SUDDEN IMPULSES AT SUBAURORAL LATITUDES - RESPONSE FOR NORTHWARD INTERPLANETARY MAGNETIC-FIELD

The response of subauroral H component magnetograms at the time of pas sage of interplanetary shocks under northward interplanetary magnetic field (IMF) conditions is used to examine the behavior of the magnetos phere when it is suddenly compressed. At subauroral latitudes, near 55 degrees or L = 3, the response has some similarities and some importa nt differences from the low-latitude response. The first effect is a p reliminary impulse which appears to be due to the Hall current driven in the ionosphere by the arrival of the first Alfven waves along the m agnetic field lines from the distant equatorial magnetosphere that has been set in motion by the penetration of the shock wave into the magn etosphere. Next there is a general increase of the magnetic field ever ywhere as the magnetosphere is compressed to its new size and the info rmation of this compression propagates to the ground. This increase ta kes about 5 min followed by a 10-min relaxation to the final asymptoti c value. This general increase is accompanied by two more localized cu rrent systems: one transient and one steady. The transient current app ears to be a dual-vortex current system launched from noon toward the nightside. The other steady system is a fixed double-cell convection s ystem. We identify the former with the propagation of the magnetopause indentation associated with the enhanced pressure in the solar wind. We attribute the latter, steady system to high-latitude reconnection w ith northward IMF.