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Abstract High‐intensity long‐duration continuous auroral electrojet (AE) activity (HILDCAA) events are associated with intensification of relativistic electron fluxes in the inner magnetosphere. The physical mechanisms of this intensification are not well established yet. We study observations by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in the near earth plasma sheet at radial distances of 10 Earth radii, at the transition region between tail and dipole‐like magnetic configurations, referred to as the nightside transition region (NTR), during a HILDCAA event. The observations revealed recurrent dipolarizations accompanied by plasma flow vortices, impulsive electric field enhancements, and increases in electron fluxes at energies of 100 keV up to 1 MeV. Electron pitch angle (PA) distributions at THEMIS showed field‐aligned flux enhancements at energies of 100 keV. This indicates a Fermi‐type energization. Arguably, electrons gain energy up to MeV via repetitive bouncing through the acceleration region. Energization of ions was insignificant which led to 1. We suggest that the increased ratio leads to a local increase of the Hall conductivity in the conjugate ionosphere, which causes ionospheric current intensification and strong , consistent with observations. 

Abstract This study contributes to improved space weather forecasting by investigating geomagnetic disturbances (GMDs) associated with rapid decreases in the SuperMAG Auroral Electrojet Lower (SML) index during strong substorms (SML  −500 nT) at high latitudes (64–75 MLAT). Through integrated analysis of two selected strong substorm events from December 2015, combining THEMIS and GOES‐13 spacecraft observations with ground magnetometer data from Arctic Canada and Spherical Elementary Current System (SECS) modeling, we reveal key relationships: (a) the SML drops during substorm expansion phase coincide with GMD onset and subsequent poleward propagation (64–75 MLAT); (b) SECS‐derived current patterns show intensified horizontal ionospheric currents (200 mA/m) and localized vertical current density (8–10 A/) that align spatially and temporally with the largest GMDs (reaching 25 nT/s) and (c) rapid SML decreases during substorm expansion phases correlate with dipolarization and earthward fast flows as observed by THEMIS‐A, E in the Earth's magnetotail and near‐Earth magnetic field reconfigurations with enhanced particle fluxes (GOES‐13). The horizontal current enhancements propagate coherently with the GMDs, while the SECS analysis reveals a structured vertical current density that connects magnetospheric drivers to ground‐level disturbances. These observations demonstrate a complete chain of processes from magnetotail reconfiguration to ionospheric current intensification and subsequent GMD development—a progression not typically observed in weaker substorms (SML  −500 nT). Our multi‐instrument approach provides a detailed characterization of substorm‐GMD coupling, identifying key parameters that could contribute to improved forecasting. 

A circuit analogy for magnetosphere-ionosphere current systems has two extremes for drivers of ionospheric currents: the “voltage generator” (ionospheric electric fields/voltages are constant, while current varies) and the “current generator” (current is constant, while the electric field varies). Here we indicate another aspect of the magnetosphere-ionosphere interaction, which should be taken into account when considering the current/voltage dichotomy. We show that nonsteady field-aligned currents interact with the ionosphere in a different way depending on a forced driving or resonant excitation. A quasi-DC driving of field-aligned current corresponds to a voltage generator, when the ground magnetic response is proportional to the ionospheric Hall conductance. The excitation of resonant field line oscillations corresponds to the current generator, when the ground magnetic response only weakly depends on the ionospheric conductance. According to the suggested conception, quasi-DC nonresonant disturbances correspond to a voltage generator. Such ultralow frequency (ULF) phenomena as traveling convection vortices and Pc5 waves should be considered as the resonant response of magnetospheric field lines, and they correspond to a current generator. However, there are quite a few factors that may obscure the determination of the current/voltage dichotomy.