Particle and field characteristics of broadband electrons observed by the FAST satellite during geomagnetic storms: A multievent study

Abstract

During geomagnetic storms, remarkable electron‐flux enhancements (>1012 eV cm−2 s−1) over a broadband energy range (∼0.05–10 keV) are sometimes observed near the equatorward edge of the auroral oval. We call such electron flux enhancements broadband electrons (BBEs). In this paper, we identified 12 BBE events from the electron‐energy spectra obtained by the Fast Auroral Snapshot (FAST) satellite during 81 large geomagnetic storms (minimum Dst index <−90 nT) between September 1996 and March 2004. Ground‐based magnetic field data show that the BBEs are observed ∼6–28 min after the onset of substorms during the main phase of large storms. During these events, the pitch angle distributions of electrons show isotropic features at a higher energy range above ∼1 keV, except for an upward loss cone. At a lower energy range below ∼1 keV, field‐aligned downward electron fluxes are most intense, and perpendicular fluxes are weakest. These results imply that a higher‐energy part of BBEs originates from higher altitudes in the inner magnetosphere and that a lower‐energy part is accelerated parallel to the local magnetic field at lower altitudes near the satellite. Intense fluctuations of electric and magnetic fields and enhanced lower‐frequency (0.1 Hz to 16 kHz) waves are observed during the BBEs. These characteristics are common for all BBE events. We compared lower‐energy (<1 keV) electron fluxes with field‐aligned Poynting fluxes of the waves at 0.06–4 Hz during the BBEs. The Poynting fluxes are systematically downward with intensity comparable to that of the lower‐energy electron fluxes, suggesting downward acceleration of lower‐energy electrons by the observed low‐frequency waves.