THE GAS-DYNAMIC MODEL OF IMPULSIVE STELLAR FLARES

We investigate the response of a plasma in a magnetically confined loo p to intense impulsive energy release during stellar flares. We carry out a numerical simulation of gasdynamic processes in an approximation to a single-fluid, two-temperature, plasma with a possible distinctio n between the ion and electron temperatures taken into account. We pre sent here results of the modelling for an initial model of the red dwa rf atmosphere including the photosphere, the chromosphere, the transit ion region and the corona. Tenuous layers of the upper chromosphere, w hich usually exist in quiescent regions on red dwarfs are also include d in this initial model. This is a fertile field for development of ou r understanding of the process of explosive evaporation. Heating of th e plasma is due to a hard electron beam with an energy of 3.10(11) erg cm(-2) s(-1); this value is based on an analysis of the soft X-ray da ta for stellar flares. The use of a new numerical technique reveals ba sic features of the gasdynamic processes for a single, elementary, hea ting lasting 10 s. In the first 0.1 - 0.2 s, the plasma is heated stro ngly in the upper chromospheric layers, followed by two disturbances w hich subsequently propagate downward and upward from the high pressure region formed. A flow quickly follows, with a temperature jump which moves slowly downwards, and ahead of which travels a radiative shock w ave. Also, hot gas moves outwards from the region of the temperature j ump. Modelling allows us to determine the physical conditions at the s ource of the emission in different spectral regions, and, in particula r, it provides evidence for the thermal origin of optical emission fro m stellar flares. We discuss possibilities for the interpretation of o bservational data of real stellar flares which consist of a set of ele mentary events. This modelling has the advantage that the behaviour of the optical, EUV and soft X-ray radiation can be explained simultaneo usly. Our gas-dynamic modelling may be applied to impulsive stellar fl ares with amplitudes Delta U < 3(m), i.e. until thermal conduction flu xes are smaller than the saturated ones and the return current doesn't limit the penetration of accelerated electrons into the chromosphere.