DataSheet4_Evidence of Kelvin-Helmholtz and tearing mode instabilities at the magnetopause during space weather events.pdf

Introduction: Kelvin-Helmholtz (KH) and tearing mode (TM) instabilities are one of the most important mechanisms of solar wind energy, momentum and plasma transport within the magnetosphere.Methods: To investigate the conditions under which KHTM instabilities occur in the Earth environment it is fundamental to combine simultaneous multipoint in situ measurements and MHD simulations. We analyzed data from the THEMIS and Cluster spacecraft considering two Space Weather (SWE) events starting with an M2.0 flare event (hereafter Case-1) that occurred on 21 June 2015 and the most-intensive flare (X9.3) of solar cycle 24 that occurred on 6 September 2017 (hereafter Case-2).Results: Our analysis utilized a 2D MHD model for incompressible and viscous flow. The results from Case-1 indicate the presence of KH and TM instabilities, suggesting existence of observed low-amplitude oscillations at the nose of the magnetopause. However, the MHD simulations for Case-2 did not show any evidence of KH vortices, but did reveal the presence of “magnetic island” structures during a low-shear condition. The reconnection rate derived from the observations is compared with the computed one in the presence of developed instabilities inside the Earth’s magnetopause.

引言：凯尔文-赫尔姆霍兹（Kelvin-Helmholtz, KH）不稳定性与撕裂模式（Tearing Mode, TM）不稳定性是太阳风在磁层内部进行能量、动量和等离子体传输的最重要机制之一。研究方法：为了探究KHTM不稳定性在地球环境中的发生条件，结合同时进行的多点原位测量和磁流体动力学（MHD）模拟是至关重要的。本研究分析了THEMIS和Cluster航天器采集的数据，针对两次空间天气（Space Weather, SWE）事件进行分析，分别是一次M2.0级耀斑事件（以下简称案例-1），该事件于2015年6月21日发生，以及太阳周期24中最为强烈的X9.3级耀斑（以下简称案例-2），该事件于2017年9月6日发生。结果：我们的分析采用了一个适用于不可压缩和粘性流动的二维MHD模型。案例-1的结果表明，存在KH和TM不稳定性，暗示在磁鞘前端观测到了低振幅的振荡现象。然而，针对案例-2的MHD模拟并未显示出KH涡旋的任何迹象，但在低剪切条件下揭示了“磁岛”结构的存在。从观测中得到的重连速率与在地球磁鞘内部发展了不稳定性的情况下计算出的速率进行了比较。