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Ann. Geophys., 17, 1535-1559, 1999
www.ann-geophys.net/17/1535/1999/
© European Geosciences Union 1999
Dynamics and local boundary properties of the dawn-side magnetopause under conditions observed by Equator-S
M. W. Dunlop1, A. Balogh1, W. Baumjohann2, G. Haerendel2, K.-H. Fornacon3, E. Georgescu2,4, R. Nakamura2, and S. Kokubin5
1Space and Atmospheric Physics Group, Imperial College, London SW7 2BZ, UK
2Max-Planck-Institute für Extraterrestrische Physik, Garching, Germany
3Institut für Geophysik und Meteorologie, TUB, Braunschweig, Germany
4Institute of Space Sciences, Bucharest, Romania
5Solar Terrestrial Environment Lab., Nagoya University, Toyokawa, Aichi 442, Japan
Abstract. Magnetic field measurements, taken by the
magnetometer experiment (MAM) on board the German Equator-S spacecraft, have
been used to identify and categorise 131 crossings of the dawn-side magnetopause
at low latitude, providing unusual, long duration coverage of the adjacent
magnetospheric regions and near magnetosheath. The crossings occurred on 31
orbits, providing unbiased coverage over the full range of local magnetic shear
from 06:00 to 10:40 LT. Apogee extent places the spacecraft in conditions
associated with intermediate, rather than low, solar wind dynamic pressure, as
it processes into the flank region. The apogee of the spacecraft remains close
to the magnetopause for mean solar wind pressure. The occurrence of the
magnetopause encounters are summarised and are found to compare well with
predicted boundary location, where solar wind conditions are known. Most scale
with solar wind pressure. Magnetopause shape is also documented and we find that
the magnetopause orientation is consistently sunward of a model boundary and is
not accounted for by IMF or local magnetic shear conditions. A number of
well-established crossings, particularly those at high magnetic shear, or
exhibiting unusually high-pressure states, were observed and have been analysed
for their boundary characteristics and some details of their boundary and near
magnetosheath properties are discussed. Of particular note are the occurrence of
mirror-like signatures in the adjacent magnetosheath during a significant
fraction of the encounters and a high number of multiple crossings over a long
time period. The latter is facilitated by the spacecraft orbit which is designed
to remain in the near magnetosheath for average solar wind pressure. For most
encounters, a well-ordered, tangential (draped) magnetosheath field is observed
and there is little evidence of large deviations in local boundary orientations.
Two passes corresponding to close conjunctions of the Geotail spacecraft are
analysed to confirm boundary orientation and motion. These further show evidence
of an anti-sunward moving depression on the magnetopause (which is much smaller
at Equator-S). The Tsyganenko model field is used routinely to assist in
categorising the crossings and some comparison of models is carried out. We note
that typically the T87 model fits the data better than the T89 model during
conditions of low to intermediate KP index near the
magnetopause and also near the dawn-side tail current sheet in the dawnside
region.
Key words. Magnetospheric physics (magnetopause · cusp
· and boundary layers; magnetosheath; magneto- spheric configuration and
dynamics).
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