IGNITION OF MHD SHOCKS ASSOCIATED WITH SOLAR-FLARES

We have selected single frequency recordings of 28 'high-frequency' ty pe II bursts characterized by a starting frequency greater than 237 MH z to estimate as accurately as possible the 'launch-time' of the flare -associated MHD shocks. We established the time associations between m etric type II burst onsets and the time characteristics of the microwa ve and X-ray fluxes of the associated flares. The associated hares wer e impulsive events with rise times most often about 1 min in the hard X-ray range and 1-2 min in the microwave wavelength range. The majorit y of the type II bursts from our sample started about 1 min after the maximum of the microwave burst. Launch times of MHD shocks producing t ype II bursts were obtained using the 10 x Saito coronal model and sho ck velocities estimated from burst characteristics at different freque ncies. Back-extrapolations of type II recordings indicate that MHD sho cks are launched in the time interval prior to the maximum of the firs t peak in the associated microwave burst, most probably at the beginni ng of the rapid increase of the microwave burst.