EDP Sciences
Free access
Issue
A&A
Volume 387, Number 2, May IV 2002
Page(s) 687 - 699
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20020491


A&A 387, 687-699 (2002)
DOI: 10.1051/0004-6361:20020491

The triggering of MHD instabilities through photospheric footpoint motions

C. L. Gerrard1, T. D. Arber2 and A. W. Hood1

1  School of Mathematics and Statistics, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, Scotland, UK
2  Space and Astrophysics Group, Physics Department, University of Warwick, Coventry, CV4 7AL, UK

(Received 9 July 2001 / Accepted 27 March 2002 )

Abstract
The results of 3D numerical simulations modelling the twisting of a coronal loop due to photospheric vortex motions are presented. The simulations are carried out using an initial purely axial field and an initial equilibrium configuration with twist, $\Phi = L B_{\theta} / r B_{z} < \Phi_{\rm crit}$. The non-linear and resistive evolutions of the instability are followed. The magnetic field is twisted by the boundary motions into a loop which initially has boundary layers near the photospheric boundaries as has been suggested by previous work. The boundary motions increase the twist in the loop until it becomes unstable. For both cases the boundary twisting triggers the kink instability. In both cases a helical current structure wraps itself around the kinked central current. This current scales linearly with grid resolution indicating current sheet formation. For the cases studied 35-40% of the free magnetic energy is released. This is sufficient to explain the energy released in a compact loop flare.


Key words: MHD -- Sun: photosphere

Offprint request: C. L. Gerrard, cath@mcs.st-and.ac.uk




© ESO 2002