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. 
V.G.Eselevich and M. V. Eselevich
astro-ph.SR