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Ann. Geophys., 22, 2391-2401, 2004
www.ann-geophys.net/22/2391/2004/
© European Geosciences Union 2004
Cluster observes formation of high-beta plasma blobs
G. Haerendel1,2, E. Georgescu2,3, K. H. Glassmeier4, B. Klecker2, Y. V. Bogdanova2,5, H. Rème6, and H. Frey7
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.
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