Properties of the current sheets in the Earth’s magnetotail electron diffusion regions
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摘要: 理论与数值模拟研究表明,在对称磁场重联过程中,电流片由单峰Harris 型向双峰分叉型的演化,而受限于卫星探测精度,尚未得到观测上的验证。基于磁层多尺度任务卫星的高时间分辨率观测,近年来已在地球磁尾报道了十余个电子扩散区事例。本文利用Harris电流片模型,对Wang等[1]给出的13个磁尾电子扩散区事例进行了系统的研究。结果表明,其中9个事例的电流片结构符合Harris电流片,其电流片厚度较薄,最薄能达到0.67个电子惯性长度;并且随着电流片厚度变薄,电子出流速度显著增大,能达到0.45个电子阿尔芬速度,表明在每个电子扩散区事例中MMS1的穿越位置逐渐远离X点。剩余4个事例表现为分叉电流片特征,呈现出电流双峰分布,其电流片厚度显著增大,最厚可达约 20个电子惯性尺度,表明其距离重联X点较远,其电子出流速度也更小。本研究从统计上确定了13个电子扩散区事例距离X点的相对空间位置分布,有助于理解电子扩散区内电子的动理学过程。
Abstract: Theoretical and numerical simulation studies suggest that, during symmetric magnetic reconnection, the current sheet evolves from a single-peaked Harris-type structure to a double-peaked bifurcated configuration. However, due to limitations in satellite measurement resolution, this evolution has not yet been observationally confirmed. Benefiting from the high-resolution observations of the Magnetospheric Multiscale (MMS) mission, more than ten electron diffusion region (EDR) events have been reported in the Earth’s magnetotail in recent years. In this study, we apply the Harris current sheet model to perform a systematic analysis of 13 magnetotail EDR events reported by Wang et al[1]. The results show that nine events are well described by a Harris-type current sheet, with relatively thin thicknesses down to 0.67 electron inertial lengths. Moreover, as the current sheet becomes thinner, the electron outflow velocity increases significantly, reaching up to 0.45 electron Alfvén speeds, indicating that the MMS1 crossing locations in each EDR are progressively farther from the X point. The remaining four events exhibit clear bifurcated current sheet features, with double-peaked current density profiles and significantly thicker current sheets, reaching up to ~20 electron inertial lengths, corresponding to locations farther away from the reconnection X point and associated with lower electron outflow speeds. This study statistically determines the relative spatial distribution of 13 electron diffusion region events with respect to the reconnection X point, which helps to improve our understanding of electron kinetic processes within EDR.
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