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Ann. Geophys., 22, 951-970, 2004
www.ann-geophys.net/22/951/2004/
© European Geosciences Union 2004
Relation of polar auroral arcs to magnetotail twisting and IMF rotation: a systematic MHD simulation study
A. Kullen1 and P. Janhunen2
1Alfvén Laboratory, Royal Institute of Technology, Stockholm, Sweden
2Geophysical Research, Finnish Meteorological Institute, Helsinki, Finland
Abstract. We investigate with the help of a magnetohydrodynamic (MHD) model how the
large-scale
topology of the magnetosphere develops for a constant interplanetary
magnetic field (IMF) with different
IMF clock angles and for an IMF By sign change during northward IMF.
A detailed examination of the topological changes in the tail and the
ionosphere for different IMF conditions shows a good agreement with
observational results.
The MHD simulations for different constant IMF clock angle cases
show the expected field-line bending and tail twisting for nonzero
IMF By.
The tail becomes longer and at its tailward end stronger twisted for
IMF Bz>∣By∣ than for IMF Bz<∣By∣. The field
lines originating in the high-latitude flank of the far-tail plasma
sheet map into the near-Earth tail lobes and to a strongly poleward
displaced polar cap boundary. A comparison with observations suggests
that an oval-aligned arc may occur on the high-latitude part of the
polar cap boundary.
An IMF By sign change causes large deformations of the tail.
After the IMF By flip the near-Earth and far-tail plasma sheet
regions are oppositely twisted which causes in the near-Earth tail a
bifurcation of the closed field line region that moves from one flank to
the other.
The bifurcated part of the closed field line region maps to a bridge
of closed field lines moving over the entire polar cap. This moving
bridge may be interpreted as the mapped region of a moving
transpolar arc. Based on earlier observations, such a type of polar
arcs is expected to occur after an IMF By sign change.
Key words. Ionosphere (auroral ionosphere; ionospheremagnetosphere
interactions). Space plasma physics (numerical
simulation studies)
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