Microwave detection of shock and associated electron beam formation

We use complementary European and Japanese solar radio ground-based observations, together with Yohkoh soft X-ray and SOHO extreme-UV images, to search for the signature of flare-related waves at different heights above the Sun. The key data set for event selection is 40-800 MHz dynamic radio spectra from the Potsdam Astrophysical Institute, whose radio spectral polarimeter is sensitive to the coronal shock waves due to the associated type II radio bursts in the range between 0.2 and 1 R-circle dot. Nobeyama Radio Heliograph images at 17 GHz show the chromosphere and the transition region to the corona with unprecedented sensitivity and time resolution (1 s image cadence). Here we focused on 17 GHz images in the time interval between flare onset and the start of the metric type II burst. The decametric-hectometric (Dm-Hm; 1-14 MHz) radio experiment on board Wind completes the radio spectral coverage. The spectra are used to check if the coronal shock wave is also continuously visible in the range 3-8 R-circle dot and if the corona is open or closed for electron beams exciting hectometric type III bursts. We selected two flare events that show metric type II bursts, but with different associated 17 GHz features. For both events we find flare disturbances in 17 GHz images that propagate earlier than the type II bursts : a hot, dense blob (event 1; 1997 April 2) or a cold, absorbing cloud (event 2; 1998 July 31). In event 1, the hot and dense blob preceded the formation of a wave front segment that appeared in SOHO/EIT images. In event 2, we observed the impact of the 17 GHz absorbing cloud on a preexisting quiescent prominence far out of the flaring active region after several minutes of propagation without being disturbed. We demonstrate that the spectral pattern, as well as the drift rate, of the given type II burst drastically changes shortly before the cloud's impact. The Dm-Hm spectra in event 2 reveal a typical shock-associated (SA) event in the outer corona during the interaction between the absorbing cloud and the prominence. Finally, we stress that there may be a common driver for the metric type II bursts and simultaneous decimeter reverse-drift bursts between 1 and 2 GHz recorded on the radio spectrograph of Astronomical Observatory Ondrejov.