TIME-VARIATION OF THE HARD X-RAY IMAGE DURING THE EARLY PHASE OF SOLAR IMPULSIVE BURSTS

The time variations of hard X-ray images of four impulsive bursts with simple source structures were investigated in a comparison with the m agnetic structure. Two of them are limb bursts. Common variations duri ng the early phase are as follows: i) The hard X-ray brightening seems to start at the top of a single coronal loop. ii) The X-ray source sp reads during the increasing phase of the burst in both directions alon g the loop, and both ends become brighter, especially at higher energi es with generally unequal brightness. The loop top is still bright, es pecially at lower energies, to show three peaks. The speed of the expa nsion of the X-ray source amounts to about 10(4) km s-1 in three cases . iii) At and after the peak of the X-ray flux, the source tends to be a single source at the loop top, especially at lower energies. iv) Th e effective temperature for quasi-thermal electrons and their number d ensity during the early phase in the vicinity of the loop top are (4-6 ) x 10(7) K and (5-2) x 10(9) cm-3, respectively, so that the electron mean free path is greater than three-times the local temperature scal e height. These observations axe consistent with the idea that anomalo us resistivity, which triggers impulsive bursts, is caused by electron plasma waves generated in the process of heat conduction.