A structured magnetospheric cusp observed by Cluster during a geomagnetic storm

Results are presented from a study of the northern magnetospheric cusp crossed by the 4 Cluster satellites during 16:25-17:55 UT on 18 April 2002 when a moderate geomagnetic storm was in progress, solar wind dynamic pressure was rather low and IMF Bz was suitable (southward) for reconnection in the dayside magnetopause. The satellite orbit and data from the FGM, CIS, PEACE, STAFF and WHISPER instruments reveal that the satellites have crossed through a structured exterior cusp. Three distinct anti-sunward ion flow events of duration about 1.5, 17.5 and 19.0 minutes, with bulk flow roughly parallel to the magnetopause surface, are detected in the cusp region. The ion density in the events is more than double that outside the events. The sharp inward/outward walls of the events are detected with expected/reversed time sequence by the 4 satellites, with time delays of up to 10 sec./4 sec. That suggests reversed accelerated plasma flows inside the events compared to the flows outside the events, with faster acceleration closer to the inward walls. The magnetic field fluctuations in By (about 50 nT) associated with the anti-sunward ion flow events are found to be nearly twice as strong as those in Bx and Bz, suggesting strong field-aligned currents