© European Geosciences Union 2004
Cluster observes formation of high-beta plasma blobs
1International University Bremen, Germany
2Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany
3Institute for Space Sciences, Bucharest, Romania
4Institut für Geophysik und Meteorologie, TU Braunschweig, Germany
5Mullard Space Science Laboratory, University College London, UK
6Centre d’Etude Spatiale des Rayonnements, Toulouse, France
7Space Sciences Laboratory, University of California, Berkeley, USA
Abstract. Late in a sequence of four moderate substorms on 26 July 2001, Cluster observed periods of a few minutes durations of high-beta plasma events (B<10nT, β=2-30), connected with dipolarizations of the magnetic field. Cluster was located near 02:45 MLT, at R=19RE and at about 5°N GSM. These events began late in the recovery phase of the second and about 5min before onset of the third substorm and lasted for three hours, way beyond the recovery phase of the fourth substorm. The most remarkable observation is that the onset coincided with the arrival of energetic (E~7keV) O+ ions and energetic electrons obviously from the ionosphere, which tended to dominate the plasma composition throughout the remaining time. The magnetic flux and plasma transport is continuously directed equatorward and earthward, with oscillatory east-west movements superposed. Periods of the order of 5-10min and strong correlations between the magnetic elevation angle and log β (correlation coefficient 0.78) are highly reminiscent of the high-beta plasma blobs discovered with Equator-S and Geotail between 9 and 11RE in the late night/early morning sector (Haerendel et al., 1999).
We conclude that Cluster observed the plasma blob formation in the tail plasma sheet, which seems to occur predominantly in the recovery and post-recovery phases of substorms. This is consistent with the finding of Equator-S and Geotail. The origin is a pulsed earthward plasma transport with velocity amplitudes of only several tens of km/s.