Are solar coronal loops in thermal equilibrium

Many authors have recently set up static models for coronal loops. In this paper we test the thermal stability of such loops, by developing two simple methods which apply to a wide class of equilibria. Stability is found to depend on the boundary conditions adopted but not critically on the form of the heating. A loop is shown to be stable if its base conductive flux is large enough that it lies on the upper of two equilibrium branches. One particular model that has attracted much attention is the thermally isolated loop, which has a vanishing conductive flux at its base: it is found to be unstable to perturbations that maintain the value of either the base temperature or the base flux. Individual coronal loops may therefore be in a dynamic state of easeless thermal activity unless some stabilizing mechanism exists. (orig.)