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Ann. Geophys., 17, 1592-1601, 1999
www.ann-geophys.net/17/1592/1999/
© European Geosciences Union 1999
High-beta plasma blobs in the morningside plasma sheet
G. Haerendel1, W. Baumjohann1, E. Georgescu1, R. Nakamura1, L. M. Kistler2, B. Klecker1, H. Kucharek1, A. Vaivads1, T. Mukai3, and S. Kokubun4
1Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
2University of New Hampshire, Durham, NH, USA
3Institute of Space and Astronautical Science, Sagamihara, Japan
4Solar-Terrestrial Environment Lab., Nagoya University, Toyokawa, Japan
Abstract. Equator-S frequently encountered, i.e. on 30%
of the orbits between 1 March and 17 April 1998, strong variations of the
magnetic field strength of typically 5–15-min duration outside about 9RE
during the late-night/early-morning hours. Very high-plasma beta values were
found, varying between 1 and 10 or more. Close conjunctions between Equator-S
and Geotail revealed the spatial structure of these "plasma blobs" and
their lifetime. They are typically 5–10° wide in longitude and have
an antisymmetric plasma or magnetic pressure distribution with respect to the
equator, while being altogether low-latitude phenomena
(≤ 15°). They drift slowly sunward, exchange plasma across
the equator and have a lifetime of at least 15–30 min. While their spatial
structure may be due to some sort of mirror instability, little is known about
the origin of the high-beta plasma. It is speculated that the morningside
boundary layer somewhat further tailward may be the source of this plasma. This
would be consistent with the preference of the plasma blobs to occur during
quiet conditions, although they are also found during substorm periods. The
relation to auroral phenomena in the morningside oval is uncertain. The energy
deposition may be mostly too weak to generate a visible signature. However,
patchy aurora remains a candidate for more disturbed periods.
Key words. Magnetospheric physics (plasma convection;
plasma sheet; plasma waves and instabilities)
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