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Ann. Geophys., 26, 1461-1477, 2008
www.ann-geophys.net/26/1461/2008/
© European Geosciences Union 2008
SWMF simulation of field-aligned currents for a varying northward and duskward IMF with nonzero dipole tilt
H. Wang1,*, A. J. Ridley1, and H. Lühr2
1Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, MI-48109, USA
2GeoForshungsZentrum Potsdam, 14473 Potsdam, Germany
*On leave from College of Electronic Informatics, Wuhan University, Wuhan 430079, P. R. China
Abstract. This study concentrates on the FACs distribution for the varying
northward and duskward interplanetary magnetic field (IMF)
conditions when the dipole tilt is nonzero. A global MHD
simulation (the Space Weather Modeling Framework, SWMF) has been
used to perform this study. Hemispheric asymmetry of the time
evolution of northward IMF Bz (NBZ) FACs is found. As the IMF
changes from strictly northward to duskward, NBZ FACs shift
counterclockwise in both summer and winter hemispheres. However,
in the winter hemisphere, the counterclockwise rotation prohibits
the duskward NBZ FACs from evolving into the midday R1 FACs. The
midday R1 FACs seem to be an intrusion of dawnside R1 FACs. In the
summer hemisphere, the NBZ FACs can evolve into the DPY FACs,
consisting of the midday R0 and R1 FACs, after the
counterclockwise rotation. The hemispheric asymmetry is due to the
fact that the dipole tilt favors more reconnection between the IMF
and the summer magnetosphere. When mapping the NBZ and DPY FACs
into the magnetosphere it is found that the NBZ currents are
located on both open and closed field lines, irrespective of the
IMF direction. For the DPY FACs the hemispheric asymmetry emerges:
the midday R1 FACs and a small part of R0 FACs are on closed field
lines in the winter hemisphere, while a small part of the midday
R1 FACs and all the R0 FACs are on open field lines in the summer
hemisphere. Both IMF By and dipole tilt cause the polar cap
hemispheric and local time asymmetric. When the IMF is northward,
the summer polar cap is closed on the nightside while the winter
polar cap is open. The polar cap boundary tends to move
equatorward as the IMF rotates from northward to duskward, except
in the summer hemisphere, the polar cap on the dawnside shifts
poleward when the clock angle is less than 10°. The
further poleward displacement of the polar cap boundary on one
oval side is caused by the twist of the tail plasma sheet, which
is in accordance with the changing open field lines topology in
the magnetotail.
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