MODEL OF THE FORMATION OF THE LOW-LATITUDE BOUNDARY-LAYER FOR STRONGLY NORTHWARD INTERPLANETARY MAGNETIC-FIELD

Observations of the low-latitude boundary layer in the subsolar region when the interplanetary magnetic field (IMF) is strongly northward indicate that the boundary layer consists of steplike multiple layers rather than a single diffusive layer. Those sublayers can be formed by spatially limited, temporally varying reconnection near the polar cusp. In this model when the interplanetary magnetic field is strongly northward a magnetosheath flux tube reconnects in the north and south beyond the cusp. The tube shortens itself and reorients to align itself with other magnetospheric field lines and eventually be assimilated with other magnetospheric field lines. Energy from the shortening of the flux tube and the reduction of magnetic energy is transferred into the magnetosphere and increases the pressure above its initial equilibrium value while the reconnected flux tube sinks into the magnetosphere. The interchange instability is one of the possible mechanisms to disperse and expand the newly captured magnetosheath flux tube azimuthally along the magnetopause current layer and to lead the system back to equilibrium. It is stable to radial interchanges and unstable to azimuthal interchanges. Thus the newly formed flux tube becomes a boundary layer. The interchange instability converts the thermal energy of the plasma into dynamic energy. The interchange front moves with a fraction of the sound speed and accelerates toward the terminator. Subsequent reconnection forms a new layer before the last one is completely dispersed. In this model, different sublayers represent different ages after reconnection.