1 Observatoire de Paris, Section de Meudon, DASOP, CNRS-UMR~8645, F-92195 Meudon, France

2 Mullard Space Science Laboratory, University College London, Holmbury Saint Mary, Dorking, Surrey, RH5 6NT, U.K.

3 Current address: Institute of Space & Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-0022, Japan

4 Astrophysikalisches Institut Potsdam, Observatorium für solare Radioastronomie, An der Sternwarte 16, D-14482 Potsdam, Germany

Large-scale shock waves in the solar corona are observed through their characteristic radio emission at decimetric and longer wavelengths ("type II bursts"). Their driver has not been identified so far. Particularly favorable observing conditions on 27 November 1997 allowed us to combine imaging and spectroscopic observations of the shock signature over a broad radio band with X-ray imaging of plasma structures at high time resolution. The data provide evidence that the shock is generated by rapidly expanding or disrupting structures in the outskirts of the flaring active region:

(1) the type II emission starts above the top of a pre-existing, highly inclined loop system at the time when a plasma blob ejected along the legs reaches the top;

(2) the alignment of type II source positions at successively lower frequencies is far from radial with respect to the associated flare, but seems to be related to the orientation of the pre-existing loop system and the motion of the plasma blob.