Fragmented Type II Burst Emission During CME Liftoff

Silja Pohjolainen1, Jens Pomoell2, Rami Vainio2

1Department of Physics and Astronomy, University of Turku, Finland
2Department of Physics, University of Helsinki, Finland

Solar radio type II bursts are rarely seen at frequencies higher than a few hundred MHz. Since metric type II bursts are thought to be signatures of propagating shock waves, it is of interest to know how these shocks, and the type II bursts, are formed. In particular, how are high-frequency, fragmented type II bursts created? Are there differencies in shock acceleration or in the surrounding medium that could explain the differences to the 'typical' metric type IIs? We analyse one unusual metric type II event in detail, with comparison to white-light, EUV, and X-ray observations. As the radio event was associated with a flare and a coronal mass ejection (CME), we investigate their connection. We then utilize numerical MHD simulations to study the shock structure induced by an erupting CME in a model corona including dense loops. We find that the fragmented part of the type II burst can be formed when a shock wave propagates through high-density loops. In the simulations, the shock becomes significantly stronger inside the dense loops. The observed plasma emission reflects the changing density in these loops and the jumps between the loops. The later, typical type II burst dies out when the shock reaches the outer boundary of the active region where the Alfven speed rises sharply and/or the orientation of the magnetic field changes abruptly.