The radio-silent start of an intense solar gamma-ray flare

Radio-silent gamma-ray flares are solar flares that lack any significant em ission in the (non-thermal) radio wave band during their impulsive hard X-r ay and gamma-ray emission phases. Flares with extremely suppressed long-wav elength spectra have previously been reported by White et al. (1992) and ha ve been discussed in different context by Hudson and Ryan (1995). A strikin g example of a radio-silent flare was observed by SMM during the onset of t he 6 March 1989 energetic gamma-ray flare. We argue that the absence of rad io emission at wavelengths longer than microwave wavelengths is an indicati on of the compactness of the flare rather than that the flare did not exhib it non-thermal properties. Probably the flare site was restricted to altitu des above the photosphere in a newly emerging loop configuration lower than the equivalent altitude corresponding to an emission frequency of 1.4 GHz. This implies the presence of a dense and highly magnetized closed field co nfiguration confining the electron component which causes the impulsive gam ma-ray continuum. Reconnection in such a configuration did not lead to open magnetic fields and streamer formation. Acceleration of particles in the g amma and hard X-ray bursts was restricted to closed field lines. Thermal ex pansion of the loop system may subsequently lead to the generation of radia lly propagating blast waves in the solar corona which are accompanied by ty pe II solar radio bursts and decimetre emissions. The emission during the o nset of the flare was dominated by a continuum originating from electron br emsstrahlung at X-ray and gamma-ray energies with only little evidence for the presence of energetic ions. It is, therefore, concluded that energetic electrons have been primary and not secondary products of the particle acce leration process.