Metastable Magnetic Configurations and Their Significance for Solar Eruptive Events

Abstract

Solar flares and coronal mass ejections (CMEs) involve the sudden release of magnetic energy that can lead to the ejection from the Sun of large masses of gas with entrained magnetic field. In dynamical systems, such sudden events are characteristic of metastable configurations that are stable against small perturbations but unstable to sufficiently large perturbations. Linear stability analysis indicates whether or not the first requirement is met, and energetic analysis can indicate whether or not the second requirement is met: if a magnetic configuration that is stable against small perturbations can make a transition to a lower energy state, then it is metastable. In this paper, we consider a long twisted flux tube, anchored at both ends in the photosphere and restrained by an overlying magnetic arcade. We argue from a simple order-of-magnitude calculation that, for appropriate parameter values, it is energetically favorable for part of the flux tube to erupt into interplanetary space, even when the configuration is stable according to linear MHD stability theory. The properties of metastable magnetic configurations may be relevant to CMEs and to other explosive astrophysical events such as solar flares.