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In the present study we examine three substorm events, Events 1-3, focusing on the spatio-temporal development of auroral electrojets (AEJs) before auroral breakup. In Events 1 and 2, auroral breakup was preceded by the equatorward motion of an auroral form, and the ground magnetic field changed northward and southward in the west and east of the expected equatorward flow, respectively. Provided that these magnetic disturbances were caused by local ionospheric Hall currents, this feature suggests that the equatorward flow turned both eastward and westward as it reached the equatorward part of the auroral oval. The auroral breakup took place at the eastward-turning and westward-turning branches in Events 1 and 2, respectively, and after the auroral breakup, the westward AEJ enhanced only on the same side of the flow demarcation meridian. The zonal flow divergence is considered as an ionospheric manifestation of the braking of an earthward flow burst in the near-Earth plasma sheet and subsequent dawnward and duskward turning. Therefore, in Events 1 and 2, the auroral breakup presumably mapped to the dawnward and duskward flow branches, respectively. Moreover, for Event 3, we do not find any pre-onset auroral or magnetic features that can be associated with an equatorward flow. These findings suggest that the braking of a pre-onset earthward flow burst itself is not the direct cause of substorm onset, and therefore, the wedge current system that forms at substorm onset is distinct from the one that is considered to form as a consequence of the flow braking.

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. Plain Language Summary: The study aims to investigate the occurrence of geomagnetic perturbations across different regions of the Earth by analyzing data from the SuperMAG collaboration. The network records changes in Earth's magnetic field, and the study endeavors to determine the extent to which these disturbances vary across different parts of the Earth. We focused on low, middle, and high latitudes in the northern hemisphere to comprehensively understand the perturbations during their intensification. The findings of the study suggest that the behavior of the perturbations varies significantly across different regions. We observed that magnetic perturbations in the auroral region intensify more in the morning compared to the evening, indicating the dynamic interplay between Earth's magnetic field and charged particles from space. Additionally, the study discovered that as the disturbances grew stronger, they did not follow the same patterns. Instead, the disruptions varied depending on the location on Earth. Moreover, the perturbations seem to intensify more during the declining solar phase. This knowledge is crucial in predicting space weather, especially in protecting satellites and power grids from the effects of space storms. Key Points: We utilized magnetic records from about 400 stations spanning four solar cycles between 1976 and 2023 across three magnetic componentsThe strongest perturbations were found in the north magnetic component associated with the dawnside westward auroral electrojetThe weakest and most significant extreme values of perturbations were observed during ascending and declining solar phases, respectively [ABSTRACT FROM AUTHOR]

During geomagnetic storms, the increase in energy input into the ionosphere in the form of Poynting flux and electron precipitation leads to an enhanced ionospheric outflow that results in an increase of the O+ content in the magnetosphere. Using different missions and instrumentation, two main ionospheric sources have been identified for the oxygen ions reaching the inner magnetosphere during geomagnetic storms: the dayside cusp, and the night side auroral region. Evidence of both pathways have been presented in the literature. However, the relative contribution of each of these pathways to the enhancement of O+ observed in near‐Earth plasma sheet, as well as the dynamics involved during the development of geomagnetic storms remains an open question. Here, we present the first statistical study to date to address this question, in the form of a superposed epoch analysis of O+ and H+ moments obtained by the Magnetospheric Multiscale (MMS) mission throughout the main phase of 90 geomagnetic storms with a minimum SYM‐H of at least −50 nT. The results show a clear increase in the oxygen density in the near‐Earth plasma sheet, with values further from Earth remaining low. Temperature values for both species show an increase with the progress of the storms. These results combined suggest that, during the main phase of geomagnetic storms, most of the oxygen ions observed in the near‐Earth plasma sheet are traveling directly from the nightside auroral region. Key Points: Superposed epoch analysis shows dynamic evolution of O+ ions during geomagnetic stormsSignificant enhancement of the O+‐to‐H+ number density ratio observed at radial distances less than 12 REScalar temperature for both species show the arrival of heated ions far from Earth throughout the analyzed period [ABSTRACT FROM AUTHOR]

A characteristic feature of the main phase of geomagnetic storms is the dawn‐dusk asymmetric depression of low‐ and mid‐latitude ground magnetic fields, with largest depression in the dusk sector. Recent work has shown, using data taken from hundreds of storms, that this dawn‐dusk asymmetry is strongly correlated with enhancements of the dawnside westward electrojet and this has been interpreted as a "dawnside current wedge" (DCW). Its ubiquity suggests it is an important aspect of stormtime magnetosphere‐ionosphere (MI) coupling. In this work we simulate a moderate geomagnetic storm to investigate the mechanisms that give rise to the formation of the DCW. Using synthetic SuperMAG indices we show that the model reproduces the observed phenomenology of the DCW, namely the correlation between asymmetry in the low‐latitude ground perturbation and the dawnside high‐latitude ground perturbation. We further show that these periods are characterized by the penetration of mesoscale bursty bulk flows (BBFs) into the dawnside inner magnetosphere. In the context of this event we find that the development of the asymmetric ring current, which inflates the dusk‐side magnetotail, leads to asymmetric reconnection and dawnward‐biased flow bursts. This results in an eastward expansion and multiscale enhancement of the dawnside electrojet. The electrojet enhancement extends across the dawn quadrant with localized enhancements associated with the wedgelet current systems of the penetrating BBFs. Finally, we connect this work with recent studies that have shown rapid, localized ground variability on the dawnside which can lead to hazardous geomagnetically induced currents. Plain Language Summary: During geomagnetic storms, electric currents in space can have a dramatic effect on the magnetic field on the ground, causing so‐called geomagnetic disturbances (GMDs). Storm‐time GMDs exhibit a lopsided asymmetry: dusk‐biased near the equator and dawn‐biased at high latitudes where aurora usually occur. This asymmetry has been interpreted as a giant wedge‐like current system, a dawnside current wedge (DCW). Using a high‐resolution supercomputer model, we successfully reproduced the DCW and showed that it occurred during a period of intense, localized flow bursts, akin to bubbles, on the nightside of near‐Earth space. The bubbles' buoyancy propels them from the nightside inwards toward dawn, driving intense currents into the Earth's atmosphere. Our simulations suggest that the causal agent of these dawnside bubbles is magnetic reconnection, typically symmetric but skewed dawnward due to asymmetry in the ring current, a crescent‐shaped population of energetic ions in space which intensifies during geomagnetic storms. Understanding the cause of stormtime GMD asymmetry is not only important to characterize how electric currents bind the magnetosphere and upper atmosphere, but also to mitigate space weather hazards, as intense GMDs can disrupt and damage power systems on Earth. Key Points: Global model reproduces correlation between ring current asymmetry and dawnside electrojet inferred from hundreds of geomagnetic stormsAnalysis of the model reveals a dawnside current wedge mediated by mesoscale flow bursts and driven by an asymmetric substorm‐like processModel reveals multiscale enhancement of dawnside electrojet with space weather implications due to rapid, localized ground variability [ABSTRACT FROM AUTHOR]