A New Model for Propagating Parts of EIT Waves: A Current Shell in a CME |
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Authors: | C Delannée T Török G Aulanier J-F Hochedez |
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Institution: | (1) Royal Observatory of Belgium, Brussels, Belgium;(2) Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK;(3) LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France |
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Abstract: | EIT waves are observed in EUV as bright fronts. Some of these bright fronts propagate across the solar disk. EIT waves are
all associated with a flare and a CME and are 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 nondimensional 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 the narrow electric currents
sheets that appear in the simulations. Both codes give similar results, which we confront with two well-known SOHO/EIT observations
of propagating EIT waves (7 April and 12 May 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 intensifies as the flux tube accelerates and it is co-spatial with weak plasma compression. The current density integrated
over the altitude has the 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. |
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Keywords: | MHD Sun: corona Sun: coronal mass ejections (CMEs) Sun: magnetic fields Sun: activity Sun: flare |
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