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Ann. Geophys., 23, 885-900, 2005
www.ann-geophys.net/23/885/2005/
© European Geosciences Union 2005
A multi-spacecraft survey of magnetic field line draping in the dayside magnetosheath
I. J. Coleman
British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK
Abstract. When the interplanetary magnetic field (IMF) encounters the Earth's
magnetosphere, it is compressed and distorted. This distortion is known as
draping, and plays an important role in the interaction between the IMF and
the geomagnetic field. This paper considers a particular aspect of draping,
namely how the orientation of the IMF in a plane perpendicular to the
Sun-Earth line (the clock angle) is altered by draping in the magnetosheath
close to the dayside magnetopause. The clock angle of the magnetosheath field
is commonly estimated from the interplanetary magnetic field (IMF) measured
by upstream monitoring spacecraft either by assuming that the draping process
does not significantly alter the clock angle ("perfect draping") or that
the change in clock angle is reasonably approximated by a gas dynamic model.
In this paper, the magnetosheath clock angles measured during 36 crossings of
the magnetopause by the Geotail and Interball-Tail spacecraft are compared to
the upstream IMF clock angles measured by the Wind spacecraft. Overall, about
30% of data points exhibit perfect draping within ±10°, and 70% are
within 30°. The differences between the IMF and magnetosheath clock
angles are not, in general, well-ordered in any systematic fashion which
could be accounted for by hydrodynamic draping. The draping behaviour is
asymmetric with respect to the y-component of the IMF, and the form of the
draping distribution function is dependent on solar wind pressure. While the
average clock angle observed in the magnetosheath does reflect the
orientation of the IMF to within ~30° or less, the assumption that
the magnetosheath field direction at any particular region of the
magnetopause at any instant is approximately similar to the IMF direction is
not justified. This study shows that reconnection models which assume laminar
draping are unlikely to accurately reflect the distribution of reconnection
sites across the dayside magnetopause.
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