Timing and occurrence rate of X-ray plasma ejections

We examined 126 limb flares between October 1991 and August 1998. X-ray plasma ejections were found in 54 flares. All the X-ray plasmoids were detected in images taken before the maximum peak of hard X-ray (HXR) emission or in each of the first image after the HXR peak. In our choice of 57 flares which the soft X-ray telescope aboard, Yohkoh started to observe before the HXR peak, X-ray plasma ejections were found in similar to 63-70% of these flares. We found X-ray plasma ejections in 100% of X-class flares and 74-82% of M-class flares, whereas only 31-38% of C-class flares have X-ray plasma ejections. It is difficult to detect X-ray plasma ejections in C-class flares, because the scale size and lifetime of ejections are short. We propose that solar flares including microflares occur through magnetic reconnection and that X-ray plasma ejections are general phenomena associated with solar flares. (C) 2000 Elsevier Science Ltd. All rights reserved.