Shock-related radio emission during coronal mass ejection lift-off?

Abstract

Aims. We identify the source of fast-drifting decimetric-metric radio emission that is sometimes observed prior to the so-called flare continuum emission. Fast-drift structures and continuum bursts are also observed in association with coronal mass ejections (CMEs), not only flares. Methods. We analyse radio spectral features and images acquired at radio, Hα, EUV, and soft X-ray wavelengths, during an event close to the solar limb on 2 June 2003. Results. The fast-drifting decimetric-metric radio burst corresponds to a moving, wide emission front in the radio images, which is normally interpreted as a signature of a propagating shock wave. A decimetric-metric type II burst where only the second harmonic lane is visible could explain the observations. After long-lasting activity in the active region, the hot and dense loops could be absorbing or suppressing emission at the fundamental plasma frequency. The observed burst speed suggests a super-Alfvenic velocity for the burst driver. The expanding and opening loops, associated with the flare and the early phase of CME lift-off, could be driving the shock. Alternatively, an instantaneous but fast loop expansion could initiate a freely propagating shock wave. The later, complexlooking decametre-hectometre wave type III bursts indicate the existence of a propagating shock, although no interplanetary type II burst was observed during the event. The data does not support CME bow shock or a shock at the flanks of the CME as the origin of the fast-drift decimetric-metric radio source. Therefore super-Alfvenic loop expansion is the best candidate for the initiation of the shock wave, and this result challenges the current view of metric/coronal shocks originating either in the flanks of CMEs or from flare blast waves.