The excitation of type II radio bursts in the corona

Simultaneous radioheliograph and orbital coronagraph observation of coronal transient activity shows metric type II radio emission originating early in the event, well below the visible white light transient. It is suggested that the shock which excites the type II emission is independent of the transient, in the sense that it is initiated in the low corona (probably in association with a flare) and travels through the already-existing transient disturbance with a propagation speed significantly greater than that of the front edge of the transient itself. Radio emission then results when the flare shock overtakes, first, the region of principal density pile-up along the sides of the expanding transient and only later the top of the transient.