Comparison of repetition period of whistler-mode chorus and relativistic electron precipitation based on conjunction events of Arase and CALET

The dynamic evolution of Earth's radiation belts is an important topic of space weather research to mitigate the possible malfunctions of satellites orbiting, especially at GEO. Relativistic electron precipitation (REP) detected at LEO indicates when, where, and how the loss process of radiation belt electrons takes place. REP is detected as the enhancement of downward electron counts in the MeV energy range. Pitch angle scattering by whistler-mode chorus emissions in the magnetosphere is a plausible mechanism responsible for REP. Previous studies revealed that the repetition periods of REP and chorus are statistically similar, but those in the simultaneous observation of REP and chorus have not been analyzed yet.\\ In this study, we investigated the repetition periods of both REP and chorus based on the conjunction events of the ISS/CALET at LEO and the Arase satellite in the magnetosphere. We defined REP by using the count rates observed with CALET’s CHarge Detector (CHD) as the ratio of CHD-X (upper layer) to CHD-Y (lower layer) count rates $\geq$1.2. The threshold energies to detect the precipitating electrons are 1.6 MeV and 3.6 MeV for CHD-X and CHD-Y, respectively. Chorus appears in the spectra with a hierarchical time scale consisting of several hundred milliseconds, corresponding to the repetition of each chorus element, and a few seconds, corresponding to a group of chorus elements. The same hierarchical time scale can be expected for REP. We used CHD count-rate data with a 1-second time cadence to investigate the fluctuations of REP in seconds. Additionally, we used higher time-resolution count-rate data with a time resolution of up to 20 Hz to investigate the fluctuations of REP in hundred milliseconds. Similarly, we analyzed the repetition periods of chorus emissions using Arase satellite data. We use Arase/WFC data, which measures 64 kHz sampled waveforms over limited time intervals, to analyze the fine-scale structure of individual chorus elements and compare them with the repetition period of REP on a millisecond time scale. The present study reveals that the repetition periods of REP occurred in a time scale corresponding to the chorus elements observed in the magnetosphere.