Study of magnetohydrodynamic instabilities in high-β plasmas in the magnetosphere by theory and modelling

本文在理想磁流体力学（MHD）和双组分流体近似下，研究了磁层中高β等离子体的磁流体力学（MHD）不稳定性的线性特征。(1)通过双组分流体分析，阐明了在日下磁层顶激发了Kelvin-Helmholtz（K-H）不稳定性，该不稳定性是由离子抗磁漂移速度的切变而不是电场漂移速度的切变驱动的。这种不稳定性是由离子抗磁漂移速度中的剪切驱动的，这是高β等离子体中的非理想MHD漂移，并且是在引导中心水平不可见的宏观效应。归一化增长率随磁鞘密度与磁层密度比值的增大而减小。(2)双分量流体分析表明，磁尾高β等离子体中的气球不稳定性不仅具有向西传播的漂移速度，而且由于电子压梯度的扰动而具有平行电场，这是极光电子加速的原因。对于亚暴发生前地磁尾高度拉伸的磁力线配置，计算了离子反弹频率的俯仰角平均值。结果表明，在15 Re从地球，平均离子反弹和磁漂移频率小于气球的增长率下获得的流体近似。因此，动力学对气球不稳定性的影响在15 Re内可以忽略不计。然而，超过15 Re的离子反弹和磁漂移频率大于气球的增长率，因此，动力学的气球不稳定性的影响是重要的超过15 Re从地球。

In this research, linear characteristics of magnetohydrodynamic (MHD) instabilities in high-β plasmas in the magnetosphere have been studied within ideal MHD and two-component fluid approximations.(1) It has been clarified by two-component fluid analysis that a Kelvin-Helmholtz (K-H) instability driven by a shear in the ion diamagnetic drift velocity and not in the electric field drift velocity is excited at the subsolar magnetopause. This instability is driven by the shear in the ion diamagnetic drift velocity, which is a nonideal MHD drift in a high-β plasma and is a macroscopic effect not visible at the guiding center level. The normalized growth rate decreases with an increase in the ratio of the magnetosheath density to the magnetospheric density.(2) It has been clarified by two-component fluid analysis that ballooning instability in the high-β plasma in the geomagnetic tail has a westward propagating drift speed and also has a parallel electric field due to a perturbation in the electron pressure gradient, which is responsible for the auroral electron acceleration. For a highly stretched field line configuration in the geomagnetic tail, which is expected before the onset of substorms, the pitch angle average of ion bounce frequency was calculated. It was shown that at 15 Re from the Earth, the average ion bounce and magnetic drift frequencies are smaller than the ballooning growth rate obtained under the fluid approximations. Therefore, kinetic influences on ballooning instability are negligible within 15 Re from the Earth. However, beyond 15 Re the ion bounce and magnetic drift frequencies are larger than the ballooning growth rate and therefore, kinetic influences on ballooning instability are important beyond 15 Re from the Earth.