KELVIN-HELMHOLTZ INSTABILITY IN BOUNDARY LAYER REGIONS OF THE PLASMA SHEET DURING MAGNETOSPHERIC SUBSTORM RECOVERY

The Kelvin-Helmholtz instability in boundary layer regions of the plasma sheet during magnetospheric substorm recovery is studied based on two idealized MHDmodels.One model consists of three uniform plasma regions-the tail lobe,the plasma sheet,and the south tail lobe.In this model the unstable mode is excited at the boundary surface between the north (or south) tail lobe and the plasma sheet.The other model consists of five uniform plasma regions-the north tail lobe,the north boundary layer of the plasma sheet,the middle layer of the plasma sheet,the south boundary layer of the plasma sheet,and the sorth tail lobe.There are two unstable modes in boundary layer regions of the plasma sheet:one is excited at the boundary surface between the north (or south) boundary layer and the middle layer of the plasma sheet (Mode-I) and the other is excited at the boundary surface between the north (or south) boundary layer of the plasma sheet and the north (or south) tail lobe (Mode-II).Mode-Iis unstable most of the time.These unstable modes may cause some features of boundary layer regions of the plasma sheet observed oil satelittes.