A GLOBAL MAGNETOHYDRODYNAMIC SIMULATION OF THE RESPONSE OF THE MAGNETOSPHERE TO A NORTHWARD TURNING OF THE INTERPLANETARY MAGNETIC-FIELD

We have used a global magnetohydrodynamic simulation model to investig ate the time series of events which occur in the magnetosphere when th e interplanetary magnetic field (IMF) changes from southward to northw ard. Within 15 min of the northward turning the magnetopause reconnect ion site moves from the subsolar point to the high-latitude tail. The high-latitude reconnection converts tail lobe field lines into closed dayside field lines and new field lines in the IMF. The removal of lob e flux plus continuing reconnection at the near-Earth neutral line cau se the field lines which thread the plasma sheet to relax to a more di polar state and a very slow tailward retreat of the near earth neutral line. The newly closed dayside field lines are convected around the f lanks of the magnetosphere and into the tail. The closed flux convecte d from the dayside to the tail is the main source of closed flux into the tail. As a result, the plasma sheet recovers first along the flank s of the magnetosphere and then nearer midnight. The near-Earth neutra l line shrinks toward midnight and then moves rapidly tailward.