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Abstract The objective of the present study is to investigate solar wind‐magnetosphere‐ionosphere coupling (SWMIC) under intense external driving in terms of dayside ground magnetic disturbances. Dayside magnetic reconnection drives a region‐1 sense current system, the SWMIC current system. This current system intensifies, before other magnetospheric currents, if the interplanetary magnetic field becomes strongly southward after sudden storm commencements. During the October 2003 storm, its effect on mid‐ and low‐latitude magnetic fields manifested as a single‐swirl pattern of equivalent currents, which is expected for the field‐aligned current closure through the auroral electrojet. During the July 2000 storm, in contrast, it manifested as a double‐swirl pattern, as expected for ionospheric closure currents extending to lower latitudes. The difference in the equivalent current pattern presumably reflected different distributions of ionospheric conductance. For each storm, it is found that the sequences of ground magnetic disturbances are well approximated as a convolution of external driving and a response function, . This result suggests that the SWMIC current system behaves like an overdamped electric circuit. The e‐folding time is inferred by fitting the convolution to ground magnetic observations, and is estimated at ∼10 min, which is consideredL/Rwith the inductor of the SWMIC current systemL, and the ionospheric resistorR. However, the estimation is more challenging if the variation of solar wind driving is not steplike. Moreover, for low‐latitude magnetic disturbances, the effect of other current systems such as the ring current and equatorial electrojet can significantly modify the response of ground magnetic disturbances. 

Abstract Substorms are known to induce global magnetosphere‐ionosphere coupling. However, the specific response of the dayside ionospheric electric field and its influence on the equatorial electrojet (EEJ) remain controversial. This study investigates the electromagnetic field response in the dayside equatorial region during isolated substorms using ground magnetic field data. Statistical analysis revealed that the H component decreased at equatorial and low‐latitude stations during isolated substorms. These decreases were of similar magnitude on average, indicating that significant changes in the EEJ caused by penetrating electric fields were not observed. However, individual events showed slight positive and negative variations. These results suggest that substorm‐associated electric fields can reach equatorial regions, but additional conditions determine the positive and negative variations. This finding provides new insights into the spatial extent of substorm‐induced electric fields. 

Abstract Dispersionless injection, involving sudden, simultaneous flux enhancements of energetic particles over a broad range of energy, is a characteristic signature of the particles that are experiencing a significant acceleration and/or rapid inward transport process. To provide clues to the physical processes that lead to the acceleration and transport of energetic ions in the dispersionless injection region, we conduct superposed epoch analyses of 75 dispersionless injection events identified by Van Allen Probes with focus on the species‐ and azimuthal angle‐ (φ) dependent signatures of ∼50–600 keV ions inside geosynchronous orbit. Our analysis shows that, on average, the light (hydrogen and helium) ion fluxes undergo a rapid, transient enhancement, while the heavy (oxygen) ion fluxes exhibit a more gradual, persisting enhancement. Such a species‐dependent behavior could be explained in terms of different gyro‐radius of the ion species. For events where the proton injection onset is 30–60 s earlier than the electron one, proton fluxes initially increase at smallφvalues (i.e., tailward guiding centers) and then at largerφvalues (earthward ones). The initial signatures suggest a result of the earthward transport of injected protons, as seen at the explosive growth phase. For events where both electron and proton fluxes increase simultaneously, on the other hand, proton fluxes isotropically increase with no significantφdependence. Such an isotropic proton flux enhancement may imply a local process in which charged protons are rapidly accelerated to higher energies at the spacecraft location. 

Abstract Enhancement of currents in Earth's ionosphere adversely impacts systems and technologies, and one example of extreme enhancement is supersubstorms. Despite the name, whether a supersubstorm is a substorm remains an open question, because studies suggest that unlike substorms, supersubstorms sometimes affect all local times including the dayside. The spectacular May 2024 storm contains signatures of two supersubstorms that occurred successively in time with similar magnitude and duration, and we explore the nature of them by examining the morphology of the auroral electrojet, the corresponding disturbances in the magnetosphere, and the solar wind driving conditions. The results show that the two events exhibit distinctly different features. The first event was characterized by a locally intensified electrojet followed by a rapid expansion in latitude and local time. Auroral observations showed poleward expansion of auroras (or aurorae), and geosynchronous observations showed thickening of the plasma sheet, magnetic field dipolarization, and energetic particle injections. The second event was characterized by an instantaneous intensification of the electrojet over broad latitude and local time. Auroras did not expand but brightened simultaneously across the sky. Radar and LEO observations showed enhancement of the ionospheric electric field. Therefore, the first event is a substorm, whereas the second event is enhancement of general magnetospheric convection driven by a solar wind pressure increase. These results illustrate that the so‐called supersubstorms have more than one type of driver, and that internal instability in the magnetotail and external driving of the solar wind are equally important in driving extreme auroral electrojet activity. 

Abstract In the present study we investigate the response of the dayside ground magnetic field to the sequence of interplanetary magnetic field (IMF)B_Ychanges during the May 2024 geomagnetic storm. We pay particular attention to its extraordinarily large (>120 nT) and abrupt flip, and use GOES‐18 (G18) magnetic field measurements in the dayside magnetosheath as a time reference. In the dayside auroral zone, the northward magnetic component changed by as much as 4,300 nT from negative to positive indicating that the direction of the auroral electrojet changed from westward to eastward. The overall sequence was consistent with the conventional understanding of the IMFB_Ydriving of zonal ionospheric flows and Hall currents, which is also confirmed by a global simulation conducted for this storm. Surprisingly, however, the time delay from G18 to the ground increased significantly in time. The delay was 2–3 min for a sharpB_Yreduction ∼30 min prior to theB_Yflip, but it became as long as 10 min for the zero‐crossing of theB_Yflip. It is suggested that the prolonged time delay reflected the travel time from G18 to the reconnection site, which sensitively depends on the final velocity at the magnetopause, that is, the inflow velocity of the magnetic reconnection. Around theB_Yflip, the solar wind number density transiently exceeded 100 cm⁻³, and should have increased further through the bow shock crossing. It is suggested that this unusually dense plasma reduced the reconnection rate, and therefore, the solar wind‐magnetosphere energy coupling due to the extraordinary IMF. 

Abstract We comprehensively analyzed geomagnetic perturbations using ground magnetic records from over 400 stations spanning four solar cycles, from 1976 to 2023. We assess the perturbations in the three magnetic components separately. Our study covers low, middle, and high magnetic latitudes in the northern magnetic hemisphere, with the primary objective of quantifying extreme values and evaluating their variability on magnetic latitude, local time, and solar cycle phases “minimum, ascending, maximum, and declining.” Our findings reveal spatial patterns to be less discernible as perturbations intensify, with distinct responses at middle and high latitudes. The extreme values, defined as percentiles 0 and 100, were observed to be localized and randomly distributed in local time, especially in the east magnetic component. Additionally, we observed dusk‐dawn asymmetries in the magnitude of perturbations related to the auroral electrojets, indicating complex interactions between the magnetosphere and ionosphere. Furthermore, the results reveal a preference for the most significant extreme values to occur in the declining phase of the solar cycle. These insights deepen our understanding of geomagnetic perturbations and their variability, contributing to space weather forecasting and mitigation strategies. 

Abstract The present study investigates mid‐ and low‐latitude ground magnetic disturbances observed following the arrival of three interplanetary (IP) shocks during the super‐geomagnetic storms of February 1958 and July 1959. One may expect that after IP shocks, the H (northward) magnetic component increases globally but especially on the dayside. However, in each event, the H component was depressed sharply for 1–2 hr in the dawn‐to‐noon sector, whereas it increased in other local time (LT) sectors. Observed magnetic deflections suggest that there existed field‐aligned currents (FACs) flowing into and out of the auroral zone around the western and eastern edges of the LT sector of the dayside H depression. These features strongly suggests that the observed H depression was a remote effect of a R1‐sense FAC system. It was previously reported that similar ground magnetic disturbances were observed after the SSC of the 2003 Halloween storm, which reveals striking similarities to the well‐known H depression observed at Colaba during the 1859 Carrington storm. It is therefore suggested that the external driving behind IP shocks, especially those associated with major storms, is most optimum for the sharp reduction of the dayside H component through the formation and intensification of the dayside FAC system. Associated magnetic disturbances are considered to be larger in magnitude with increasing magnetic latitude, and oriented azimuthally as well as meridionally. Such magnetic disturbances in dayside midlatitudes may not be discussed very often as a target of space weather, but their potential impacts on ground infrastructures probably require closer attention. 

Bursty bulk flows (BBFs) play a significant role in transporting plasma earthward in the magnetotail. While their properties have been extensively studied, their behavior during geomagnetic storms needs further understanding. In this study, we investigate the stormtime characteristics of BBFs, and compare them to non‐stormtime, by performing a superposed epoch analysis using data from ISAS/NASA's Geotail mission. Our results show that the properties of BBFs during stormtime and non‐stormtime remain largely consistent relative to the background plasma sheet conditions. The convection electric field is higher for stormtime BBFs which is primarily associated with an elevated magnetic field in the plasma sheet during storms. Moreover, stormtime plasma sheet conditions, such as an enhanced magnetic field and an elevated ion temperature, are reflected in the properties of BBFs indicating the strong influence of the background plasma environment on BBF dynamics. 

The outflow of low‐energy ions into inner magnetosphere is a key contributor to the warm plasma cloak, and O⁺torus, which can also contribute to storm time ring current particularly after local acceleration. Although outflows are well observed, their source and transport in the inner magnetosphere remain unclear. Using Helium Oxygen Proton and Electron mass spectrometer data from the Van Allen Probes, we systematically examine outflows during geomagnetic storms. We developed an automated event identification method to detect outflows, estimate onset times, and record occurrence details. With this event set, we performed a statistical analysis of occurrence characteristics and storm phase dependence, separately for outflows parallel and anti‐parallel to the magnetic field. Our findings show: (a) outflows are mostly detected off the magnetic equator; (b) with the understanding that the parallel outflows are from the Southern Hemisphere, while anti‐parallel from the Northern Hemisphere, we observed more events in the hemisphere within which the source of O⁺outflow lies; (c) the peak of the occurrence rate in MLT is derived at mid‐night with skewing toward dawn; (d) during the main phase of the storms, outflows events are concentrated at post‐midnight and dawn. Decrease in the occurrence rate is observed in the recovery phase. Hemispheric and dawn‐dusk asymmetries are evident in the event distribution patterns; (e) a preference for outflows from the summer hemisphere. These results offer new insights into nightside, low‐energy outflows. Our large event data set enables us to explore the sources of energization and their transport better. 

During substorms, Earth's magnetotail undergoes rapid dipolarization, driving Earthward plasma flows that decelerate and dissipate energy upon encountering the dipole magnetic field in the nightside transition region. This region mediates the interaction between the magnetotail, inner magnetosphere, and the ionospheric auroral zone, though significant mapping uncertainties obscure the precise link and particle acceleration processes. Using data from THEMIS, TREx, and ELFIN, we analyze a storm‐time substorm on 4 September 2022, establishing a relationship, that is, not a causation, between magnetospheric and ionospheric dynamics. Following a dipolarization, the auroral bulge overlapped with the footprints of the electron isotropy boundary (IB) and the outer radiation belt. Notably, the precipitating electron energies reached at least 2 MeV in the bulge, exceeding previous reports. By comparing the latitudes of the electron IB with respect to the auroral bulge, we deduce that the sources of both auroral and relativistic precipitation were confined in the dipolarized region.