Magnetic skeletons and 3D magnetic reconnection

Abstract

The upper atmosphere of the sun, the solar corona, is approximately 1,000,000K hotter than the surface of the Sun, a property which cannot be explained by the normal processes of heat conduction and radiation. It is now commonly believed that the magnetic fields which fill the solar atmosphere, and propagate down into the interior of the Sun, are important for transferring and transforming energy from the strong plasma flows inside the Sun into the corona as heat. I have investigated an elementary flux interaction which forms a fundamental building block of the coronal heating process. This interaction involves two opposite polarity sources on the Sun's surface in the presence of an overlying magnetic field. To fully understand how this interaction transfers heat into the solar corona, the magnetic skeleton is required, which shows possible sites of heating that are due to magnetic reconnection.

A magnetic field is best described by its magnetic skeleton. The most important parts of the magnetic skeleton to find are the null points, from which separatrix surfaces extend that divide magnetic flux of different topology. Part of this thesis proposes a new method of finding null points, for which the accuracy is shown and then compared with another commonly used method (which gave false results).

Using these techniques for finding the magnetic skeleton in the magnetic interaction above, the evolution of the skeleton was found to head through seven distinct states, some of which were far more complicated than expected. This included a high number of separators (the intersection of two separatrix surfaces), which are a known location of magnetic reconnection. This separator reconnection was shown to be the main heating mechanism in this interaction, from which the total amount and rates of reconnection in the experiment was calculated. This led to the discovery of recursive reconnection, a process where magnetic flux is reconnected before reconnecting back to its original state, to allow for the process to repeat again. This recursive reconnection was shown to allow far more reconnection than would have been previously expected, all of which releases heat into the neighbouring areas of the atmosphere.

Finally, the interaction was modelled with sources of different magnetic radii but of equal flux. This showed that when the antisymmetric nature of the previous interactions was removed, there was little change in the reconnection rates, but when the strength of the overlying magnetic field was increased, the reconnection rates were found to increase. This increase in the overlying magnetic field strength also produced a new magnetic feature called a bald-edge, which was found to replace some of the null points. These bald-edges were found to be associated with surfaces similar to separatrix surfaces that divide flux of different topology but do not extend from a null point. Also features similar to separators extend from these bald-edges.