Electron dropout events and flux enhancements associated with geomagnetic storms observed by PROBA-V/EPT from 2013 to 2019

Abstract:

Electron flux variations for E>500 keV during geomagnetic storms are investigated using the Energetic Particle Telescope (EPT). This detector launched in May 2013 on board the satellite PROBA-V at an altitude of 820 km was designed to provide uncontaminated spectra of electrons, protons and alpha particles. Electron flux dropout events are observed during the main phase of each storm and even during substorms: a rapid reduction of the electron flux is noted throughout the outer electron radiation belt at all energies above about 0.5 MeV on timescales of a few hours. The electron spectrograms measured by the EPT between 2013 and 2019 show that after each geomagnetic storm, dropout events are followed by a flux enhancement starting first at low L values, and reaching the slot or even the inner belt for the strongest storms. We determine the link between Disturbed Storm Time (Dst) and the minimum value of the L-shell where the dropouts deplete the outer belt, as well as the non-linear relation between Dst and the minimum L-shell where the flux penetrates in the slot region or even the inner belt during the storms. Dropouts appear at all energies measured by EPT and penetrate down to L~3.5 for the strongest events. Dropouts are observed at LEO each time Dst has an inverted peak < -40 nT. Flux enhancements appear at lower L only for big storm events with Dst < -50 nT. They penetrate down to an impenetrable barrier with a minimum L-shell related to Dst and to the energy. For E > 1 MeV, this limit is also linked to the plasmapause position.