Different Behaviors of Outer Radiation Belt keV and MeV Electrons During Two Sequential Geomagnetic Storms

Electron flux dropout is an extraordinary phenomenon in the outer radiation belt, which is characterized by a drastic depletion in trapped electron populations over a broad range of energies, pitch angles, and L-shells. Using data from multiple satellites, we study a 5-day dropout of MeV electrons and a several-hour dropout of keV electrons in geostationary Earth orbit (GEO) during two sequential geomagnetic storms. Outward radial diffusion caused real loss of MeV electrons, while asymmetric and depressed magnetic fields led to adiabatic variations in keV electron fluxes. In addition to time scale differences, low-energy keV, medium-energy keV and high-energy MeV electron fluxes behaved differently, involving an increasing trend, an initial decreasing trend followed by an increasing trend, and a decreasing trend, respectively, as observed by Radiation Belt Storm Probes (RBSPs) and low-altitude satellites during the first storm. Substorm injections accompanied by inward radial diffusion initially produced an enhanced flux of electrons with energies up to similar to 100 keV. Chorus waves generated in morning side of magnetosphere were responsible for scattering part of this seed population into the atmosphere. In turn, these enhanced but short-lived chorus waves accelerated lower-energy keV electrons to achieve a moderate energy level of hundreds of keV within one day. Additionally, the initial slow outward radial diffusion was associated with MeV electron loss, and there was insufficient time for acceleration by chorus waves, so only MeV electrons could maintain long-term dropout. Competition between energy- and L-shell-dependent acceleration and loss mechanisms caused different keV and MeV electron behaviors.