On two distinct shocks during the flare of 9 July 1996

Due to the emission of shock-accelerated electrons, broadband radio observations display propagating super Alfvénic shock waves in the low corona ('type II bursts'). We study the 9 July 1996 flare (AR NOAA 7978) focusing on the aspect of shock generation. This event's radio spectrogram shows two different type II bursts in sequence. Radio imaging data (Paris, Meudon Observatory) reveal that both bursts appear at different sites above the H flare. The driver of the first type II burst seems to propagate with twice the speed of the second one. The projected source site of the first type II burst (seen earlier and at higher frequencies) is spatially situated further away from the H flare site than the source of the second type II burst. We try to understand this by comparing with Yohkoh soft X-ray images. The first shock source occurs near the top of high soft X-ray loop structures. Its driver can be a guided fast mode magnetic disturbance. The second type II source appears in-between two high soft X-ray loop systems. This might be a piston-driven disturbance powered by an evaporation front. We get a consistent picture only by assuming a very inhomogeneous Alfvén speed in the active region's atmosphere.