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We have simulated the cooling of coronal flare plasma (T e > 10 7 K) using a numerical model of a vertical magnetic flux tube containing an idealized flare chromosphere, transition region, and corona. The model solves the set of one-dimensional, two-fluid hydrodynamic equations. The cooling of the flux tube is calculated for a specific case beginning with an initial atmosphere in hydrostatic equilibrium and a maximum temperature of about 18 x 10 6 K. The behavior of temperature, density, and velocity is calculated as a function of height as the system cools. Early in the cooling, energy is transported by conduction into the transition region and chromosphere where it is radiated away. Later, the transition region-corona interface moves upward into the tube at velocities of about 20 km s-1 , while the chromosphere cools and the coronal component cools by both conduction and radiation. Coronal downflow velocities of about 60 km s _1 are evident during this phase. The expected spectral line emission from the system in X-ray lines of Fe xxv, Fe xxiv, Fe xxn, O vm, and O vu is also calculated and compared to recent observational results. Some observational results can be explained as a consequence of simple cooling of flare flux tubes. The expected spectral line emission from certain transition region lines is also briefly considered. The dependence of our results on flare size is discussed, and our results are compared with similar previous work.

We investigate the stability of plasmas at temperatures and densities typical of the solar transition region and corona using both a linear analysis and nonlinear time-dependent numerical simulations. Growth rates, decay rates, and oscillation frequencies of the perturbations determined from the linear analysis are in excellent agreement with the simulations. The nonlinear regime is characterized by a bifurcation of the plasma into a cool dense condensation surrounded by a hot tenuous corona. The condensation may then be accelerated by forces in the plasma such as those arising from gravity or differential heating. Finally, the results of the detailed simulation show that the transition region is a dynamically stable structure which is the result of the nonlinear evolution of the condensational instability. Subject headings: instabilities-plasmas-Sun: corona

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