Reconnection of Flux Tubes—Specifics of High Plasma ${\varvec{\beta }}$

Abstract

Two fundamental mechanisms of magnetic reconnection worked out by Sweet-Parker tandem (Parker (1957); Sweet (1958) and Petschek (1964)), were both aimed to explain an enormous amount of energy produced by coronal flares. Since then extensive studies of a topological change of magnetic field resulted in a huge body of refined reconnection theories, beautiful laboratory experiments, and numerical modeling. One thing, however, remained for a long time unchanged: the subject of studies, magnetized plasma, has been a priori considered magnetically dominated, i.e., having very low plasma beta, $\beta =8\pi p/B^2\ll 1$. It is just this condition that provides release of a huge amount of energy stored in magnetic fields due to their topological change. This simple fact dominated so strongly that during almost six decades there was no attempt to investigate the opposite situation, i.e., when topologically favorable conditions for reconnection appear in gas-dominated plasma with finite or even larger than unity plasma beta, $\beta =8\pi p/B^2\ge 1$. As the quest was for a large amount of magnetic energy, the case of high beta plasma (with magnetic energy less that gas-kinetic energy) did not seem promising. This, however, proved to be wrong. It turned out that in case of small-scale magnetic flux tubes under real conditions of solar photosphere, high $\beta$ reconnection is unavoidable process. In the photosphere magnetic flux, tubes buffeted by convective motions collide and reconnect. True that the photospheric reconnection does not give the immediate gain in energy, but it sets the system in strongly unstable state. Now the central problem becomes to understand how the post-reconnection products evolve. In this chapter we shall study the peculiarities of high $\beta$ reconnection, and what is more important, the post-reconnection processes. We shall see that high beta reconnection triggers various nonlinear processes that are responsible for a wide range of observed phenomena.