The extreme compression of the magnetosphere on May 4, 1998, as observed by the POLAR spacecraft Original Research Article

On May 4, 1998 the velocity and density of the solar wind were high and the interplanetary magnetic field was strong and southward. The POLAR spacecraft crossed the dayside magnetopause well inside geosynchronous orbit, at 5.3 RE and a solar zenith angle of 19°. After this crossing, POLAR spent most of the rest of its outbound orbit in the magnetosheath and for brief periods crossed into the solar wind at distances from 7.3 RE and a solar zenith angle of 32° to a distance of only 8.5 RE and a solar zenith angle of 45°. This corresponds to subsolar distances of only 6.8 to 7.5 RE for the shock. During this very disturbed period of time, predictions of the locations of the magnetopause by both Shue and co-workers and by Petrinec and co-workers indicate extreme distortions of the magnetopause location. Because of the importance of such events to the understanding of space weather, we recommend that this event be pursued as a special IACG 2 campaign.