The Formation of Solar Prominences

Abstract

The formation of solar prominences is an interesting example of development of cool condensations that are not due to gravitational forces. It is widely believed that the physical processes operating in this case may also be relevant in different astrophysical systems, like the supernova remnants or the nuclei of active galaxies. The basic mechanism that drives the formation of filamentary structures is the thermal instability, that can be physically described as the tendency of a radiating plasma to react to a cooling condensation by an increase in the radiative output. This in turn produces a further cooling, until a state is reached where this tendency is reversed. In an optically thin plasma of solar composition the thermally unstable behaviour can occur for temperatures roughly in the range 4x104≲T≲107K. Thermal conduction, on the other hand, strongly opposes the formation of regions of different temperatures. We thus conclude that situations in which thermal conduc-tion is inhibited are those in which most likely prominences are formed. There are two further physical parameters that influence the possibility of prominence formation and the rate at which they form. These are, of course, gravity and magnetic fields. The weight of the filament must be supported against gravitational forces and this can be done by the magnetic field. This latter has also the effect of channelling the heat flux along field lines and thus to act as an effective thermal insula-tor for perturbations across \(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{B} \).KeywordsField LineThermal InstabilitySupernova RemnantFilamentary StructureUnstable BehaviourThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.