SAMPEX observations of storm‐associated electron flux variations in the outer radiation belt

Abstract

Flux variations of the outer radiation belt electrons (> 1‐MeV) during the main phase and early recovery phase of 25 geomagnetic storms are studied using data obtained by the Heavy Ion Large Telescope (HILT) experiment onboard the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite. Employing a simple model for the ring current field, we examine the degree to which the decrease of electron flux during the early main phase is attributable to adiabatic deceleration processes in response to changes in the magnetic field. Such an adiabatic response is shown to be detected most clearly for 4 < L < 5. In the lower L region (2 < L < 4) the electron flux decrease is less prominent and at times increases during the main phase of intense storms. On the other hand, in the region 5 < L < 7 the level of the electron decrease is larger than that expected from the adiabatic response alone. These observations suggest that the energetic electrons are trapped effectively near the inner edge of the outer radiation belt probably because of sudden inward transport and acceleration of the electrons during the main phase. The reduced flux of electrons returns to the normal level during the early recovery phase, even exceeding the prestorm level after about 1–2 days for intense storms. An outward diffusion process of the electrons at the inner edge, which are trapped during the main phase, could at least account partly for this observation. The low‐altitude observation of precipitating electrons supports the recirculation model for radiation belt electron dynamics during magnetic storms.