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Guillaume Aulanier   Submitted: 2007-09-26 02:32

EIT waves are observed in EUV as bright fronts. Some of these bright fronts propagate across the solar disc. EIT waves are all associated with a flare and a CME and flare commonly interpreted as fast-mode magnetosonic waves. Propagating EIT waves could also be the direct signature of the gradual opening of magnetic field lines during a CME. We quantitatively addressed this alternative interpretation. Using two independent 3D MHD codes, we performed non-dimensional numerical simulations of a slowly rotating magnetic bipole, which progressively result in the formation of a twisted magnetic flux tube and its fast expansion, as during a CME. We analyse the origins, the development and the observability in EUV of narrow electric currents sheets which appear in the simulations. Both codes give similar results which we confront with two well-known SoHO/EIT observations of propagating EIT waves (April 7 and May 12, 1997), by scaling the vertical magnetic field components of the simulated bipole to the line of sight magnetic field observed by SoHO/MDI and the sign of helicity to the orientation of the soft X-ray sigmoids observed by Yohkoh/SXT. A large-scale and narrow current shell appears around the twisted flux tube in the dynamic phase of its expansion. This current shell is formed by the return currents of the system, which separate the twisted flux tube from the surrounding fields. It intensities as the flux tube accelerates and it is co-spatial with weak plasma compression. The current density integrated over the altitude has a shape of an ellipse which expands and rotates when viewed from above, reproducing the generic properties of propagating EIT waves. The timing, orientation and location of bright and faint patches observed in the two EIT waves are remarkably well reproduced. We conjecture that propagating EIT waves are the observational signature of Joule heating in electric current shells, which separate expanding flux tubes from their surrounding fields during CMEs or plasma compression inside this current shell. We also conjecture that the bright edges of halo CMEs show the plasma compression in these current shells.

Authors: C. Delannee, T. Torok, G. Aulanier, J.-F. Hochedez

Publication Status: Solar Physics (published)
Last Modified: 2008-02-15 00:59
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