Simple emergence structures from complex magnetic fields

School of Mathematics and Statistics, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, UK e-mail: michelle@mcs.st-and.ac.uk

Received: 7 February 2007
Accepted: 17 April 2007

Abstract

Context.There has been suggestions that the initial subsurface magnetic field in flux emergence models is too organised. Thus, it may be that the results they yield are very specific and cannot be applied to the generic flux emergence process occurring on the Sun.

Aims.We will demonstrate that the interaction of two flux tubes can increase the complexity of the solar interior's magnetic field. However, the subsequent emergence of this complex field results in the atmospheric flux having a simple and generic structural form.

Methods.Using a numerical 3D MHD code, we consider the evolution of two flux tubes within a vertically stratified domain, representing the upper layers of the solar interior through to the corona. The flux tubes are initially situated in the solar interior, one tube lying directly above the other. By choosing the lower tube to be buoyant, whilst the upper tube is in mechanical equilibrium with its surroundings, we encourage the interaction of the two tubes within the solar interior. Thus, we will create a more complex magnetic field lying just below the photospheric surface than would be generated with a single flux tube model. By studying the subsequent emergence and structure of the resulting atmospheric field we will be able to identify if there are any significant changes due to the increased complexity of the field below the surface.

Results.The reconnection process modifies the topology of the subsurface magnetic field as indicated by the results of previous simulations. Since reconnection is occurring in a high plasma-β region, where the sound speed is greater than the Alfvén speed, we observe no significant jets and also find no significant heating. When this reconnected field comes to the surface it is complex but the emergence occurs in much the same manner as for a single flux tube and, therefore, produces very little change in the atmospheric field. This strongly indicates that the results of previous emergence simulations are valid for both simple and complex subsurface fields.