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 observe d information concerning the dimensions and energy-loss rate of each l oop, a range of loop models with different temperatures, emission meas ures, 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, al though the uncertainties can be quite large. For cool loops (T approxi mate to 2 x 10(6) K), the model cannot reproduce the observed temperat ure 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 materi al is injected into the cooler loops by a mechanism that is not direct ly related to heating (e.g., not chromespheric evaporation).