Abstract:
The observations of the 1980 March 28, 2B flare in optical, radio microwave and metric and x-ray wavelengths are presented. In the dynamic radio spectrum, unusual and rare U-shaped type II radio bursts associated with this flare have been observed. The normal type II bursts display negative frequency drift, due to the flare-triggered shock wave travelling outwards in the solar corona. But in this case the type II emission first showed the usual negative frequency drift, and then after reaching a plateau level, showed a positive drift, thus giving the shape of an inverted 'U'. We propose a model for this type II emission. We suggest that the shock wave generated by the flare or the flare spray propagates through ducting and repeated reflections, which takes place within a large scale coronal loop or an arcade of loops, thus resulting in reversed frequency drift in type II emission.
