POST-FLARE LOOPS OF 26-JUNE-1992 .2. GRADUAL EVOLUTION OF COOL AND HOT LOOPS

We observed the large post-flare loop system, which developed after th e X 3.9 flare of 25 June 1992 at 20:11 UT, in H alpha with the Multich annel Subtractive Double Pass Spectrograph at Pic-du-Midi and in X-ray s with the it Yohkoh/SXT instrument. Following the long-term developme nt of cool and hot plasmas, we have determined the emission measure of the cool plasma and, for the first time, the temporal evolution of th e hot-loop emission measure and temperature during the entire gradual phase. Thus, it was possible to infer the temporal variation of electr on densities, leading to estimates of cooling times. A gradual decreas e of the hot-loop emission measure was observed, from 4 x 10(30) cm(-5 ) at 23:00 UT on 25 June 1992 to 3 x 10(28) cm(-5) at 13:10 UT on 26 J une 1992. During the same period, the temperature decreased only slowl y from 7.2 to 6.0 x 10(6) K. Using recent results of NLTE modeling of prominence-like plasmas, we also derive the emission measure of cool H er loops and discuss their temperature and ionisation degree. During t wo hours of H alpha observations (11-13 hours after the flare) the ave raged emission measure does not show any significant change, though th e amount of visible cool material decreases and the volume of the loop s increases. The emission measure in H alpha, after correction for the Doppler-brightening effect, is slightly lower than in soft X-rays. Si nce the hot plasma seems to be more spatially extended, we arrive at e lectron densities in the range n(e)(hot) less than or equal to n(e)(co ol) similar to 2 x 10(10) cm(-3) at the time of the H alpha observatio ns. These results are consistent with the post-flare loop model propos ed by Forbes, Malherbe, and Priest (1989). The observed slow decrease of the emission measure could be due to an increase of the volume of t he loops and a gradual decrease of the chromospheric ablation driven b y the reconnection, which seems to remain effective continuously for m ore than 16 hours. The cooling time for hot loops to cool down to 10(4 ) K and to appear in H alpha would be only a few minutes at the beginn ing of the gradual phase but could be as long as 2 hours at the end, s everal hours later.