[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

*新井 まどか¹、加藤 雄人¹、片岡 龍峰²、寺本 万里子³、熊本 篤志¹、土屋 史紀⁴、笠羽 康正⁴、三好 由純⁵、笠原 禎也⁶、松田 昇也⁷、篠原 育⁸、山本 和弘⁵、松岡 彩子⁹、堀 智昭⁵、新堀 淳樹⁵、鳥居 祥二¹⁰、赤池 陽水¹⁰ (1.東北大学大学院理学研究科地球物理学専攻、2.国立極地研究所、3.九州工業大学、4.東北大学大学院理学研究科惑星プラズマ・大気研究センター、5.名古屋大学宇宙地球環境研究所、6.金沢大学学術メディア創成センター、7.金沢大学、8.宇宙航空研究開発機構／宇宙科学研究所、9.京都大学理学研究科地磁気世界資料解析センター、10.早稲田大学)

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.