Particle and field characteristics of broadband electrons observed by the FAST satellite during a geomagnetic storm

Abstract

Broadband electrons (BBEs) are remarkable flux enhancements (>1013 eV cm−2 s−1) of precipitating electrons over a broad energy range (0.03–30 keV) near the equatorward edge of the auroral oval during geomagnetic storms. We show characteristics of particles (energy spectra and pitch angle distribution) and fields (electric field, magnetic field, and wave spectra) during a BBE event observed by the Fast Auroral SnapshoT (FAST) satellite. The BBEs were observed at an altitude of ∼2000 km at 59°–61° invariant latitudes (ILATs) and 21 h magnetic local time (MLT). The event was observed at ∼7 min after the onset of a substorm during the main phase of the Bastille Day geomagnetic storm (minimum Dst = −301 nT) on 15 July 2000. The precipitation region of the BBEs corresponded to a localized intensification of auroral emission, lasting ∼14 min, observed by the Polar UVI images at 50°–60° geomagnetic latitudes (MLATs) and 20–21 MLTs. These results suggest that rapid particle acceleration was occurring in the inner magnetosphere associated with a storm‐time substorm. The pitch angle distribution of BBEs was isotropic except for a loss cone feature around the field‐aligned upward direction at a higher energy range above ∼1 keV, while field‐aligned electron fluxes were larger than the perpendicular fluxes below ∼1 keV. These results imply that a higher energy part of the BBEs originated from higher altitudes in the inner magnetosphere and that a lower energy part was accelerated parallel to the local magnetic field at lower altitudes near the satellite. Intense fluctuations of electric and magnetic fields were observed during this BBE event. From these results, we discuss possible acceleration of the lower energy part of BBEs through wave–particle interaction.