Relativistic electron flux growth during storm and non-storm periods as observed by ARASE and GOES satellites

Abstract Variations of relativistic electron fluxes and wave activity in the Earth magnetosphere are studied in order to determine the contribution of different acceleration mechanisms of the outer radiation belt electrons: ULF mechanism, VLF mechanism, and adiabatic acceleration. The electron fluxes were measured by Arase satellite and geostationary GOES satellites. The ULF power index is used to characterize the wave activity of the magnetosphere in the Pc5 range. To characterize VLF wave activity in the magnetosphere, we use data from Arase satellite (PWE instrument). We consider strongest magnetic storms during the Arase satellite era: May 27-29, 2017; September 7-10, 2017; and August 25-28, 2018. Also, non-storm intervals with a high solar wind speed before and after these storms are considered as well. The magnitudes of relativistic electron fluxes during these magnetic storms (with an average solar wind speed) are found to be greater than that during non-storm intervals with a high solar wind streams. The substorm activity, as characterized AE index, is found to be a necessary condition for the increase of relativistic electron fluxes, whereas a high solar wind speed alone is not always necessary for the relativistic electron growth. The enhancement of relativistic electron fluxes by 1.5-2 orders of magnitude is observed 1-3 days after the ULF index growth and growth of the VLF radiation power. The analysis shows that the growth of VLF and ULF wave activity occurs approximately at the same time and coincides with the growth of substorm activity. Therefore, it is not easy to separate the contribution of these acceleration mechanisms over time. These mechanisms can act on a first phase of electron acceleration. During magnetic storms, the flux intensity maximum shifts to lower L-shells compared to intervals without magnetic storms. The acceleration mechanism associated with the injection of electrons into the region of the magnetic field weakened by the ring current and their subsequent betatron acceleration during the restoration of the magnetic field can work effectively at the second phase of electron acceleration.