THERMALLY UNSTABLE PERTURBATIONS IN STRATIFIED CONDUCTING ATMOSPHERES

We investigate the thermal stability of isobaric perturbations in a st ratified isothermal background atmosphere with solar abundances, as re sulting from the competition of optically thin plasma radiative coolin g and of heating conducted from the surrounding atmosphere. We have an alyzed the threshold line between stable and unstable perturbations, i n the plane of the two important control parameters: the initial size of the perturbation and the temperature of the unperturbed medium; thi s line changes with the pressure of the unperturbed atmosphere. We hav e extended the results of linear perturbation analysis by means of num erical calculations of the evolution of spherical isobaric perturbatio ns, using a two-dimensional hydrodynamic code including Spitzer heat c onduction. We explore a wide range of the parameters appropriate to th e solar and stellar upper atmospheres: the background uniform temperat ure is between 10(5) K and 10(7) K, the initial pressure between 0.1 a nd 10 dyn cm-2, and the perturbation size between 10(5) and 10(10) cm. The numerical results are in substantial agreement with the linear an alysis. We discuss possible implications of our results also in terms of observable effects, especially concerning plasma downflows, and pro pose thermal instability as a possible candidate to explain the observ ed redshifts in solar and stellar transition region lines.