a): Source region of the type III bursts (red square) over the open magnetic field lines (in green) inferred using the PFSS extrapolation model by Schrijver & De Rosa (2003). b): Superposition of two open magnetic field lines (in green) close to the positions of three successive individual type III bursts (red diamonds) on the 19.5 nm EUVI synoptic map from STEREO B. The blue lines outline the EUVI wave front at different times: 13:56, 14:06 (near the time of the type III burst), and 14:16 UT.