Arase Observations of the Radiation Belts During the May 2024 Geomagnetic Storm

During the largest geomagnetic storm of Solar Cycle 25 that happened in May 2024, the Arase satellite has successfully operated all instruments and observed a number of remarkable phenomena in the inner magnetosphere and radiation belts. Due to significant compression of the magnetosphere caused by an interplanetary shock, the satellite's apogee was located outside the magnetosphere, indicating a substantial degree of compression. Following the main phase of the storm, a rapid flux increase of high-energy electrons with energies of several MeV was observed in the region of L < 3. This was the largest flux increase event recorded ever since the launch of the Arase satellite.

Additionally, the plasmasphere contracted inward more than usual, with the plasmapause reaching L ~ 2. The enhanced flux of high-energy electrons at L < 3 persisted for an extended period of 10 to 30 days or even more, significantly changing the radiation environment near the Earth. Analyzing observation data from the Arase satellite, we estimated the decay time constant of the electrons and compared it with the rates of pitch-angle scattering  caused by various plasma waves, such as hiss waves, VLF transmitters, and lightning whistlers. The results suggest that continuous scattering driven by plasmaspheric hiss primarily governs the decay of those high-energy electrons. In this presentation, we report on the variations in the radiation belts and inner magnetosphere observed by the Arase satellite during this historic geomagnetic storm.