YOHKOH/HXT EVIDENCE FOR A HYPERHOT LOOP-TOP SOURCE IN THE PRE-IMPULSIVE PHASE OF A LOOP FLARE

A loop flare that occurred on 22 April 1993 near the disk center is examined using the Yohkoh Hard X-ray Telescope (HXT). We specifically looked into the faint early phase of the flare prior to the start of the strong impulsive phase. The pre-impulsive phase, though weak in intensity, is expected to contain essential clues to the mechanism of loop flares according to the causality principle, but it has not received attention previously, probably due to the insufficient dynamic range and cadence of observations by the instruments on earlier satellites. Observations with Yohkoh/HXT can clarify what occurs in this phase. This flare, like many other flares of this type, shows a relatively weak emission with a smooth and gradual increase during this pre-impulsive phase, followed by impulsive bursts, and then turns into a smooth decay phase without impulsive bursts. First, we found that the spectrum for the initial smooth rise part is consistent with a thin-thermal source at a temperature around 80 MK. Imaging of this phase in the HXT/L and M bands shows a single source between the footpoint sources that will come up in the impulsive phase following this phase, suggesting that this hyperhot source is located at a high part of the loop between the footpoints, since this flare takes a form of a loop. Furthermore, as we go up to the earliest times of the flare before this `hyperhot' source phase, two fainter sources are found near the footpoint sources that will appear later in the impulsive phase. The spectra of these sources at this earliest time of the flare, in contrast to the `hyperhot' source, cannot be determined from the HXT because the instrument was not in flare mode, and HXT/M1, M2, and H-band data are, unfortunately, not available at this very initial time. We can guess, however, that they are also of thermal character because the time profile is smooth without any spikes just as in the following `hyperhot' thermal phase, and in the post-impulsive `superhot' thermal phase coming up much later. These findings suggest that there is an important, and probably dynamic, early phase in loop flares that has been unnoticed in the still dark pre-impulsive phase, because the very early footpoint sources change into the loop top source in a matter of 20–30 s, comparable to the dynamic Alfvén time scale. Some implications of our new findings are discussed.