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Abstract: Aims: We identify the source of fast-drifting decimetric-metric radioemission that is sometimes observed prior to the so-called flare continuumemission. Fast-drift structures and continuum bursts are also observed inassociation with coronal mass ejections CMEs, not only flares. Methods: Weanalyse radio spectral features and images acquired at radio, H-alpha, EUV, andsoft 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 amoving, wide emission front in the radio images, which is normally interpretedas a signature of a propagating shock wave. A decimetric-metric type II burstwhere only the second harmonic lane is visible could explain the observations.After long-lasting activity in the active region, the hot and dense loops couldbe absorbing or suppressing emission at the fundamental plasma frequency. Theobserved burst speed suggests a super-Alfvenic velocity for the burst driver.The expanding and opening loops, associated with the flare and the early phaseof CME lift-off, could be driving the shock. Alternatively, an instantaneousbut fast loop expansion could initiate a freely propagating shock wave. Thelater, complex-looking decametre-hectometre wave type III bursts indicate theexistence of a propagating shock, although no interplanetary type II burst wasobserved during the event. The data does not support CME bow shock or a shockat the flanks of the CME as the origin of the fast-drift decimetric-metricradio source. Therefore super-Alfvenic loop expansion is the best candidate forthe initiation of the shock wave, and this result challenges the current viewof metric-coronal shocks originating either in the flanks of CMEs or from flareblast waves.

Autor: S. Pohjolainen


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