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

Frequent transient X-ray brightenings occur in solar active regions. The Yohkoh Soft X-ray Telescope observed 142 transient brightenings during an interval of time in late 1991 October. We classify them in terms of morphology and time evolution: (1) simultaneous multiple loop brightenings are more often seen than brightenings of single and pointlike structures; (2) for multiple-loop brightenings, the loops tend to brighten from their footpoints and/or the apparent contact point in the initial phase of transient brightenings, followed by the brightening of the entire loops; (3) more than one-half of the multiple-loop brightenings 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 them are due to the magnetic interaction of multiple loops. X-ray emission from the footpoints in the early phase suggests that the hot plasma in the brightening loops comes from chromospheric matter or low-temperature coronal matter present around the bases of the coronal loops prior to the brightening. Enhanced X-ray emission at the contact points implies local plasma heating by magnetic interaction. The predominance of the Y-type configuration suggests that the interaction of coronal loops tends to occur near the footpoints.