Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003a%26a...406.1043t&link_type=abstract
Astronomy and Astrophysics, v.406, p.1043-1059 (2003)
Astronomy and Astrophysics
Astrophysics
83
Mhd, Sun: Activity, Sun: Corona, Sun: Coronal Mass Ejections (Cmes), Sun: Flares, Sun: Magnetic Fields
Scientific paper
We simulate the twisting of an initially potential coronal flux tube by photospheric vortex motions, centred at two photospheric flux concentrations, using the compressible zero-beta ideal MHD equations. A twisted flux tube is formed, surrounded by much less twisted and sheared outer flux. Under the action of continuous slow driving, the flux tube starts to evolve quasi-statically along a sequence of force-free equilibria, which rise slowly with increasing twist and possess helical shape. The flux bundle that extends from the location of peak photospheric current density (slightly displaced from the vortex centre) shows a sigmoidal shape in agreement with observations of sigmoidal soft X-ray loops. There exists a critical twist, above which no equilibrium can be found in the simulation and the flux tube ascends rapidly. Then either stable equilibrium ceases to exist or the character of the sequence changes such that neighbouring stable equilibria rise by enormous amounts for only modest additions of twist. A comparison with the scalings of the rise of flux in axisymmetric geometry by \cite{Stur:al-95} suggests the former. Both cases would be observed as an eruption. The critical end-to-end twist, for a particular set of parameters describing the initial potential field, is found to lie in the range 2.5pi
Kliem Bernhard
Torok Tibor
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