Structure and stability of the solar corona

Abstract

A personal review of the structure and stability of solar coronal plasmas is given. Firstly the structure of the corona is presented in two parts, namely models for coronal arcades and coronal loops. Each topic is dealt with using two different approaches. One approach is to solve the equilibrium equations directly and the other is to solve the time dependent equations with a slow photospheric motion prescribed. In this latter approach, the plasma evolves through a sequence of approximate equilibria with dynamic behaviour occurring only when the photospheric field distribution is sufficiently complex. The evolution of coronal loops shows that the internal magnetic field tends to contract whereas the external field tends to expand. Here it is interesting to note that the simple twisting of a straight cylindrical field generates substantial structuring in the coronal field, with a strong current concentration on the magnetic axis. Secondly, the stability of the above equilibrium structures is discussed. Three-dimensional disturbances are considered and some of the numerical methods used are described. The main focus of attention here is the ideal and resistive MHD stability properties of the magnetic field. General results are hard to come but numerical results have suggested that all force-free coronal arcades are stable unless a magnetic island exists. Some references to other non-ideal instabilities, in particular thermal instabilities, are mentioned.