The response of the magnetotail to changes in the IMF orientation: The magnetotail's long memory

We have used a three dimensional global magnetohydrodynamic simulation of the interaction between the solar wind and the magnetosphere to investigate the changes which occur in the magnetotail when the interplanetary magnetic field (IMF) changes orientation. For IMF B-y not equal 0 the entire magnetotail twists when the IMF changes from southward to northward. The twisting at a given location on the tail magnetopause occurs shortly after the IMF change reaches that location. inside the magnetosphere the response time is much longer. New field lines with open ends formed by the reconnection of tail lobe field at a near-Earth neutral line also twist as the field lines convect tailward. Initially formed with a southward B-z they end up with a northward B-z. The response of the tail to a change in the IMF is controlled by the penetration of the solar wind electric field into the magnetosphere. (C) 1998 Elsevier Science Ltd. All rights reserved.