A multievent study of broadband electrons observed by the DMSP satellites and their relation to red aurora observed at midlatitude stations

Abstract

Broadband electrons during magnetic storms are characterized by an unusually intense flux of precipitating electrons in the broadband energy range from 30 eV to 30 keV near the equatorward edge of the auroral oval (47°–66° magnetic latitude). Broadband electrons were first reported by Shiokawa et al. [1996]. In this paper, we report a multievent study of broadband electrons, using particle data obtained by the Defense Meteorological Satellite Program (DMSP) satellites during 23 magnetic storms from January 1989 through May 1992. Twelve broadband electron events are identified. Most of them are observed in the night sector, but some are observed in the morning sector. Particle data for successive polar passes of the DMSP multisatellites are used to show that broadband electrons generally last for less than 30 min and that for some events, they precipitate over a wide range of local times simultaneously. On the basis of a quantitative calculation of optical emissions from electrons in the neutral atmosphere, we conclude that broadband electrons are a possible cause of red auroras observed at midlatitude ground stations. We suggest that broadband electrons are associated with certain substorms during the main phase of magnetic storms. This conjecture comes from observations of H component positive bays and Pi 2 pulsations observed at low‐latitude magnetic stations and from magnetic field variations observed at geosynchronous satellites. We conclude that the magnetospheric source of broadband electrons lies within the inner part of the plasma sheet. This conclusion is based on the facts that broadband electrons appear in latitudes where plasma sheet particles were observed before the event and that broadband electrons are observed poleward of the subauroral ion drifts, a position that corresponds to the inner edge of the injected particle layer during storms. High‐energy particle data obtained at geosynchronous satellites show that both strong magnetopause compressions in the dayside and decrease of the particle fluxes in the nightside occur in association with the broadband electron events. Possible mechanisms of broadband electron production are discussed.