Synchrotron Radio Emission from a Fast Halo Coronal Mass Ejection

An interplanetary (IP) type-II-like radio burst is analyzed. It occurred on 2003 June 17-18 in association with a fast halo coronal mass ejection (CME), an M6.8 soft-X-ray (SXR) flare, and produced a solar proton event. Unlike coronal type II bursts and the ma jority of IP type II radio emissions, the IP type-II-like event associated with the fast halo CME on June 17-18 varies smoothly in time and frequency and has a frequency bandwidth that is several times larger than is typical for coronal and IP type II emissions. Moreover, the frequency change with time is inconsistent with that expected from plasma radiation associated with a CME-driven shock. I suggest that this IP type-II-like event, referred to here as an IP type II-S event, is not due to plasma radiation but, rather, incoherent synchrotron radiation from near-relativistic electrons entrained in the CME magnetic field, or in the sheath region between the shock and the CME driver. This event may be an example of a new and distinct class of interplanetary radio phenomenon.