地球磁层顶的无碰撞Kelvin-Helmholtz不稳定性（Ⅰ）

本文用无碰撞等离子体的CGL近似,讨论了地球磁层顶的Kelvin-Helmholtz（K-H）不稳定性问题。同可压缩MHD近似一样,CGL等离子体也存在下、上两个临界速度vc和vu。不稳定性仅当vc0 cosαu时才能激发,v0为磁鞘等离子体流速,α为v0和切向波矢kt间的夹角。在向阳面低纬区,时间增长率ε和空间增长率κ与可压缩MHD的相应值相近。当v0 cosα≥vu时,不稳定表面波转化为稳定的体波。各向异性对K-H不稳定性有显著影响。在近日点低纬处,若磁鞘各向异性参数S2接近水龙带不稳定性的阈值,很小的v0便能激发不稳定表面波。S2低于极小值S2min时,不稳定性不可能产生。所谓Overstability现象是不存在的。

THE COLLISIONLESS KELVIN-HELMHOLTZ INSTABILITY AT EARTH'S MAGNETOPAUSE

The Kelvin- Helmholtz instability at earth's magnetopause is investigated by using CGL approach. Similar to the results obtained in the compressional MHD approximation,the K-H instability can be excited only when vc0 cosαu,where vc and vu are the lower and upper critical velocity,respectively,v0 is the velocity of the magnetosheath flow with α being the angle between v0 and kt,the tangential wave vector. The time growth rate and spatial growth rate are both slightly lower titan those in the compressional MHD approximation near dayside equator. Surface waves are changed to body waves as long as v0cosα0《A1,the Alfven speed in the magnetosphere,provided the anisotropy in the magnetosheath achieves the thresholds for generating the firehose and mirro instability,resulting unstable K-H waves with low phase velocity. It is impossible to excite unstable surface waves if S=P∥/P⊥min a critical value of S in the magnetosheath. So called "overstability" given in previous references can be proved not to exist both analytically and numerically.