磁层顶电子剪切流K-H不稳定性

Kelvin-Helmholtz instability of sheared electron flow at the magnetopause

磁层顶是由经过舷激波的太阳风和地磁层相互作用而形成的边界层,其上有电流流动.本文将这种电流看成电子剪切流,来讨论夜侧磁层顶的电子剪切流所引起的Kelvin-Helmholtz(K-H)不稳定性问题.电子剪切流受到的洛仑兹力中包括了电场的作用,正是这种作用使得剪切不稳定性要复杂一些.本文对电子剪切流激发的K-H不稳定性进行了细致的分析,从而得知,电子流剪切引起的K-H不稳定性倾向于在磁层顶内侧激发,并且只有当剪切速率在上下两阈值之间时,才可能激发K-H不稳定性.本文对比了夜侧近地磁层顶和磁尾磁层顶,发现当磁层顶两侧电子数密度之比恒定时,高电子数密度的磁层顶侧易产生K-H不稳定性.而磁鞘侧电子数密度n2与磁层侧电子数密度n1之比n2/n1的改变对不稳定性的影响也很显著:较大的n2/n1产生的K-H波主要是在低速区域,其有效增长率较小.

Magnetopause is the boundary layer formed by the interaction of solar wind with magnetosphere, and current flows upon it. The current was considered as the sheared electron flow, and the Kelvin- Helmholtz （K-H） instability induced by the sheared electron flow of nightside magnetopause was discussed. Because of the effect of the electric field, instabilities of the shear electron flow are a little more complicated than those of the shear plasma flow. From the results obtained from the paper, K-H instability rising from sheared electron flow is easy to occur at the inner boundary of the magnetopause, and, it is likely that K-H instability happens when the velocity of sheared electron flow is between two thresholds. And, comparing the nightside near earth magnetopause with the magnetotail, we found that when the ratio of electron number densities of magnetopause is constant, and high electron number density can raise K-H instability. It is obvious that the variations of n2/n1 influenced the effective growth rates： K-H wave centralizes the lower velocity region with bigger n2/n1, and its effective growth rate is smaller.