Radio evidence for electron acceleration in a cusp-like structure

In the spectrum of the type IV/II radio burst of 1994 July 18 we found a rare superposition of fast and slowly drifting features in the frequency range 100–300 MHz about 1 min before the onset of the shock induced meter wave type II burst. We take this as signatures of the passage of an MHD-like disturbance and the simultaneous injection of two contradirected electron beams. These beams are manifested as an ordinary type III burst and a reverse drift burst starting at the same frequency. Remarkably, the reverse drift burst is visible only due to its absorbtion trace in an underlying continuum patch. We argue that the superimposed burst features are emitted from a common source which covers the top of a closed magnetic field structure and the immediately superposed part (cusp) of the helmet streamer configuration. The radio source is situated immediately at the electron acceleration site.