Gas dynamics in the impulsive phase of solar flares. II - The structure of the transition region - A diagnostic of energy transport processes

Abstract

Optically thin line emissiosn formed in the solar transition region provides sensitive diagnostics of the atmospheric structure at these levels. Observations of such emission lines during the impulsive phase of solar flares show that a good correlation exists between the energy input rate (as evidenced by, for example, the hard X-ray flux) and the intensity I of a given line, and thus also the differential emission measure quantity DEM = n/sup 2/dx/d ln T, where n is the density and dx/d ln T is the temperature scale length. In this paper, we calculate and contrast the predicted temporal behavior of DEM for two models of energy transport in a flaring loop. We find that a model in which the principal mechanism of energy transport is thermal conduction cannot satisfactorily reproduce the basic observational trend noted above. On the other hand, a model in which the energy is transported throughout the flaring loop principally by collisional degradation of a beam of accelerated suprathermal electrons does adequately reproduce observed behavior. We therefore conclude that electron beam heating is a likely candidate for energy transport in the impulsive phase of flares.