Self-similar magnetohydrodynamics. IV. The physics of coronal transients

In this theoretical study, the white-light coronal transient is regarded to be a fully developed magnetohydrodynamic flow that plows into a preexisting ambient atmosphere. To keep the mathematical problem simple, a model is considered where the ambient atmosphere has no magnetic field whereas the outflow carries a substantial axisymmetric magnetic field. A contact surface forms to drive a strong gasdynamic shock that travels ahead and compresses the ambient atmosphere. Such a global flow, with the effect of gravity included, is illustrated with a set of exact, analytic, self-similar solutions of magnetohydrodynamics. The governing nonlinear equations, derived in the first paper of this series, are solved with a general technique, constructing explicitly the gasdynamic shock and the trailing contact surface. Various types of magnetic field configurations 1 in the outflow behind the contact surface are shown to give rise to plasma structures which resemble those commonly observed in white-light transients, such as loops, voids, and blobs. It is advocated that the coronal transient is a result of a hydromagnetic system becoming gravitationally unstable in the low corona. This takes place when the magnetic tension force and solar gravity fail to counter the natural tendency of a magnetized plasma to expand. A physical picture of this dynamic process is described, and some quantitative properties to relate to observation are pointed out. The original self-similar theory requires an adiabatic index γ = 4/3. It is shown that this constraint can be relaxed to γnot = 4/3, raising interesting questions of heating and cooling in an expanding plasma