Investigations of the acceleration region of energetic electrons associated with decimetric type III and X-ray bursts

Preliminary results of the association between type III decimetric bursts, mainly having center frequency above 1000 MHz and recorded by Phoenix radio spectrometer, and seven hard X-ray flares observed by the Yohkoh/HXT from September, 1992 to October, 1993 are reported here. (a) For an assumed improved density model of the solar chromosphere, an average electron beam velocity (similar to 0.16 c) and hence an average electron energy (similar to 7 keV) was inferred from the average frequency drift rate (similar to 1350 MHz/s) of 160 isolated type III bursts. Assuming the electrons lose energy primarily by collisions, the height of injection of the energetic electrons was estimated (1.2 x 10(9) - 4.5 x 10(9) cm). (b) In two flares at the onset of the bursts, the correlation between X-rays and radio was better at lower frequencies (less than or equal to 500 MHz). However, during the time evolution of those flares, the correlation improved for higher and higher frequencies (greater than or equal to 800 MHz), suggesting that the acceleration region was displaced towards the photosphere. The estimated velocity of the acceleration region is similar to 3 x 10(3) - 8 x 10(3) km/s. (c) In two flares, the enhancement of the radio decimetric emission above 500 MHz started earlier (similar to 10 s) than the X-ray emissions, suggesting in case of these two flares that the acceleration region is located near to where the decimetric emission is generated. (C) 2000 COSPAR. Published by Elsevier Science Ltd.