Self-Similar Magnetohydrodynamics - Part Four - the Physics of Coronal Transients
Abstract
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 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 γ ≠ 4/3, raising interesting questions of heating and cooling in an expanding plasma. For future interest, an appendix is attached to extend the self-similar theory to allow for three-dimensional distributions of the magnetic field and plasma and to incorporate Newtonian self-gravity.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 1984
- DOI:
- 10.1086/162110
- Bibcode:
- 1984ApJ...281..392L