A Statistical Survey of Microburst Precipitation based on FIREBIRD-II CubeSat Measurements
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摘要: 微爆型电子沉降是指辐射带高能电子呈现脉冲式沉降,持续时间通常小于1秒,是地球辐射带高能电子损失的重要机制,通常认为是由哨声模合声波散射导致的。系统研究微爆型电子沉降,对于理解磁层空间波粒相互作用过程、揭示地球辐射带动态变化规律、量化磁层-电离层物质和能量输运均具有重要意义。本文基于FIREBIRD-II卫星计划中FU3卫星(2015年2月1日至2019年10月7日)和FU4卫星(2015年2月1日至2022年3月16日)的观测数据,统计分析了200–900 keV电子的微爆型沉降事件对磁鞘值(L)、磁地方时(MLT)、地理经纬度、地磁活动水平的依赖性。结果表明微爆型电子沉降事件主要分布在晨侧的外辐射带,发生率峰值出现在L=6-7,MLT=6-7范围。发生率随电子能量的增加而降低,265.4 keV电子的发生率峰值可以达到3‰,而481.2 keV能级电子的发生率峰值降低到约1‰。同时还发现微爆型电子沉降事件对地磁活动水平有较强依赖性,地磁活动越剧烈发生率越高,265.4 keV电子平静时期发生率约为2‰,地磁活动期间最高可以达到6‰。发生率峰值随着地磁活动加剧向低L值移动,可能与磁暴时期等离子体层层顶位置向地球侧移动有关。统计结果显示微爆型电子沉降事件发生率的全球分布与哨声模合声波波幅的全球分布具有较强相关性,表明哨声模合声波可能是能量小于MeV电子微爆型沉降的重要驱动机制,这为合声波对数百keV电子的散射效应补充了关键的统计观测证据。Abstract: Microbursts are pulsed precipitation events of energetic electrons from the outer radiation belt, characterized by durations less than 1s. This phenomenon is a crucial loss mechanism for energetic electrons in Earth's radiation belts and is commonly thought to be driven by whistler-mode chorus waves. A comprehensive investigation of energetic electron precipitation is of great significance for better understanding of the Earth's radiation belts dynamic and space weather forecasting. In this study, using the data from FIREBIRD-II FU3 satellite from 1 February 2015 to 7 October 2019 and FU4 satellite from 1 February 2015 to 16 March 2022, we statistically analyze the dependence of microburst events (at energies of 200 – 900 keV) on L values, MLTs, geographic distributions, and geomagnetic activity levels. The results show that microburst events mostly occurred in the outer radiation belt at the dawn side, with peak occurrence rate observed at L=6-7 and MLT=6-7. The occurrence rates of microburst events decrease with increasing electron energies, reaching approximately 3‰ for 265.4 keV electrons and 1‰ for 481.2 keV electrons. Microburst events also show a strong dependence on geomagnetic activity levels. The occurrence rate of microburst events increases with geomagnetic activity intensity. For 265.4 keV electrons, it rises from 2‰ during quiet periods to a maximum of 6‰ during active periods. The peak of occurrence rate shifts to lower L value during active periods. The global distribution characteristics of microburst occurrence rate are consistent with the amplitude distribution of whistler-mode chorus waves, indicating that the pitch angle scattering caused by chorus is an important driver of microburst. This provides key statistical observational evidence for the scattering effect of chorus waves on hundreds of keV electrons.
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