaks are identifiedInteraction regions occur at L‐shells between about 4 and 6 and have dimensions of about 0.6–1.2 Earth radiiIndividual wave elements exhibit a fine inner structure corresponding to the wave bouncing between the hemispheres [ABSTRACT FROM AUTHOR]

The evolution of the low‐latitude magnetosphere over the substorm cycle is reconstructed based on a new high‐resolution 3D representation of the magnetic field and nearest‐neighbor data mining. The study covers radial distances 2.5–25RE and employs a record‐large pool of spacecraft data taken during 1995–2019. The magnetospheric state is quantified by four indices, representing the ground geomagnetic activity and its temporal trends in the entire ±90° range of geomagnetic latitude: the SuperMAG SMR, the midlatitude positive bay MPB, the auroral SML, and the polar cap PC index. The developed technique has been tested on specific substorm events, with the results presented in the form of 5‐min cadence diagrams and animations of the magnetic field line configurations and electric current distributions. In all the analyzed events, the initial intensification and radial expansion of the inner tail current is accompanied by a gradual stretching of the magnetic field, followed by its sudden collapse, dramatic depletion of the current beyond R∼12RE, and a large‐scale dipolarization of the field around the time of MPB peak, after which the system recovers and tends to its pre‐substorm state. Plain Language Summary: The dynamical structure of the Earth's magnetosphere during geomagnetic substorms is reconstructed, based on (a) a large multi‐year database of satellite data taken during the last quarter century, (b) a pool of concurrent ground geomagnetic activity indices, covering full range of latitudes, (c) a new magnetic field model with enhanced spatial resolution, and (d) an advanced "nearest‐neighbor" data mining approach. Based on a synthesis of the above methods and data, we explore the ability of our approach to extract maximum information from past observations and reproduce the principal phases of magnetospheric substorms in terms of time sequences of the magnetic field and electric current diagrams, from the beginning to active and recovery phase of the disturbance. Key Points: A new high‐resolution B‐field representation combined with dynamical data mining reveals magnetosphere behavior on the substorm‐time scaleFull cycle of magnetosphere evolution is reconstructed based on 25‐year archive of satellite data and a set of ground‐based activity indicesInitial growth of the distant magnetotail current and its subsequent sudden collapse during the substorm onset are consistently reproduced [ABSTRACT FROM AUTHOR]

In the publication Troshichev et al. (2006) (https://doi.org/10.1029/2005JA011402) on the polar cap (PC) indices, PCN and PCS, an error was made by using components of the interplanetary magnetic field (IMF) in their geocentric solar ecliptic (GSE) representation instead of the prescribed geocentric solar magnetospheric (GSM) representation for calculations of index scaling parameters. The mistake has caused a trail of incorrect relations and wrong conclusions extending since 2006 up to now (2020) which should be discontinued, for instance, by issuing a corrigendum note from the authors. The present contribution explains the error and discusses in an extended example its consequences for one of the publications that have referred to the invalid scaling parameter set. Further investigations reported here of the PC index versions recommended by the International Association for Geomagnetism and Aeronomy (IAGA) indicate occurrences of similar problems in the present derivation of index scaling parameters. Plain Language Summary: The publication Troshichev et al. (2006) (https://doi.org/10.1029/2005JA011402) on the polar cap (PC) indices, PCN and PCS, has by mistake used components of the interplanetary magnetic field (IMF) in their geocentric solar ecliptic (GSE) representation instead of the prescribed geocentric solar magnetospheric (GSM) representation for calculations of index scaling parameters. The mistake has caused a trail of incorrect relations and wrong conclusions extending since 2006 up to now (2020) which should be discontinued. The examinations have been extended to the PCN version recently endorsed by the International Association for Geomagnetism and Aeronomy (IAGA). The analyses indicate that the scaling parameters for this PC index version are also questionable. Key Points: The use of invalid scaling parameters for Polar Cap (PC) index calculations is disclosedRelated adverse consequences for derived publications on PC indices are analyzedA summary of publications devaluated by their use of inconsistent PC index versions is provided [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]

The process of magnetic reconnection in a flux tube can occur in a time‐stationary fashion as slippage reconnection or in a time‐dependent manner based on, for example, the tearing instability. However, it is not well known under which conditions a system can sustain slippage reconnection. Likewise, it is unclear whether systems can exhibit slippage reconnection and time‐dependent reconnection simultaneously. In order to investigate these questions, we employ a set of 3D magnetohydrodynamic simulation. Using these simulations, we model a twisted flux tube with a spatially localized resistive region in the center of the simulation box, and an applied driver on one end plane of the simulation box. As a result, the Poynting flux injected at the boundary propagates to the resistive region and dissipates there. If the driver is nearly circular, we find a predominance of slippage reconnection, which decouples the applied driver entirely from one half of the flux tube. Second, a tearing‐like instability occurs with slippage reconnection in the resistive region when the resistivity is strong enough and the cross section of the flux tube, as defined by the shape of the applied velocity perturbation, is elliptical enough. In this case we find the surprising feature that magnetic field perturbation generated by the tearing‐like instability radiates away from the resistive region to influence the entire flux tube shape. Key Points: Magnetic reconnection in a twisted magnetic flux tube can be time‐stationary, leading to slippage reconnectionIn case the flux tube profile is elliptical enough, tearing‐like reconnection can occur simultaneously with slippageThese results pertain to a variety of plasma systems involving dissipation in twisted flux tubes [ABSTRACT FROM AUTHOR]

A novel algorithm for estimating both the linear and quadratic gradients of physical quantities based on multiple spacecraft observations using the least squares method is put forward. Using 10 or more spacecraft constellation measurements as input, this new algorithm can yield both the linear and quadratic gradients at the barycenter of the constellation. Iterations were used in the algorithm. Tests on cylindrical flux ropes, dipole magnetic field, and modeled geomagnetospheric field were carried out. The results of these tests indicate that the linear gradient obtained is of second-order accuracy, while the quadratic gradient is of first-order accuracy. The test on the modeled geomagnetospheric field showed that, the greater the number of spacecraft in the constellation, the greater the accuracy of the quadratic gradient calculated. However, the accuracy of the linear gradient obtained was independent of the number of spacecraft. The feasibility, reliability, and accuracy of this algorithm have been successfully verified. This algorithm could find wide applications in the design of future multiple spacecraft missions as well as in the analysis of multiple-point measurement data. [ABSTRACT FROM AUTHOR]

We present the first multi-wavelength imaging observations of strong thermal emission velocity enhancement (STEVE) using an all-sky imager at Athabasca (magnetic latitude = 61.5°N), Canada. This study is based on three STEVE events which were accompanied by picket fence structures in the green-line. Although the STEVE arc was dominant in 630 and 557.7-nm, weak emissions were also found in other wavelengths including OI at 844.6, Hβ, Na, and the nominal background filter at 572.5-nm. As observed at 630 and 557.7-nm, the STEVE arc started as a faint arc close to the auroral oval and moved equatorward. The 557.7-nm arc exhibited picket fence structure at later times after it moved equatorward. The picket fence was sometimes found to persist even after the 630-nm arc had disappeared. During a particular event, the STEVE arcs in both the 630 and 557.7-nm were found to carry a ribbon-like motion moving along the arc. We have found that STEVE arcs are embedded in a region of weak diffuse auroral emissions. The STEVE arcs have sharp boundaries and these boundaries are different in red- and greenline. The sharp decrease in the intensity at the immediate poleward edge of the STEVE arc appears as a "dark-band" in the green-line images. Based on the horizontal component of the geomagnetic field at Fort Smith (magnetic latitude 67.28°N), we find that the STEVE arc detachment, its equatorward motion, and its brightness coincided with changes in the magnetic activity during the recovery phase of a substorm. [ABSTRACT FROM AUTHOR]

The Global-scale Observations of the Limb and Disk (GOLD) mission, launched in 2018, aims to investigate the low latitude ionosphere from a geostationary orbit at 47.5°W. It uses two identical spectrometers measuring the wavelength range from 134.0 to 163.0 nm. The configuration of the Earth's magnetic field shows that the largest offset between geographic and geomagnetic equators occurs in the longitude sectors sampled by GOLD. In an attempt to investigate the longitude dependence of the occurrence rate and time of onset of plasma bubbles, or plasma depletions, GOLD data were separated in three sectors: 65°-55°W, 50°-40°W, and 10°W--0°. Observations of the nighttime emissions in 135.6 nm on November 2018 and March 2019 show plasma depletions occurring very frequently at these longitudes. The growth rate of the Rayleigh-Taylor instability was computed at these longitudes under similar low solar activity conditions, assuming an empirical model of upward plasma drifts. The time and value of the maximum growth rates obtained cannot always explain the observations. On average, the observed occurrence rate of plasma depletions is high, with a maximum of 73% (observed during November 2018 at ~45°W). Most of the depletions observed in November at 45°W and 60°W occur within 1 h after sunset. When compared with the November 2018 observations, depletions in March 2019 occur at later times. [ABSTRACT FROM AUTHOR]

Two‐dimensional hybrid particle‐in‐cell (PIC) simulations are carried out on a constant L‐shell (or drift shell) surface of the dipole magnetic field to investigate the generation process of near‐equatorial fast magnetosonic waves (a.k.a equatorial noise; MSWs hereafter) in the inner magnetosphere. The simulation domain on a constant L‐shell surface adopted here allows wave propagation and growth in the azimuthal direction (as well as along the field line) and is motivated by the observations that MSWs propagate preferentially in the azimuthal direction in the source region. Furthermore, the equatorial ring‐like proton distribution used to drive MSWs in the present study is (realistically) weakly anisotropic. Consequently, the ring‐like velocity distribution projected along the field line by Liouville's theorem extends to rather high latitude, and linear instability analysis using the local plasma conditions predicts substantial MSW growth up to ±27° latitude. In the simulations, however, the MSW intensity maximizes near the equator and decreases quasi‐exponentially with latitude. Further analysis reveals that the stronger equatorward refraction at higher latitude due to the larger gradient of the dipole magnetic field strength prevents off‐equatorial MSWs from growing continuously, whereas MSWs of equatorial origin experience little refraction and can fully grow. Furthermore, the simulated MSWs exhibit a rather complex wave field structure varying with latitude, and the scattering of energetic ring‐like protons in response to MSW excitation occurs faster than the bounce period of those protons so that they do not necessarily follow Liouville's theorem during MSW excitation. Key Points: The 2‐D hybrid PIC simulations are carried out on a constant L‐shell surface to simulate the excitation of MSWs for the first timeDespite the extended unstable region in latitude, MSWs do not grow well if they get latitudinally out of resonanceScattering of ring‐like protons during MSW excitation is local so that those protons do not necessarily follow Liouville's theorem [ABSTRACT FROM AUTHOR]

Understanding the day‐to‐day variability of equatorial plasma bubble (EPB) irregularities has been a major challenge for many decades that makes the deterministic prediction of EPBs remain elusive. In the present study, we report a case of intense and periodic EPBs observed during 8 and 9 April 2013 by the 47‐MHz Equatorial Atmosphere Radar at Kototabang, Indonesia. The periodic EPBs separated by about 200–250 km were initiated before sunset. The presunset onset and development of these periodic EPBs were discussed in light of the gravity waves (GWs) excited in connection with the deep convection due to the tropical cyclone (TC) Victoria. The outgoing long‐wave radiation measurement by very high‐resolution radiometer (VHRR) onboard Indian meteorological satellite Kalpana‐1 shows the occurrence of deep convective activity during these days. The presence of upward propagating GWs from the deep convective region associated with TC Victoria was confirmed using the GPS radio occultation (GPS‐RO) observations in the troposphere and stratosphere. The GW signatures at ionospheric altitudes were also observed from the Ionosonde observations over magnetic equator, and medium‐scale (~300 km) GWs were observed from the GPS‐TEC (Total Electron Content) data near to the magnetic equator and cyclone center. From the GW parameters observed from GPS‐TEC and GPS‐RO, we surmise that the secondary GWs generated by the dissipation of primary GWs associated with TC Victoria could have served as a seeding source on the generation of periodic EPBs during these two consecutive days. Key Points: GW seeding of periodic and intense presunset EPBs was observed on 8 and 9 April 2013 in connection with the deep convection associated with tropical cyclone VictoriaThe GPS‐TEC observation shows clear GW signatures with ~300‐km wavelength, ~23‐min periodicity, and horizontal velocity of ~225 m/sThe observed GW parameters indicate the possibility of EPB seeding due to the secondary GWs generated by the dissipation of primary GWs at the middle atmosphere [ABSTRACT FROM AUTHOR]

Recent work by Oberhagemann and Mann (2020, https://doi.org/10.1029/2019GL085271) has shown that ballooning instabilities in the transition region between dipolar and taillike fields associated with substorm onset may be triggered by an anisotropic pressure distribution that moves from an initially mild perpendicular anisotropy toward more parallel anisotropy. Here we examine the mechanism which can result in this transition toward more parallel pressure during growth phase tail stretching. We trace particles through a two‐dimensional model magnetic field to examine the evolution of particle distributions as they periodically drift through a tail which changes from a less stretched, quiet time to a more stretched late growth phase configuration. Our findings show that these particle pitch angle distributions become more parallel anisotropic through a combination of drift shell splitting and preferential de‐energization in the perpendicular direction. Drift shell splitting causes low pitch angle particles from a high‐pressure region to mix with high pitch angle particles from a low‐pressure region in a radially localized area in the tail transition region where high earthward pressure gradients exist. Preferential de‐energization occurs due to a transfer of particles from less stretched to more stretched field lines that reverses the usual balance between Fermi and betatron acceleration and causes particle pitch angle distributions to become more field aligned. Our model predicts onset characteristics and location in good agreement with observations, with the most unstable region consistent with the location of the auroral beads, which accompany the onset of the expansion phase of many substorms. Key Points: We examine the substorm onset paradigm of increasingly parallel pressure anisotropic ballooning using a growth phase model magnetotailDuring tail stretching, drifting energetic particle distributions develop more parallel anisotropy, increasing ballooning instabilityThe results can explain why onset occurs when it does, an observationally testable hypothesis, with forecasting potential [ABSTRACT FROM AUTHOR]

The Jovian magnetosphere assumes a disc‐like geometrical configuration ("magnetodisc") owing to the persistent presence of a system of azimuthal currents circulating in a washer‐shaped volume aligned with, or near, the magnetic equatorial plane. A Voyager era empirical model of the magnetodisc is fitted to vector magnetic field measurements obtained during the Juno spacecraft's first 24 orbits. The best fitting (within 30 Jovian radii) magnetodisc model is characterized by an inner and outer radius of 7.8 and 51.4 Jovian radii, a half‐thickness of 3.6 Jovian radii, with a surface normal at 9.3° from the Jovigraphic pole and 204.2° System 3 west longitude. We supplement the magnetodisc model with a second current system, also confined to the magnetic equatorial plane, consisting of outward radial currents that presumably effect the transfer of angular momentum to outward flowing plasma. Allowing for variation of the magnetodisc's azimuthal and radial current systems from one 53‐day orbit to the next, we develop an index of magnetospheric activity that may be useful in interpretation of variations in auroral observations. Key Points: An empirical magnetodisc model is fitted to the Juno magnetic field observationsThe magnetodisc model provides a more accurate representation of the magnetic field in the inner and middle magnetosphere of JupiterThe model is independently tested via observations of charged particle interactions with the Jovian satellites [ABSTRACT FROM AUTHOR]

Based on the Mars Atmosphere and Volatile EvolutioN (MAVEN) observations, we have analyzed the role of the crustal magnetic field on ion loss driven by the direct interaction of the solar wind with the Mars ionosphere. Crustal magnetic fields significantly attenuate the ion ionospheric motions and raise the flux of returning ions. On the other hand, since the ion densities in the ionosphere with strong crustal field are significantly higher than in the ionosphere with a weak crustal magnetic field, the net escape fluxes from the ionosphere with the crustal sources remain vital. The crustal magnetic field also leads to the expansion of the ionosphere and increase of the area exposed to solar wind. As a result, fluxes from higher altitudes essentially contribute to the flow pattern in Martian tail producing an excess of ion loss rate (∼15%) through the southern part of the tail. Thus, effects of inhibition and enhancement of the escape rate by the crustal magnetic field at Mars operate in competition producing a minor influence on the total ion loss. Key Points: Crustal magnetic field at Mars has a twofold effect on atmospheric erosionAt lower altitudes the crustal fields form a protective shield around the ionized atmosphereDue to expansion of ionosphere to higher altitudes, a larger cross‐section area is exposed to the solar wind that increases ion losses [ABSTRACT FROM AUTHOR]

Understanding the energization processes and constituent composition of the plasma and energetic particles injected into the near‐Earth region from the tail is an important component of understanding magnetospheric dynamics. In this study, we present multiple case studies of the high‐energy (≳40 keV) suprathermal ion populations during energetic particle enhancement events observed by the Energetic Ion Spectrometer (EIS) on NASA's Magnetospheric Multiscale (MMS) mission in the magnetotail. We present results from correlation analysis of the flux response between different energy channels of different ion species (hydrogen, helium, and oxygen) for multiple cases. We demonstrate that this technique can be used to infer the dominant charge state of the heavy ions, despite the fact that charge is not directly measured by EIS. Using this technique, we find that the energization and dispersion of suprathermal ions during energetic particle enhancements concurrent with (or near) fast plasma flows are ordered by energy per charge state (E/q) throughout the magnetotail regions examined (~7 to 25 Earth radii). The ions with the highest energies (≳300 keV) are helium and oxygen of solar wind origin, which obtain their greater energization due to their higher charge states. Additionally, the case studies show that during these injections the flux ratio of enhancement is also well ordered by E/q. These results expand on previous results which showed that high‐energy total ion measurements in the magnetosphere are dominated by high‐charge‐state heavy ions and that protons are often not the dominant species above ~300 keV. Key Points: In the magnetotail during injections, the charge states of suprathermal He and O ions can be inferred with a correlation analysisEnergization of ionospheric and solar wind ions during injections in the magnetotail is remarkably coherent and ordered by charge stateThe highest energy ions (≳300 keV) observed are heavies of solar wind origin and reach higher energies due to their higher charge states [ABSTRACT FROM AUTHOR]

Two THEMIS satellites, separated by only 0.38 Earth radii (RE), traversed two consecutive dayside magnetopause (MP) current sheets at 3.5 < ZGSM < 3.8 RE on 15 November 2010. An early‐stage crater‐like flux transfer event (FTE) with weakly enhanced total pressure is sampled at the first, complete outbound MP crossing. A mature FTE flux rope is observed just 70 s later across the second, inbound crossing of the MP current sheet. Two counterstreaming magnetosheath ion beams at the interface of two converging reconnection exhausts earthward of the MP current sheet provide direct evidence of two X‐lines across the early‐stage crater FTE. A D‐shaped ion beam observed along the southward magnetic field in the magnetosheath boundary layer and northward exhausts observed earthward of the MP provide evidence of two X‐lines across the mature FTE flux rope. Tripolar out‐of‐plane magnetic field perturbations (ΔBM) of substantial magnitudes were sampled across the MP current sheet in both stages of the FTE flux rope evolution. These dayside observations suggest an association between a tripolar ΔBM perturbation with two magnetic reconnection X‐lines and FTE flux rope formation. A dedicated particle‐in‐cell numerical simulation of magnetic reconnection reproduces similar ion velocity distributions and out‐of‐plane magnetic field perturbations at an asymmetric current sheet as those observed across a nascent flux rope between two X‐lines at the dayside asymmetric MP. Key Points: Counterstreaming ion beams observed by THEMIS confirm two reconnection X‐lines around a nascent FTE flux rope in agreement with a PIC simulationTHEMIS observed tripolar out‐of‐plane (BM) magnetic fields across a nascent flux rope and a mature FTE flux ropePIC simulations suggest that a tripolar BM at a nascent flux rope is due to unipolar Hall magnetic fields from two X‐lines at an asymmetric current sheet [ABSTRACT FROM AUTHOR]