Type II radio emission in coronal transients

Recent observations demonstrate that some type II radio bursts (a) occur below the top of coronal white light loops in the early stages and (b) travel faster than white light transients when both data sources are recorded concurrently. These characteristics are examined with numerical simulations of a coronal transient in combination with the suggestion by Holman and Pesses (1983) that shock drift acceleration may be the originating mechanism for type II emission. The simulated angular relation between the transient shock normal and the upstream magnetic field, along with requirements on this orientation in order that shock drift be effective, lead naturally to the observed spatial relationship (in the lower corona) and relative velocities of white-light transients and type II bursts. The large type II velocities do not directly correspond to either material or shock motion, but are due to the production of emission at different locations along the shock surface. In addition, the model coincides with the hypothesis that the shocks generating the coronal type II emission also produce interplanetary SA (shock-accelerated) events.The National Center for Atmospheric Research is sponsored by the National Science Foundation.