Fragmented type II burst emission during CME liftoff

We have performed multiwavelength analysis on an event with a metric type II burst, which appeared first as fragmented emission lanes in the radio dynamic spectrum. The start frequency was unusually high. Since type II bursts are thought to be signatures of propagating shock waves, it is of interest to know how the shocks, and the type II bursts, are formed. This radio event was associated with a flare and a coronal mass ejection (CME), and we investigate their connection. Observations suggested that a propagating shock was formed due to the erupting structures, and the observed radio emission reflects the high densities in active region loops. We then utilised numerical MHD simulations, to study the shock structure induced by an erupting CME, in a model corona including dense loops. Our simulations show that the fragmented part of the type II burst can be formed when a coronal shock driven by a CME passes through a system of dense loops overlying an active region. To produce fragmented emission, the conditions for plasma emission have to be more favourable inside the loop than in the inter-loop area. The obvious hypothesis, consistent with our simulation model, is that the shock strength decreases significantly in the space between the denser loops. Outside the active region, the type II burst dies out when the changing geometry no longer favours the electron shock-acceleration.