Type II radio precursor and X-ray flare emission

Abstract

The formation of a radio-emitting shock wave and its precursor above a flaring active region is investigated during the flare on 12 April 2001. Using combined spectral and imaging observations of radio bursts with Yohkoh soft and hard X-ray imaging observations, we confirm earlier findings that the type II bursts are generated in association with and above expanding soft X-ray loops. The type II precursor is identified as a signature of the reconnection process above the expanding soft X-ray loops which later generate the type II burst. The precursor emission consists of fast reverse drift bursts that reveal electron beams propagating towards the expanding loops and the hard X-ray sources. There is some degree of similarity between the radio bursts and the rising hard X-ray time profile. This suggests a close relation between the precursor and the soft and hard X-ray sources, i.e., signatures of the primary energy release during the flare. We found that the speeds of the expanding loops (≤ 700 km s−1), of the precursor (≃ 950 km s−1), and of the type II exciter (≃ 900–1400 km s−1) are comparable. But the expanding loops have the lowest speed.