Modeling of the Hydromagnetic Instability

1. 由于零阶磁场在磁层顶的连续旋转，当磁鞘磁场向北时比向南时更容易激发开尔文-亥姆霍兹（K-H）不稳定性。当磁鞘磁场向北时，由于K-H不稳定性的动量输运，磁层顶速度边界层厚度增大。当磁鞘磁场不在正北方向时，在涡旋附近形成一个缓慢的稀薄区，在此区域内等离子体被稀薄并沿着磁场线加速。导出了二维可压缩等离子体（流体）横向磁场运动的熵守恒定律。对二维横向构型的MHD模拟表明，在非线性K-H不稳定性阶段，由于涡旋对的存在，输运动量和异常粘度比线性增长最快的涡旋大得多。在涡旋内部，等离子体在快速磁子稀薄作用下被强烈稀薄，涡旋内部的磁压减小。由于K-H不稳定性的影响，在涡的外围处流动速度加快。在三维磁层-电离层耦合系统中，当电离层电导率高于一个临界电导率时，K-H不稳定性被完全抑制，该临界电导率几乎等于沿场线平均的Alfven电导率。这是由于磁层中横向磁场的稳定影响，这是由引起磁层中剪切速度流的横向电场产生的。

1. Owing to the continuous rotation of the zero-th order magnetic field across the magnetopause the Kelvin-Helmholtz (K-H) instability is more easily excited when the magnetosheath magnetic field is northward than when it is southward. The thickness of the velocity boundary layr at the magnetopause becomes larger when the magnetosheath magnetic field is northward owing to the momentum transport by the K-H instability. When the magnetosheath magnetic field is not due north, a slow rarefaction region is formed near the vortex, wherein the plasma is rarefied and accelerated along the field line.2. The conservation law of the enstrophy is derived for the 2-D compressible plasma (fluid) motion transverse to the magnetic field. An MHD simulation for the 2-D transverse configuration, where the magnetic field is transverse to the flow velocity, shows that owing to the vortex pairing in the nonlinear stage of the K-H instability the transported momentum and the anomalous viscosity become much larger than those due to the linearly fastest growing vortex. Inside the vortex the plasma is rarefied strongly by the fast magnetosonic rarefaction and the magnetic pressure is reduced inside the vortex. The flow velocity is enhanced at the periphery of the vortex, which is formed by the K-H instability.3. In the 3-D magnetosphere-ionosphere coupling system the K-H instability is suppressed completely when the ionospheric conductivity is higher than a critical conductivity, which is nearly equal to the Alfven conductance averaged along the field line. This is due to a stabilizing influence of the transverse magnetic field in the magnetosphere, which is produced by the transverse electric field causing the sheared velocity flow in the magnetosphere.