THE OSCILLATORY SHRINKAGE IN TRACE 195 A LOOPS DURING A FLARE IMPULSIVE PHASE

摘要

An altitude decrease of the flare loop-top X-ray sources during the early impulsive phase has recently been discovered with the RHESSI observations. The relevant evidence for the loop shrinkage has also recently been provided based on the Nobeyama radio images at 34 GHz. In this Letter, through analyzing the images observed with TRACE and RHESSI for the M2.5 flare on 2002 April 16, we present a new argument for the loop shrinkage: TRACE 195 A loop shrinkage. The results show that in the early impulsive phase of about 5 minutes, the 195 A loop in the running difference images shrank by about 30%, suggesting a mean apparent downward speed of approximately 15.3 km s~(-1). This speed is nearly the same as that deduced from the RHESSI 12-25 keV energy band but is greater than 7.7 km s~(-1) deduced from the RHESSI 6-12 keV energy band. The temporal behavior of 195 A loop shrinkage is consistent with that revealed from the RHESSI X-ray sources. Further study shows that the TRACE 195 A loop shrinkage is in the form of an oscillation, with a period of about 150 s and an amplitude of about 300 km. We discuss this oscillatory shrinkage with respect to a theoretical reconnection model.