Abstract
On the basis of previous works, we investigated coronal mass ejection (CME) propagations and the consequent type II radio bursts invoked by the CME-driven shocks. The results indicate that the onset of type II bursts depends on the local Alfvén speed (or the magnetoacoustic wave speed in the non-force-free environment), which is governed by both the magnetic field and the plasma density. This determines that the type II burst cannot appear at any altitude. Instead, its onset positions can never be lower than a critical height for the given coronal environment, which consequently determines the start frequencies of the emission: for an eruption taking place in the magnetic configuration with a background field of 100 G, the onset of type II bursts should occur at around 0.5 R☉ from the solar surface, and the corresponding start frequency of the fundamental component is about 150 MHz. This result is consistent with similar estimates based on observations that bring the corresponding frequency to a few hundred MHz. Our results further indicate that the onset of type II bursts depends on the rate of magnetic reconnection as well. When magnetic reconnection during the eruption is not fast enough, a type II burst may not occur at all even if the associated CME is fast (say, faster than 800 km s-1). This may account for the fast and radio-quiet CMEs. Related to these results, properties of the associated solar flares and type III radio bursts, especially those used as the precursors of the type II radio bursts, are also discussed.