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Ann. Geophys., 17, 1560-1573, 1999
www.ann-geophys.net/17/1560/1999/
© European Geosciences Union 1999
Identification of magnetosheath mirror modes in Equator-S magnetic field data
E. A. Lucek1, M. W. Dunlop1, A. Balogh1, P. Cargill1, W. Baumjohann2, E. Georgescu2,4, G. Haerendel2, and G.-H. Fornacon3
1The Blackett Laboratory, Imperial College, London SW7 2BZ, UK
E-mail: e.lucek@ic.ac.uk
2Max-Planck Institute für extraterrestrische Physik, Garching, Germany
3Institut für Geophysik und Meteorologie, Technische Universtät Braunschweig, Braunschweig, Germany
4Institute of Space Sciences, Bucharest, Romania
Abstract. Between December 1997 and March 1998
Equator-S made a number of excursions into the dawn-side magnetosheath, over a range of local times between
6:00 and 10:40 LT. Clear mirror-like structures, characterised by compressive fluctuations in |B| on occasion
lasting for up to 5 h, were observed during a significant
fraction of these orbits. During most of these passes the
satellite appeared to remain close to the magnetopause
(within 1–2 Re), during sustained compressions of the
magnetosphere, and so the characteristics of the mirror
structures are used as a diagnostic of magnetosheath
structure close to the magnetopause during these orbits.
It is found that in the majority of cases mirror-like
activity persists, undamped, to within a few minutes of
the magnetopause, with no observable ramp in |B|,
irrespective of the magnetic shear across the boundary.
This suggests that any plasma depletion layer is typically
of narrow extent or absent at the location of the satellite,
at least during the subset of orbits containing strong
magnetosheath mirror-mode signatures. Power spectra
for the mirror signatures show predominately field
aligned power, a well defined shoulder at around 3–10 x 10
–2 Hz and decreasing power at higher frequencies.
On occasions the fluctuations are more sinusoidal,
leading to peaked spectra instead of a shoulder. In all
cases mirror structures are found to lie approximately
parallel to the observed magnetopause boundary. There
is some indication that the amplitude of the compressional
fluctuations tends to be greater closer to the
magnetopause. This has not been previously reported in
the Earth's magnetosphere, but has been suggested in
the case of other planets.
Key words.
Magnetospheric physics (magnetosheath;
plasma waves and instabilities; magnetopause, cusp
and boundary layers)
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