Coronal mass ejections and type II radio bursts

The simultaneous availability of white light data on CMEs from the Solar and Heliospheric Observatory (SOHO) and radio data on shock waves from the Radio and Plasma Wave experiment on board the Wind spacecraft over the past decade have helped in making rapid progress in understanding the CME-driven shocks. I review some recent developments in the type II-CME relationship, focusing on the properties of CMEs as shock drivers and those of the medium supporting shock propagation. I also discuss the solar cycle variation of the type II bursts in comparison with other eruptive phenomena such as CMEs, flares, large solar energetic particle events, and shocks detected in situ. The hierarchical relationship found between the CME kinetic energy and wavelength range of type II radio bursts, non-existence of CMEless type II bursts, and the explanation of type II burst properties in terms of shock propagation with a realistic profile of the fast mode speed suggest that the underlying shocks are driven by CMEs, irrespective of the wavelength domain. Such a unified approach provides an elegant understanding of the entire type II phenomenon (coronal and interplanetary). The blast wave scenario remains an alternative hypothesis for type II bursts only over a small spatial domain (within one solar radius above the solar surface) that is not accessible to in situ observation. Therefore the existence of blast waves cannot be directly confirmed. CMEs, on the other hand, can be remote sensed from this domain.