A Nanoflare Explanation for the Heating of Coronal Loops Observed by Yohkoh

Abstract

The nanoÑare model of Cargill (1994a) is used to model active region loops observed by the Y ohkoh Soft X-ray Telescope (SXT). Using observed information concerning the dimensions and energy-loss rate of each loop, a range of loop models with di erent temperatures, emission measures, and Ðlling factors is generated. For hot loops (T [ 4 ] 106 K), it is shown that Ðlling factors less than 0.1 can Ðt the data, although the uncertainties can be quite large. For cool loops (T B 2 ] 106 K), the model cannot repro-duce the observed temperature and emission measure for any value of the Ðlling factor. Earlier work of Porter & Klimchuk suggested that some of these loops cannot be explained by a steady state heating model either. It is proposed that there may exist two distinct classes of loops and that coronal material is injected into the cooler loops by a mechanism that is not directly related to heating (e.g., not chromo-spheric evaporation).