On the Time Variability of Coronal Heating

We derive constraints on the time variability of coronal heating from observations of the so-called active-region moss by the Transition and Coronal Explorer (TRACE). The moss is believed to be due to million-degree emission from the transition regions at the footpoints of coronal loops whose maximum temperatures are several million degrees. The key point of the TRACE observations is that in the moss regions one generally sees only moss, and not million degree loops. TRACE movies showing this result will be presented. We will demonstrate using both analytic and numerical calculations, that the lack of observable million-degree loops in the moss regions places severe constraints on the possible time variability of coronal heating in the loops overlying the moss. In particular, the heating in the hot moss loops cannot be truly flare-like with a sharp cutoff, but instead, must be quasi-steady to an excellent approximation. The implications of this result for coronal heating models will be discussed. This work was supported in part by NASA and ONR