Response of the magnetosheath‐cusp region to a coronal mass ejection

Abstract

Cluster made an unusual magnetosheath‐exterior cusp crossing during the first 2.5 hours of a coronal mass ejection (CME) that flowed past Earth for about 7 hours on 24 October 2003. During the first 2.5 hours (1525–1802 UT) the solar wind dynamic pressure remained high and stable though the CME had a discontinuity after 40 min (1605 UT), when the azimuthal flow turned dawnward up to −100 km s−1 and IMF By and Bz changed from highly negative to positive up to 25 nT. The responses of the magnetosheath‐cusp region during the unusual event are presented using Cluster and ground‐based (EISCAT VHF radar; 69.6°N, 19.2°E) observations. The unusual Cluster crossing (compared to the usual midaltitude cusp crossing at this time of the year) occurred owing to a large compression of the magnetosphere. Cluster, which was in the southern magnetospheric lobe, suddenly found itself in the magnetosheath at the arrival of the CME at 1524:45 UT. Cluster then crossed through the compressed magnetosheath (highly compressed after the discontinuity in the CME) for about 1.5 hours (1525–1655 UT), magnetopause with strong signatures of lobe reconnection (≈1655 UT), stagnant but compressed exterior cusp for about an hour (1700–1802 UT), and then entered the dayside magnetosphere. The observations also show strong signatures of magnetosphere‐ionosphere coupling through a late afternoon (≈17 MLT) cusp during the first 40 min (1525–1605 UT) of the event when IMF Bz remained negative. Strong magnetic waves are also generated in the magnetosheath‐cusp region.