Plasma parameters derived from MSDP observations of cool flare loops

Abstract

MSDP observations of 16 May 1981 two-ribbon flare are used to study the physical structure and dynamical behaviour of cool flare loops. The loops have been detected in the Ha line just after the flare maximum and they appeared in absorption against the disk. Using the first-order differential cloud model (DCMI) technique, we derive empirically some basic plasma parameters at 15 points along one loop leg. The flow velocities and true heights have been reconstructed with respect to a geometrical projection. Subsequently, detailed non-LTE models of cool loops have been constructed in order to fit Ha source-function values previously derived from DCMI analysis. It is demonstrated that this source function is rather sensitive to the radial component of the flow velocity (the so-called Doppler brightening) and to the enhanced irradiation of the loops from the underlying flare ribbons. In this way, we have been able to estimate quantitatively all plasma parameters which determine the physical structure of cool loops (i.e. the temperature, pressure, density, etc.), as well as the momentum-balance condition within the loops. For these dark loops we have arrived at relatively low gas pressures of -2the order of 0. I - 0.5 dyn cm, with corresponding electron densities around I0 II cm -3. Pressure-gradient forces have been found to be of small importance in the momentum-balance equation. However, the classical momentum equation is not capable of explaining significant departures from a free-fall motion under the gravity force, found from our MSDP data. We propose three possible explanations to this problem.