On the consequences of a non-equilibrium ionisation balance for compact flare emission and dynamics

Abstract

We carry out a hydrodynamic simulation of a compact flare and find significant non-equilibrium distributions for the ionisation balance during the impulsive and gradual phases, which can strongly alter the radiative emission. This has major implications for attempts to derive the theoretical intensities of emission lines used for spectroscopic diagnostic analyses of the plasma properties. During the impulsive phase we find that the emissivities of He I, He II and C IV in the transition region can be strongly enhanced above their expected equilibrium values, followed by a significant reduction which increases the amount of chromo- spheric plasma ablated into the corona. Furthermore, during the flare heating the overall charge state of the coronal ions can be significantly lower than is suggested by an equilibrium ionisation balance and, therefore, line ratio measurements will yield plasma temperatures that are much greater than the formation temperature of the emitting ion. During the gradual phase the emissivity at transition region temperatures remains suppressed, compared with its equilibrium value, with correspondingly reduced downflow velocities and increased radiative cooling time-scales. Finally, we synthesise the emission as it would be detected by TRACE in its 171 A and 195 A wavelength bands, and find that the filter ratio technique can give reasonably good estimates of the plasma temperature in quiescence, though when the populations of Fe VIII, Fe IX, Fe X and Fe XII exhibit departures from equilibrium the temperatures derived from filter ratio measurements become unreliable.