Nobeyama Radioheliograph and RHESSI Observations of the X1.5 Flare of 2002 April 21

Abstract

We present an overview of the radio observations of the X1.5 flare of2002 April 21 and complementary data from other wavelengths. This flarewas fairly well observed by the Ramaty High Energy Spectroscopic Imager(RHESSI) spacecraft and fully observed by the Nobeyama Radioheliograph(NoRH) at 17 and 34 GHz. This long-duration event lasted more than 2 hrand featured a beautiful arcade of rising loops on the limb visible atX-ray, EUV, and radio wavelengths. The main flare was preceded by asmall event 90 minutes earlier showing a long EUV loop connectingwell-separated radio and hard X-ray sources. The main flare itselfstarts with a compact radio and hard X-ray source at the eastern end ofthe region that develops into emission close to the solar surface (andwell inside the solar limb) over a large region to the northwest. As theflare proceeds, a large set of loops is seen to rise well above thesolar limb. Distinct regions of radio emission with very different timebehavior can be identified in the radio images, and, in particular, apeculiar nonthermal source seen in radio and hard X-rays low in thecorona at the base of the arcade is seen to turn on 30 minutes after thestart of the impulsive phase. At about the same time, an extremelyintense burst of coherent radio emission is seen from 500 to 2000 MHz;we speculate that this lower-frequency burst is produced by electronsthat are accelerated in the nonthermal source at the base of the arcadeand injected into the loop system where they radiate plasma emission inthe 1010 cm-3 density plasma at the top of thearcade of loops. This event is striking as a demonstration of the manyways in which a flare can produce radio emission, and the combined dataat different wavelengths reveal a diversity of energy release andnonthermal acceleration sites.