| Summary: | [1] 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 B-y 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 degrees N, 19.2 degrees 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 (approximate to 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 (approximate to 17 MLT) cusp during the first 40 min (1525-1605 UT) of the event when IMF B-z remained negative. Strong magnetic waves are also generated in the magnetosheath-cusp region.
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