High-frequency type II radio emissions associated with shocks driven by coronal mass ejections

We investigate the causal relationship between type II radio emissions in the frequency range from 1 to 14 MHz observed by the WAVES experiment on the Wind spacecraft and metric type II bursts observed by ground-based radio telescopes. We do this by extrapolating, to lower radio frequencies, fits to the frequency drift rates of metric type II bursts, These fits, which describe the shock propagation through the solar corona, are based on coronal density laws derived from white light observations. In order to establish whether the type II radio emissions were generated from the same shock, we explicitly chose events for which the type II radio emissions in the frequency range from 1 to 14 MHz were temporally separated from the associated metric type II bursts. The analyses indicate that some type II radio events observed by WAVES may not be low-frequency continuations of the apparently associated metric type II bursts, We conclude that two temporally separated shocks are required to account for the radio observations for these events, one shock being temporally well associated with the flare maximum, In two cases the observed GOES X-ray flux confirms the presence of two closely spaced solar events. We argue that the type II radio emissions observed by Wind/WAVES are likely generated by coronal mass ejection-driven shocks that form as low as 2 R-S in the solar corona.