THEMIS Observations of Relativistic Electrons at the Nightside Transition Region During HILDCAA Events

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 $\sim$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 $\sim$MeV via repetitive bouncing through the acceleration region. Energization of ions was insignificant which led to $T_i/T_e\approx$ 1. We suggest that the increased $T_e^{1/2}/n_e$ ratio leads to a local increase of the Hall conductivity in the conjugate ionosphere, which causes ionospheric current intensification and strong $dB/dt$, consistent with observations.