A formation mechanism for the type II radio emission in the solar corona unrelated to shock waves

Mark 4, COR1/STEREO and LASCO/SOHO data analysis shows that at least a portion of type II radio bursts observed in the corona occurs in the presence of a CME, but in the absence of a shock ahead of them. A drift current instability in the CME frontal structure is discussed as a possible cause of such bursts.