Decimeter Burst Emission and Particle Acceleration

Abstract

The radio emission of solar flares at decimeter wavelengths includes a variety of emission processes of a plasma thought to have a high beta. Very intense coherent emissions are observed at frequencies smaller than about 9 GHz. They are caused by plasma instabilities driving various wave modes that in turn may emit observable radio waves. Particularly important are type III bursts, caused by electron beams exciting Langmuir waves. Their sources may be used to trace the path of the electrons back in the corona to the acceleration region. Less known are radio emissions from trapped electrons driving loss-cone unstable waves, suspected for type IV bursts. These types of coherent radio emission give clues on the geometry and plasma parameters near the acceleration region. More speculative are emissions that are directly produced by the acceleration process. A possible group of such phenomena are narrowband, short peaks of emission. Narrowband spikes are seen sometimes at frequencies above the start of metric type III events. There is mounting evidence for the hypothesis that these spikes coincide with the energy release region. Much less clear is the situation for decimetric spikes, which are associated with hard X-ray flares. More frequently than spikes, however, there is fluctuating broadband decimetric emission during the hard X-ray phase of flares. The use of these coherent radio emissions as a diagnostic tool for the primary energy release requires a solid understanding of the emission process. At the moment we are still far away from an accepted theory. Complementary observations of energetic electrons and the thermal coronal background in EUV lines and soft X-rays can put coherent emissions into context and test the different emission scenarios. In combination with other wavelengths, spectroscopic imaging by FASR would provide exciting possibilities for the diagnostics of the acceleration process. 203 204 SOLAR AND SPACE WEATHER RADIOPHYSICS 1. Introduction The total solar radio emission in meter and decimeter waves occasionally brightens up by more than an order of magnitude during flares. These radio sources thus outshine the thermal radiation of the rest of the Sun and are called bursts. Meterwave emissions have been studied since the late 1940s and are reviewed in the book by McLean & Labrum (1985). Observationally, they are classified as type I through V bursts, and there are more or less accepted scenarios for each of them. First spectral observations in the decimeter range were made already in the early 1960s by (Young et al. 1961), exploring the spectral region beyond 300 MHz up to 1 GHz. In addition to type III bursts continuing into the decime-ter range, these observers noted a rich variety of " type III-like " features later termed pulsations and fine structures in type IV bursts, such as drifting parallel bands and intermediate drift bursts 1 . The decimeter range has been systemati-cally studied only in the 1980s when digitally recording spectrometers became available resolving the fast temporal changes and narrowband spectral structure. In the 1990s the spectral coverage was extended finally beyond 3 GHz where coherent emissions were found to become weak and rare. Today several spec-trometers observe in the decimeter range and the spectral properties of coherent emissions have been well explored in the past decade. Some of the decimeter wave observations have been reviewed by Bastian et al. (1998) and in Benz (2002). These observations have revealed a new field of solar emissions with potential diagnostic value for flare physics, CME origin, particle acceleration and propagation, coronal heating and more. We still do not know much about the locations of decimeter bursts, and thus cannot superpose them easily on images of the thermal corona, such as observed in EUV lines and soft X-rays, since imaging instruments in this wave-lenths are sparse. The lack of imaging severely limits progress in the field and will hopefully change in the next cycle of solar activity, when solar ded-icated interferometers such as FASR will be put into operation. This review thus is a summary of current observations which are still so incomplete that the inferences on flare physics and particle acceleration are limited. 2. The Decimeter Range