EDP Sciences
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Volume 400, Number 1, March II 2003
Page(s) 355 - 367
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20021887

A&A 400, 355-367 (2003)
DOI: 10.1051/0004-6361:20021887

Solar coronal heating by relaxation events

P. K. Browning1 and R. A. M. Van der Linden2

1  Department of Physics, UMIST, Manchester, UK
    e-mail: Philippa.Browning@umist.ac.uk
2  Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
    e-mail: Ronald.VanderLinden@oma.be

(Received 11 November 2002 / Accepted 20 December 2002 )

A coronal heating model is proposed which predicts heating by a series of discrete events of various energies, analogous to the observed range of events from large scale flares through various transient brightening phenomena down to the often discussed "nanoflares". We suggest that an energy release event occurs when a field becomes linearly unstable to ideal MHD modes, with dissipation during the nonlinear phase of such an instability due to reconnection in fine-scale structures such as current sheets. The energy release during this complex dynamic period can be evaluated by assuming the field relaxes to a minimum energy state subject to the constraint of helicity conservation. A model problem is studied: a cylindrical coronal loop, with a current profile generated by slow twisting of the photospheric footpoints parameterised by two values of $\alpha$ (the ratio of current density to field strength). Different initial $\alpha$ profiles, corresponding to different footpoint twisting profiles, lead to energy release events of a wide range of magnitudes, but our model predicts an observationally realistic minimum size for these events.

Key words: Sun: corona -- Sun: magnetic fields -- magnetohydrodynamics (MHD) -- plasmas

Offprint request: P. K. Browning, Philippa.Browing@umist.ac.uk

© ESO 2003