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Extrapolation of a nonlinear force-free field containing a highly twisted magnetic loop View all abstracts by submitter

Bernhard Kliem   Submitted: 2004-12-15 15:33

The stress-and-relax method for the extrapolation of nonlinear force-free coronal magnetic fields from photospheric vector magnetograms is formulated and implemented in a manner analogous to the evolutionary extrapolation method. The technique is applied to a numerically constructed force-free equilibrium that has a simple bipolar structure of the normal field component at the bottom but contains a highly twisted loop and a shear (current) layer, with a smooth but strong variation of the force-free parameter α in the magnetogram. A standard linear force-free extrapolation of this magnetogram, using the so-called α _mathrm{best} value, is found to fail in reproducing the twisted loop (or flux rope) and the shear layer; it yields a loop pair instead and the shear is not concentrated in a layer. With the nonlinear extrapolation technique, the given equilibrium is readily reconstructed to a high degree of accuracy if the magnetogram is sufficiently resolved. A parametric study quantifies the requirements on the resolution for a successful nonlinear extrapolation. Permitting magnetic reconnection by a controlled use of resistivity improved the extrapolation of a magnetogram which contained structures with scales at the resolution limit.

Authors: G. Valori, B. Kliem, R. Keppens
Projects: None

Publication Status: A&A, in press
Last Modified: 2004-12-15 15:33
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