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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 22, issue 3
Ann. Geophys., 22, 951–970, 2004
https://doi.org/10.5194/angeo-22-951-2004
© Author(s) 2004. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 22, 951–970, 2004
https://doi.org/10.5194/angeo-22-951-2004
© Author(s) 2004. This work is distributed under
the Creative Commons Attribution 3.0 License.

  19 Mar 2004

19 Mar 2004

Relation of polar auroral arcs to magnetotail twisting and IMF rotation: a systematic MHD simulation study

A. Kullen1 and P. Janhunen2 A. Kullen and P. Janhunen
  • 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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