[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-P02] Repetition period of whistler-mode chorus and relativistic electron precipitation in conjunction events of Arase and ISS/CALET

*Madoka Arai¹, Yuto Katoh¹, Ryuho Kataoka², Mariko Teramoto³, Atsushi Kumamoto¹, Fuminori Tsuchiya⁴, Yasumasa Kasaba⁴, Yoshizumi Miyoshi⁵, Yoshiya Kasahara⁶, Shoya Matsuda⁷, Iku Shinohara⁸, Kazuhiro Yamamoto⁵, Ayako Matsuoka⁹, Tomoaki Hori⁵, Atsuki Shinbori⁵, Shoji Torii¹⁰, Yosui Akaike¹⁰ (1.Department of Geophysics, Graduate School of Science, Tohoku University, 2.National Institute of Polar Research, 3.Kyushu Institute of Technology, 4.Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, 5.Institute for Space-Earth Environmental Research, Nagoya University, 6.Emerging Media Initiative, Kanazawa University, 7.Kanazawa University, 8.Japan Aerospace Exploration Agency/Institute of Space and Astronautical Science, 9.Data Analysis Center for Geomagnetism and Space Magnetism, Graduate School of Science, Kyoto University, 10.Waseda University)

Relativistic electron precipitation (REP) is the enhancement of downward electron counts in the MeV energy range detected at the LEO orbit. Pitch angle scattering by whistler-mode chorus emissions is a plausible mechanism responsible for REP. Kandar et al. (2023) statistically analyzed the distribution of repetition periods of REP, where the repetition period refers to time spacing between REP. They showed that the results were consistent with the statistical distribution of repetition periods of rising tone chorus elements reported by Shue et al. (2015). Chorus emissions are also responsible for the precipitation of electrons in the keV energy range, contributing to diffuse/pulsating aurora. Ozaki et al. (2018) indicated that the successive generation of chorus elements on a time scale of several hundred milliseconds and the rapid intensity modulation of pulsating aurora on a time scale of less than one second, which is referred to as the 3 Hz modulation, were observed simultaneously. Furthermore, previous studies have revealed that the successive generation of chorus groups (clusters of chorus elements) on a time scale of several seconds is similar to the temporal variations of pulsating aurora, with both exhibiting periodicity on a time scale of a few seconds and accompanying the 3 Hz modulation. Such hierarchical structures in the time scales of chorus and pulsating aurora suggest that REP also possesses a similar hierarchical property.
In this study, we investigated the repetition periods of both REP and chorus observed during conjunction events of the ISS/CALET and the Arase satellite to identify their hierarchical correspondence as observed in the relationship between pulsating aurora and chorus. We defined the criteria of the conjunction between ISS and Arase that the Arase footprint was within ±2.5 degrees of latitude and within ±15 degrees of longitude from ISS when REP was detected. Among 103 conjunction events identified in the observation data from February 2017 to December 2023, we focus on an event on May 12, 2021, in which the Arase satellite observed whistler-mode waves at -9 to -13 degrees of the magnetic latitude, L = 4.09-4.66, and 0-1 MLT ranges. 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 ≧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. Based on the detection criteria for REP developed by O'Brien et al. (2003), we determined the repetition periods of REP. Similarly, we calculated the repetition periods of the chorus group using Arase/PWE data, which is available continuously in time with a time resolution of 1 sec, and those of individual chorus elements using Arase/WFC data, which produces 64 kHz sampled waveform in the limited time interval. The present study reveals that REP occurred in a time scale corresponding to the group of chorus emissions observed in the magnetosphere.