Magnetic Dipolarizations and Energetic Electron Flux Variations at the Nightside Geostationary Orbit

Abstract

We consider energetic electron (EE, 30–600 keV) flux variations, connected with magnetic dipolarizations, at geosynchronous orbit. Two types of these variations, due to injections and drift shell crossing (DSC), have been known since the 1960s, but no methods to separate them were suggested. To reach this goal we apply the hodogram Je(Bz) technique (EE flux vs. local magnetic field) to data of three spacecraft, GOES-15 observing a sharp dipolarization and thus being in the injection region, and GOES-13, GOES-14, being inside the drifting electron cloud (DEC). Analysis of 45 sharp dipolarizations and corresponding EE flux variations in 2013–2017 showed that hodograms look quite different (a) inside and outside of the injection region and (b) for medium (30–200 keV) and high (200–600 keV) energies, allowing to identify the variation type as either DSC or true injection. EE fluxes and energy spectra were compared inside the injection region and in the DEC 4 hours eastward. EE fluxes are well correlated before the injection, but strongly scattered at its peak suggesting a highly structured drifting injected cloud. Spectra at the injection peak appeared similar, demonstrating hardening with increased geomagnetic activity. The best correlations between different injection and dipolarization measures are observed between peak Je values and ∆(MPB)^1/2, where MPB is the ground mid-latitude positive bay index; correlations of approximately 0.7 are attained inside the injection region, being lower in the DEC. The peak injected EE flux increases by ∼1–1.5 orders of magnitude with increase in dipolarization strength from low to high geomagnetic activity.