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Ann. Geophys., 22, 2151-2169, 2004
www.ann-geophys.net/22/2151/2004/
© European Geosciences Union 2004
Observed and simulated depletion layers with southward IMF
N. C. Maynard1, W. J. Burke2, J. D. Scudder3, D. M. Ober1, G. L. Siscoe4, W. W. White1, K. D. Siebert1, D. R. Weimer1, G. M. Erickson4, J. Schoendorf1, and M. A. Heinemann2
1ATK Mission Research, Nashua, New Hampshire, USA
2Air Force Research Laboratory, Hanscom Air Force Base, Massachusetts, USA
3Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA
4Center for Space Physics, Boston University, Boston, Massachusetts, USA
Abstract. We present observations from the Polar satellite that confirm the existence
of two types of depletion layers predicted under southward interplanetary
magnetic field (IMF) conditions in magnetohydrodynamic simulations. The first
depletion type occurs along the stagnation line when IMF BX and/or dipole
tilt are/is present. Magnetic merging occurred away from the equator (Maynard
et al., 2003) and flux pile-ups developed while the field lines drape to the
high-latitude merging sites. This high-shear type of depletion is consistent
with the depletion layer model suggested by Zwan and Wolf (1976) for low-shear
northward IMF conditions. Expected sites for depletion layers are associated
with places where IMF tubes of force first impinge upon the magnetopause. The
second depletion type develops poleward of the cusp. Under strongly driven
conditions, magnetic fields from Region 1 current closure over the lobes
(Siscoe et al., 2002c) cause the high-latitude magnetopause to bulge outward,
creating a shoulder above the cusp. These shoulders present the initial
obstacle with which the IMF interacts. Flow is impeded, causing local flux
pile-ups and low-shear depletion layers to form poleward of the cusps.
Merging at the high-shear dayside magnetopause is consequently delayed. In
both low- and high-shear cases, we show that the depletion layer structure is
part of a slow mode wave standing in front of the magnetopause. As suggested
by Southwood and Kivelson (1995), the depletions are rarefactions on the
magnetopause side of slow-mode density compressions. While highly sheared
magnetic fields are often used as proxies for ongoing local magnetic merging,
depletion layers are prohibited at merging locations. Therefore, the
existence of a depletion layer is evidence that the location of merging must
be remote relative to the observation.
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