Magnetohydrodynamic Turbulence in the Earth’s Magnetotail From Observations and Global MHD Simulations
Magnetohydrodynamic Turbulence in the Earth’s Magnetotail From Observations and Global MHD Simulations
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DOI：
10.3389/fspas.2021.620519
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发表时间：
2021-03
期刊：
影响因子：
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通讯作者：
M. El‐Alaoui;R. Walker;J. Weygand;G. Lapenta;M. Goldstein
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文献类型：
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作者：
M. El‐Alaoui;R. Walker;J. Weygand;G. Lapenta;M. Goldstein
在太阳风、磁鞘和磁尾等离子体片中发现了磁流体动力学（MHD）湍流。在本文中，我们回顾了磁尾湍流的观测和理论证据。对南行星际磁场（IMF）的全球磁层的MHD模拟显示，在磁场重联的向地流出中存在与湍流一致的嵌套涡旋。类似的模拟北IMF也表现出增强涡度与湍流一致。这是由开尔文-亥姆霍兹（KH）不稳定性引起的。然而，与重联相关的湍流填充了磁尾的大部分区域，而与KH不稳定性相关的湍流仅限于磁层顶附近的较小区域。分析磁尾中的湍流是困难的，因为磁尾的范围有限，而且那里的流动通常是亚磁音速的。磁尾湍流的观测分析通常假设泰勒冻结流假设是有效的，并将功率谱密度与频率的关系与柯尔莫哥洛夫在1941年推导的流体湍流谱指数进行比较。对尾部实际磁层亚暴进行的全球模拟使模拟结果能够直接与观测到的功率谱进行比较。这两种技术之间的协议提供了信心，等离子体片等离子体实际上是湍流。磁流体动力学的结果也使我们能够计算功率与波数的关系;这些结果也支持尾翼是湍流的观点。
Magnetohydrodynamic (MHD) turbulent flows are found in the solar wind, the magnetosheath and the magnetotail plasma sheet. In this paper, we review both observational and theoretical evidence for turbulent flow in the magnetotail. MHD simulations of the global magnetosphere for southward interplanetary magnetic field (IMF) exhibit nested vortices in the earthward outflow from magnetic reconnection that are consistent with turbulence. Similar simulations for northward IMF also exhibit enhanced vorticity consistent with turbulence. These result from Kelvin-Helmholtz (KH) instabilities. However, the turbulent flows association with reconnection fill much of the magnetotail while the turbulent flows associated with the KH instability are limited to a smaller region near the magnetopause. Analyzing turbulent flows in the magnetotail is difficult because of the limited extent of the tail and because the flows there are usually sub-magnetosonic. Observational analysis of turbulent flows in the magnetotail usually assume that the Taylor frozen-in-flow hypothesis is valid and compare power spectral density vs. frequency with spectral indices derived for fluid turbulence by Kolmogorov in 1941. Global simulations carried out for actual magnetospheric substorms in the tail enable the results of the simulations to be compared directly with observed power spectra. The agreement between the two techniques provides confidence that the plasma sheet plasma is actually turbulent. The MHD results also allow us to calculate the power vs. wave number; results that also support the idea that the tail is turbulent.