The Van Allen Probes mission contributed to the discovery of the relativistic (∼500 keV–2 MeV) and ultra-relativistic (∼>2 MeV) electron three-belt structure in Earth's radiation belts. This structure results from the partial depletion of the preexisting outer belt and the replenishment of a new outer belt. Ultra-low frequency and very-low frequency waves are believed to play important roles in these processes, and substorm injections are usually not responsible for the formation of the external outer belt. In this study, based on observations from the Arase and NOAA-18 (National Oceanic and Atmospheric Administration-18) satellites, we report an electron three-belt event in the energy range of ∼100–200 keV (i.e., sub-relativistic electrons). According to the evolution of the phase space density and the dawn-dusk asymmetric flux enhancement during this event, we conclude that the depletion of the upper part of the original outer belt was due to outward radial diffusion accompanied by magnetopause shadowing effect and convection, and the formation of the external outer belt was due to increased electron flux from convection as well as substorm injections. This discovery and its preliminary explanation may help understand the electron three-belt structure in radiation belts more comprehensively.

中文翻译：

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地球辐射带中的亚相对论电子三带事件：观测与解释

范艾伦探测器任务有助于发现地球辐射带中的相对论性（∼500 keV-2 MeV）和超相对论性（∼>2 MeV）电子三带结构。这种结构是由于先前存在的外带的部分耗尽和新的外带的补充而产生的。超低频和甚低频波被认为在这些过程中发挥着重要作用，而亚暴注入通常不负责外部外带的形成。在这项研究中，基于Arase和NOAA-18（美国国家海洋和大气管理局-18）卫星的观测，我们报告了能量范围~100-200 keV的电子三带事件（即亚相对论电子） ）。根据该事件期间相空间密度的演化和黎明-黄昏不对称通量增强，我们得出结论，原始外带上部的损耗是由于伴随着磁层顶阴影效应和对流的向外径向扩散造成的，并且外部外带的形成是由于对流和亚暴注入的电子通量增加所致。这一发现及其初步解释可能有助于更全面地了解辐射带中的电子三带结构。