地球辐射带电子快速损失现象的观测和数值模拟研究

Radiation belt dropout is one of the most dramatic variations in Earth's magnetosphere, during which the electron fluxes are observed to drop by several orders of magnitude in just a few hours. Investigating the radiation belt dropout can improve the understanding of radiation belt dynamics, analyzing the relationship between the magnetosphere and other space environments, avoiding the hazard effect due to extreme space weather. It is now widely accepted that both the mechanism of magnetopause shadowing combined with outward radial diffusion and the mechanism of atmospheric precipitations due to wave-particle interaction can contribute to radiation belt dropouts. The inward movement of magnetopause due to enhanced solar wind dynamic pressure can substantially deplete electrons at high L shells where electrons find themselves on the open drift shells, while electron fluxes at lower L shells can continue to drop through outward radial diffusion after the magnetopause returns to the quiet time position. Various magnetospheric waves can scatter radiation belt electrons into the loss cone via wave-particle interactions, including whistler mode chorus, plasmaspheric hiss, electron magnetic ion cyclotron waves, and magnetosonic waves. However, the key question regarding the relative contributions of these two mechanisms to the observed dropouts still remains to be fully understood. In this study, we will use multi-satellite simultaneous observations to investigate the dominant loss mechanism of dropout events during 2012-2019. The dependence of loss mechanisms on solar wind parameters and geomagnetic indices will be analyzed. Simulation results based on the Fokker-Planck equation will be performed to verify the conclusions based on satellite observations.

地球辐射带电子快速损失现象是指地球的外辐射带电子通量在几个小时内出现几个数量级的下降。该现象的研究对于理解地球辐射带动态变化，分析地球磁层与其它空间环境的耦合关系，避免空间天气灾害等均具有重要意义。磁层顶阴影和波粒相互作用引起的大气沉降被认为是导致辐射带电子快速损失的两种主要物理机制。磁层顶阴影是指增强的太阳风动压向内压缩地球磁层顶，导致原本被地球磁场捕获的高能电子穿过磁层顶逃逸到星际空间。磁层等离子体波和电子的回旋共振也可使电子沉降到两极大气而损失。目前关于这两种损失机制哪一种是主导损失机制以及什么情况下居主导地位等问题一直没有得到解决，这是开展本项目工作的主要原因。本项目将利用多颗卫星的联合共时观测，判断2012年至2019年期间地球辐射带电子快速损失事件的主导损失机制，揭示不同损失机制的统计规律及对太阳风参数和地磁活动的依赖性，并通过数值模拟的方法验证和补充观测结论。

地球辐射带电子快速损失现象是辐射带中最剧烈的变化现象之一，对于它的研究可以加深对辐射带动态变化的理解、帮助分析地球磁层和其它空间环境的耦合关系以及避免空间天气灾害。围绕辐射带电子快速损失主导物理机制这一关键科学问题，项目负责人主要开展了三个方面的研究工作：（1）基于电子相空间密度的时空分布深入分析了2014年9月12日磁暴期间辐射带电子快速损失在不同时期、不同空间区域和不同电子能级的主导物理机制；（2）系统量化了ECH、Chorus、Hiss、MS、EMIC等空间等离子体波动通过回旋共振和弹跳共振对辐射带高能电子产生的散射作用；（3）自主发展了一个完全由太阳风参数驱动的地球辐射带动态演化模型。通过这些研究发现：（1）磁暴初相辐射带电子快速损失的主导机制是磁层顶阴影，而在磁暴主相，波粒相互作用是导致电子快速损失的关键物理机制；（2）极低频Chorus波可以和辐射带电子发生弹跳共振散射投掷角高于85度能量高于 100 keV的电子；（3）ECH、Hiss、MS、EMIC这四种波动可以同时发生并产生联合散射效应，共同作用的结果强于单一波动的散射效果，并且MS波可以大幅减弱Hiss波产生的top-hat投掷角分布。这些研究结果揭示了不同地磁条件和太阳风环境下辐射带电子快速损失的主导物理机制，为辐射带电子通量预测和空间天气预报提供了关键理论依据。