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

Abstract

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 (∼ 0.16 c) and hence an average electron energy (∼ 7 keV) was inferred from the average frequency drift rate (∼ 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 × 10 9 – 4.5 × 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 (≤ 500 MHz). However, during the time evolution of those flares, the correlation improved for higher and higher frequencies (≥ 800 MHz), suggesting that the acceleration region was displaced towards the photosphere. The estimated velocity of the acceleration region is ∼ 3 × 10 3 – 8 × 10 3 km/s. (c) In two flares, the enhancement of the radio decimetric emission above 500 MHz started earlier (∼ 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.