Superposed epoch analysis of relativistic electron flux depletion during moderate CIR storms

Busola  Olugbon; Elijah Oyedola  Oyeyemi; Andrew Oke-Ovie  Akala; Mei-Ching Hannah  Fok

Busola Olugbon Department of Physics, University of Lagos, Lagos, Nigeria
Elijah Oyedola Oyeyemi Department of Physics, University of Lagos, Lagos, Nigeria
Andrew Oke-Ovie Akala Department of Physics, University of Lagos, Lagos, Nigeria
Mei-Ching Hannah Fok Geospace Physics Laboratory, NASA Goddard Space Flight Center, Maryland, USA

Keywords: flux depletions, ionospheric precipitation, superposed-epoch analysis

Abstract

A superposed epoch analysis of radiation belt electron flux depletions during September 2012 - December 2018 identified 46 clear dropout events of duration 24 hours and which extended below L = 5. Of this number, 58% were associated with co-rotating interaction region (CIR) storms. The dropouts associated with CIR storms during SYM/H index - 80 nT intervals were energy dependent with depleted 1.8 MeV fluxes returning to pre-depletion levels after the storm, and true losses occurring for fluxes of higher energy electrons. However, the population of the source electrons (54 keV) increased during the same intervals while the 742 keV electrons population appeared to be driven by competing enhancement and loss processes. The highest depletion levels were recorded for electrons of energy 1.8 MeV at 4.6 L 4.9. A superposed epoch analysis showed that onset of 1.8 MeV flux depletions was triggered by substorm expansion phase and southward orientation of the field-aligned component of interplanetary magnetic field (IMF Bz). On the other hand, recovery of fluxes coincided with substorm recovery and IMF Bz northward reversal. Analysis of cosmic noise absorption data during a selected flux depletion event showed evidence of ionosperic precipitation.