Dynamics of braided coronal loops

II. Cascade to multiple small-scale reconnection events

¹ Division of MathematicsUniversity of Dundee, Nethergate, Dundee, DD1 4HN, UK
e-mail: dpontin@maths.dundee.ac.uk
² Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen Ø, Denmark

Received: 30 March 2010
Accepted: 18 October 2010

Abstract

Aims. Our aim is to investigate the resistive relaxation of a magnetic loop that contains braided magnetic flux but no net current or helicity. The loop is subject to line-tied boundary conditions. We investigate the dynamical processes that occur during this relaxation, in particular the magnetic reconnection that occurs, and discuss the nature of the final equilibrium.

Methods. The three-dimensional evolution of a braided magnetic field is followed in a series of resistive MHD simulations.

Results. It is found that, following an instability within the loop, a myriad of thin current layers forms, via a cascade-like process. This cascade becomes more developed and continues for a longer period of time for higher magnetic Reynolds number. During the cascade, magnetic flux is reconnected multiple times, with the level of this “multiple reconnection” positively correlated with the magnetic Reynolds number. Eventually the system evolves into a state with no more small-scale current layers. This final state is found to approximate a non-linear force-free field consisting of two flux tubes of oppositely-signed twist embedded in a uniform background field.