AN OBSERVATIONAL REVISIT OF BAND-SPLIT SOLAR TYPE-II RADIO BURSTS

The band split of solar type II radio bursts, discovered several decades ago, is a fascinating phenomenon, with the type II lanes exhibiting two almost parallel sub-bands with similar morphology. The underlying split mechanism remains elusive. One popular interpretation is that the splitting bands are emitted from the shock upstream and downstream, respectively, with their frequency ratio (gamma) determined by the shock compression ratio. This interpretation has been taken as the physical basis of many published references. Here we report on an observational analysis of type II events with a nice split selected from ground-based RSTN data from 2001 to 2014, in the metric-decametric wavelength. We investigate the temporal variation and distribution of gamma, and conduct correlation analyses on the deduced spectral values. It is found that gamma varies in a very narrow range with > 80% of gamma (one-minute averaged data) being between 1.15 and 1.25. For some well-observed and long-lasting events, gamma does not show a systematic variation trend within observational uncertainties, from the onset to the termination of the splits. In addition, the parameters representing the propagation speed of the radio source (presumably the coronal shock) show a very weak or basically no correlation with gamma. We suggest that these results do not favor the upstream-downstream scenario of band splits.