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M. G. Linton (Naval Research Laboratory), G. H. Fisher (Space Sciences Laboratory, UC Berkeley), D. W. Longcope (Montana State University), R. B. Dahlburg (Naval Research Laboratory), Y. Fan (HAO/UCAR)
The current driven kink instability may be the cause of both the unusual morphology of solar \delta-spot active regions and the tendency of these regions to be significantly more flare active than most active regions. We investigate the current driven kink instability of flux tubes in the solar interior both with a linear stability analysis and with nonlinear MHD simulations. The linear analysis shows that there is a critical twist, which depends on the axial magnetic field profile, that a flux tube needs to become kink unstable. This critical twist decreases as the tube expands, so twisted flux tubes will become increasingly unstable as they rise through the convection zone. The nonlinear simulations show that a twisted tube excited by a single unstable kink mode will evolve to a helical equilibrium state. The emergence through the photosphere of such a kinked tube would create an active region which was tilted with respect to Hale's law and which would rotate as it evolved, as \delta-spots are observed to do. We then find that, when excited by multiple unstable kink modes, highly twisted flux tubes develop concentrated kinks. These concentrated kinks would produce more of the observed characteristics of \delta-spot active regions. They would create active regions which, in addition to emerging tilted and then rotating, would remain compact as they evolved, and develop strong shear along their magnetic neutral line. Finally, we find that a strong concentrated kink develops a current sheet at which the magnetic field reconnects, which may be the cause of the high flare activity of \delta-spots.
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