Determination of flare heating and cooling using the Transition Region andCoronal Explorer

We describe how the Transition Region and Coronal Explorer 171 Angstrom obs ervations can be used to determine the properties of flare-loop heating. Th e key point is that the evolution of a loop transition region (TR) is much easier to measure quantitatively than the bulk flare plasma because the TR emission originates from an unobscured source with simple geometry. We deri ve general analytic expressions for the evolution of a flare-loop TR that, in principle, permit a determination of the heating function from the obser vations. These results are compared with observations of the 1998 September 20 flare. We find that the observed evolution of the flare ribbons is in g ood agreement with our model for the evaporative cooling of flare loops and that the heating in these loops is incompatible with the assumption of spa tial uniformity.