A nanoflare explanation for the heating of coronal loops observed by Yohkoh

The nanoflare model of Cargill (1994a) is used to model active region loops observed by the Yohkoh Soft X-ray Telescope (SXT). Using observed information concerning the dimensions and energy-loss rate of each loop, a range of loop models with different temperatures, emission measures, and filling factors is generated. For hot loops (T > 4 x 10(6) K), it is shown that filling factors less than 0.1 can fit the data, although the uncertainties can be quite large. For cool loops (T approximate to 2 x 10(6) K), the model cannot reproduce the observed temperature and emission measure for any value of the filling 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 chromespheric evaporation).