Jun Elin Wiik

POST-FLARE LOOPS OF 26 JUNE 1992 – IV. Formation and Expansion of Hot and Cool Loops

Observations of the post-flare loops after the X3.9 flare which occurred on 25 June, 1992 at 20:11 UT by the Yohkoh/SXT in X-rays, as well as in Hα obtained at 5 different observatories, have provided a unique, longest ever, set of data for a study of the relationship between the 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× 106 K and the cool Hα loops of 1.5× 104 K is related to the expansion rate of the loop systems and their cooling time. Therefore, measurements of the expansion rate and relative height of hot and cool loops can provide direct observational values for their cooling times. We measured the altitude of hot and cool loops for 15 and 19 hours, respectively, and found that the cooling time increased as the density of the loops decreased. We found a reasonably good agreement between the observed cooling times and those obtained from model calculations, although the observed values were always somewhat longer than the theoretical ones. Taking into account evolutionary effects, we also found similar shapes and configurations of hot and cool loops during the entire observing period and confirmed that, at any time, hot loops are at higher altitude than cool loops, suggesting that cool loops indeed evolve from hot loops. These results were used to check the validity of the reconnection model.

Post-Flare Loops in the Chromosphere and Corona

Post-flare loops, which may last for several hours during the gradual phase of powerful flares, can provide spectacular limb events during solar eclipses. According to models, on-going magnetic reconnection keeps creating new loops at an ever-increasing altitude. The newly formed hot loops can cool down to chromospheric temperatures quickly, in a few minutes, or slowly, over a few hours, depending on their density. A good test of the model is to measure the altitude difference between loops observed at different wavelengths which show the loops at different stages of the cooling process. Studying a unique, longest ever, series of X-ray (6 − 7 × 106 K) and Hα (1.5 × 104 K) data on the X3.9 flare which occurred on June 25, 1992 at 20:11 UT we found similar shape and configuration for hot and cool loops during the entire observing period, and thus confirmed that cool loops indeed evolve from hot loops. On the other hand, we find that the computed cooling times are somewhat shorter than the cooling times we deduct from the relative altitudes of hot and cool loops. This discrepancy may imply that loops do not stay at the same altitude but slirink during the cooling process. Whether this is true or not could conceivably be answered through an analysis of multiwavelength post-flare loop observations, perhaps as a joined space (SOHO, YOHKOH) and ground-based observing effort during the 1999 total eclipse.

The Postflare Loops and the Nearby Active Chromosphere of 1992 June 26: Addendum

Video segments are presented that were processed from a quantitative study of the dynamics of the evolution of Hα postflare loops developed after a large solar flare. The high spatial resolution of the Swedish Vacuum Solar Telescope (SVST) at La Palma provided a unique set of data for such an event.