We present an explanation of the shock formation related to coronal and interplanetary type II radio bursts associated with coronal mass ejections (CMEs) on the basis of the CSHKP model. Coronal type II bursts are usually observed in metric wavelength range (metric type II bursts) and interplanetary bursts are observed in decametric-hectometric wavelength range (decametric-hectometric bursts). Our research shows that decametric-hectometric type II radio bursts are produced by the piston-driven fast-mode MHD shock which is formed in front of an eruptive plasmoid (a magnetic island in the 2-dimensional sense or a magnetic flux rope in the 3-dimensional sense), while the metric radio bursts are produced by the reverse fast-mode MHD shock which is formed through the collision of strong reconnection jet to the bottom of the plasmoid (Figure 5). This reverse shock moves apparently upward as long as the reconnection jet is sufficiently strong, and dies away when the energy releases by the reconnection stops or weakens significantly. On the other hand, the piston-driven fast-mode MHD shock continues to travel outward.
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