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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. 

Ground-based magnetometers used to measure magnetic fields on the Earth’s surface (B) have played a central role in the development of Heliophysics research for more than a century. These versatile instruments have been adapted to study everything from polar cap dynamics to the equatorial electrojet, from solar wind-magnetosphere-ionosphere coupling to real-time monitoring of space weather impacts on power grids. Due to their low costs and relatively straightforward operational procedures, these instruments have been deployed in large numbers in support of Heliophysics education and citizen science activities. They are also widely used in Heliophysics research internationally and more broadly in the geosciences, lending themselves to international and interdisciplinary collaborations; for example, ground-based electrometers collocated with magnetometers provide important information on the inductive coupling of external magnetic fields to the Earth’s interior through the induced electric field (E). The purpose of this white paper is to (1) summarize present ground-based magnetometer infrastructure, with a focus on US-based activities, (2) summarize research that is needed to improve our understanding of the causes and consequences of B variations, (3) describe the infrastructure and policies needed to support this research and improve space weather models and nowcasts/forecasts. We emphasize a strategic shift to proactively identify operational efficiencies and engage all stakeholders who need B and E to work together to intelligently design new coverage and instrumentation requirements. 

Abstract To understand magnetosphere‐ionosphere conditions that result in thermal emission velocity enhancement (STEVE) and subauroral ion drifts (SAID) during the substorm recovery phase, we present substorm aurora, particle injection, and current systems during two STEVE events. Those events are compared to substorm events with similar strength but without STEVE. We found that the substorm surge and intense upward currents for the events with STEVE reach the dusk, while those for the non‐STEVE substorms are localized around midnight. The Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite observations show that location of particle injection and fast plasma sheet flows for the STEVE events also shifts duskward. Electron injection is stronger and ion injection is weaker for the STEVE events compared to the non‐STEVE events. SAID are measured by Super Dual Auroral Radar Network during the STEVE events, but the non‐STEVE events only showed latitudinally wide subauroral polarization streams without SAID. To interpret the observations, Rice Convection Model (RCM) simulations with injection at premidnight and midnight have been conducted. The simulations successfully explain the stronger electron injection, weaker ion injection, and formation of SAID for injection at premidnight, because injected electrons reach the premidnight inner magnetosphere and form a narrower separation between the ion and electron inner boundaries. We suggest that substorms and particle injections extending far duskward away from midnight offer a condition for creating STEVE and SAID due to stronger electron injection to premidnight. The THEMIS all‐sky imager network identified the east‐west length of the STEVE arc to be ~1900 km (~2.5 h magnetic local time) and the duration to be 1–1.5 h. 

Abstract The extreme substorm event on 5 April 2010 (THEMIS AL = −2,700 nT, called supersubstorm) was investigated to examine its driving processes, the aurora current system responsible for the supersubstorm, and the magnetosphere‐ionosphere‐thermosphere (M‐I‐T) responses. An interplanetary shock created shock aurora, but the shock was not a direct driver of the supersubstorm onset. Instead, the shock with a large southward IMF strengthened the growth phase with substantially larger ionosphere currents, more rapid equatorward motion of the auroral oval, larger ionosphere conductance, and more elevated magnetotail pressure than those for the growth phase of classical substorms. The auroral brightening at the supersubstorm onset was small, but the expansion phase had multistep enhancements of unusually large auroral brightenings and electrojets. The largest activity was an extremely large poleward boundary intensification (PBI) and subsequent auroral streamer, which started ~20 min after the substorm auroral onset during a steady southward IMFB_zand elevated dynamic pressure. Those were associated with a substorm current wedge (SCW), plasma sheet flow, relativistic particle injection and precipitation down to the D‐region, total electron content (TEC), conductance, and neutral wind in the thermosphere, all of which were unusually large compared to classical substorms. The SCW did not extend over the entire nightside auroral activity but was localized azimuthally to a few 100 km in the ionosphere around the PBI and streamer. These results reveal the importance of localized magnetotail reconnection for releasing large energy accumulation that can affect geosynchronous satellites and produce the extreme M‐I‐T responses.