POST-FLARE LOOPS OF 26 JUNE 1992 .4. FORMATION AND EXPANSION OF HOT AND COOL LOOPS

Observations of the post-flare loops after the X3.9 flare which occurr ed on 25 June, 1992 at 20:11 UT by the Yohkoh/SXT in X-rays, as well a s in Ha: obtained at 5 different observatories, have provided a unique , longest ever, set of data for a study of the relationship between th e hot and cool post-flare loops as they evolve. At any given time, the altitude difference between the hot X-ray loops of 6-7 x 10(6) K and the cool Ha: loops of 1.5 x 10(4) K is related to the expansion rate o f the loop systems and their cooling time. Therefore, measurements of the expansion rate and relative height of hot and cool loops can provi de direct observational values for their cooling times. We measured th e altitude of hot and cool loops for 15 and 19 hours, respectively, an d found that the cooling time increased as the density of the loops de creased. We found a reasonably good agreement between the observed coo ling times and those obtained from model calculations, although the ob served values were always somewhat longer than the theoretical ones. T aking into account evolutionary effects, we also found similar shapes and configurations of hot and cool loops during the entire observing p eriod and confirmed that, at any time, hot loops are at higher altitud e than cool loops, suggesting that cool loops indeed evolve from hot l oops. These results were used to check the validity of the reconnectio n model.