Metastability of magnetically supported atmospheres and their relaxation

The linear and nonlinear stability of a hydrodynamic atmosphere against adiabatic perturbations is determined by the well-known Schwarzschild criterion: an atmosphere is stable if its entropy increases with height [1]. However, the generalisation of this criterion to atmospheres that are partially supported by magnetic pressure only guarantees linear stability. In this talk, we demonstrate that there exist “metastable” magnetised atmospheres, which are unstable to large perturbations despite being stable to small ones. We show how the density, pressure and magnetic-flux profiles of metastable atmospheres can be derived analytically, demonstrate their nonlinear relaxation using numerical simulations, and develop a statistical mechanical theory to predict their nonlinearly relaxed states. We discuss applications of the metastability phenomenon to explosive releases of energy in astrophysical and fusion plasmas.

[1] Schwarzschild, K., 1906, Gottinger Nachr., 41.