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

本研究在理想磁流体动力学和双组分流体近似下，研究了磁层中高β等离子体磁流体动力学（MHD）不稳定性的线性特性。(1)双分量流体分析表明，亚太阳磁层顶激发了由离子反磁漂移速度剪切而非电场漂移速度剪切驱动的开尔文-亥姆霍兹（K-H）不稳定性。这种不稳定性是由离子反磁漂移速度的剪切驱动的，这是高β等离子体中的非理想MHD漂移，是在引导中心水平不可见的宏观效应。归一化增长率随着磁鞘密度与磁层密度之比的增大而减小。(2)双分量流体分析表明，地磁尾高β等离子体中的气球不稳定性具有向西传播的漂移速度，并且由于电子压力梯度的扰动而具有平行电场，这是造成极光电子加速的原因。对于亚暴发生前地磁尾高度拉伸的场线结构，计算了离子弹跳频率的俯仰角平均值。结果表明，在距离地球15re处，平均离子弹跳和磁漂移频率小于流体近似下的气球增长率。因此，在距离地球15re范围内，动力学对气球不稳定性的影响可以忽略不计。然而，在15re以上，离子反弹和磁漂移频率大于气球生长速率，因此，在离地球15re以上，对气球不稳定性的动力学影响是重要的。

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.