MORPHOLOGY OF ACTIVE-REGION TRANSIENT BRIGHTENINGS WITH THE YOHKOH SOFT-X-RAY TELESCOPE

Frequent transient X-ray brightenings occur in solar active regions. T he Yohkoh Soft X-ray Telescope observed 142 transient brightenings dur ing an interval of time in late 1991 October. We classify them in term s of morphology and time evolution: (1) simultaneous multiple loop bri ghtenings are more often seen than brightenings of single and pointlik e structures; (2) for multiple-loop brightenings, the loops tend to br ighten from their footpoints and/or the apparent contact point in the initial phase of transient brightenings, followed by the brightening o f the entire loops; (3) more than one-half of the multiple-loop bright enings have Y-type configurations in which the apparent contact points are located close to their footpoints. Though transient brightenings show great variety in morphology, these results suggest that most of t hem are due to the magnetic interaction of multiple loops. X-ray emiss ion from the footpoints in the early phase suggests that the hot plasm a in the brightening loops comes from chromospheric matter or low-temp erature coronal matter present around the bases of the coronal loops p rior to the brightening. Enhanced X-ray emission at the contact points implies local plasma heating by magnetic interaction. The predominanc e of the Y-type configuration suggests that the interaction of coronal loops tends to occur near the footpoints.