Electron Distributions in Kinetic Scale Field Line Resonances: A Comparison of Simulations and Observations
Electron Distributions in Kinetic Scale Field Line Resonances: A Comparison of Simulations and Observations
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DOI：
10.1029/2018gl077748
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发表时间：
2018-06-28
影响因子：
5.2
通讯作者：
Johnson, J. R.
中科院分区：
文献类型：
--
作者：
Damiano, P. A.;Chaston, C. C.;Johnson, J. R.
在动力学尺度的场线共振或地磁场的本征模式中的观测，揭示了高度场向平坦的电子分布。通过将来自Van Allen探测器和星团航天器的观测与混合动力学回转流体模拟相结合，我们展示了这些分布是如何由电子与波场的非局域自洽相互作用引起的。这种相互作用表现为驻波势中的电子陷阱。这个过程沿着大部分场线运行，定性地解释了赤道平面附近和更高纬度的电子观测。伴随着高度磁场排列的高原，损失锥特征也是明显的，这是向上定向波平行电场对未被捕获的电子布居作用的结果。简单地说，动力学尺度场线共振(KFLR)是沿着闭合磁力线的驻波，在地磁活动强烈的时候在内部磁层中突出。这些波对于磁层-电离层耦合和促进电子和离子在磁力线上的扩散是重要的，这是理解辐射带动力学的基础。卫星观测显示，KFLR内的电子分布在平行于背景磁场的方向上高度拉伸。在这项工作中，通过将KFLR波场内电子的计算机模拟与Van Allen探测器和星团卫星的观测结果进行比较，我们首次阐明了这些分布的时空结构自然是由驻波电势俘获电子造成的。这项工作也是首次将KFLR与模拟进行多卫星比较，并另外说明，在观察到的分布函数中看到的损失锥特征(特别是在电离层附近)也自然地在模拟中重现。这些特征与波场内未捕获的电子向电离层的沉淀有关。因此，KFLR对于磁层和电离层之间的能量转移很重要。
Observations in kinetic scale field line resonances, or eigenmodes of the geomagnetic field, reveal highly field-aligned plateaued electron distributions. By combining observations from the Van Allen Probes and Cluster spacecraft with a hybrid kinetic gyrofluid simulation we show how these distributions arise from the nonlocal self-consistent interaction of electrons with the wavefield. This interaction is manifested as electron trapping in the standing wave potential. The process operates along most of the field line and qualitatively accounts for electron observations near the equatorial plane and at higher latitudes. In conjunction with the highly field-aligned plateaus, loss cone features are also evident, which result from the action of the upward-directed wave parallel electric field on the untrapped electron populations.Plain Language Summary Kinetic scale field line resonances (KFLRs) are standing waves along closed magnetic field lines that are prominent in the inner magnetosphere at times of strong geomagnetic activity. These waves are important for magnetosphere-ionosphere coupling and for facilitating the diffusion of electrons and ions across magnetic field lines, which is fundamental to understanding radiation belt dynamics. Satellite observations reveal that electron distributions within KFLRs are highly stretched in the direction parallel to the background magnetic field. In this work, by comparing computer simulations of electrons within the KFLR wavefields with observations from both the Van Allen Probes and Cluster satellites, we for the first time illustrate that the spatial and temporal structure of these distributions naturally result from the trapping of electrons by the standing wave electric potential. This work is also the first multisatellite comparison of KFLRs with simulations and additionally illustrates that loss cone features seen in observed distribution functions (particularly closer to the ionosphere) are also naturally reproduced in the simulations. These features are related to the precipitation to the ionosphere of untrapped electrons within the wavefield. KFLRs are thus important for energy transfer between the magnetosphere and ionosphere.