Lightning‐induced energetic electron flux enhancements in the drift loss cone

Abstract

With its relatively low altitude (520 × 670 km) orbit, SAMPEX is mostly below the stable trapping region where charged particles repeatedly drift around the Earth, especially at midlatitudes. Recent analyses of SAMPEX data have revealed a surprisingly common set of observations of enhanced energetic (>150 keV) electron fluxes at L < 3, during times when SAMPEX was located such that any electrons that it observed were in the drift loss cone (and were thus destined to be precipitated upon reaching the longitude of the South Atlantic Magnetic Anomaly in the course of their eastward drift). The data were acquired with the Heavy Ion Large Telescope on board SAMPEX, which provides high time resolution measurements (30 ms sample rate) of electrons above energy thresholds of 150 keV and 1 MeV. Preliminary examination of 1995 SAMPEX data (when the >150 keV detector was available) revealed hundreds of cases of newly enhanced drift loss cone fluxes in localized L shell regions often associated with individual thunderstorms. In one case, SAMPEX data from three consecutive days (95/196, 95/197, and 95/198) were analyzed as the satellite proceeded northbound over the same ground track from west of New Zealand and Hawaii toward Alaska, with the underlying lightning activity documented by the Optical Transient Detector on board the OrbView‐1 satellite (750 km, 70° inclination circular orbit). Enhanced fluxes observed on SAMPEX during day 95/197 were directly associated with an oceanic storm just to the west of the SAMPEX ground track, which was well placed to generate the observed drift loss cone flux enhancements. The drift loss cone electron flux enhancements were also observed 20 min later as SAMPEX crossed the same L shells in the north and in subsequent orbits, indicating that lightning‐induced precipitation of electrons into the drift loss cone persisted at least for a few hours. Data from UARS satellite, passing through the same region within the same hour, also confirmed the presence of L‐dependent structure and further allowed the determination of the electron energy spectra, which exhibited a general shape and range strikingly similar to previously documented spectral characteristics of lightning‐induced electron precipitation (LEP) events in the bounce loss cone [Voss et al., 1998]. This similarity lends support to the argument that the observed drift loss cone features are produced by the LEP process. In summary, SAMPEX data indicate that globally distributed thunderstorms may continually precipitate energetic electrons from the radiation belts, producing transient enhancements in the drift loss cone that are detected within the few hour periods as they drift around the Earth and precipitate in the South Atlantic Magnetic Anomaly.