Radio Signature of the Strong Compression between a Streamer and a Coronal Hole Boundary

We present evidence of the first detection of the radio signature at metric wavelengths of the strong compression between a helmet streamer (HS) and the boundary of a coronal hole (CH) using radio observations from the Callisto MEXICO-LANCE and ALASKA-HAARP systems and white-light observations obtained by the STEREO-A/COR1-COR2 coronagraphs. The event occurred very close to the Sun (∼3.4 solar radii) and produced an intense and unusually broad drifting radio feature at metric wavelengths after a downward-drifting band of emission related to a metric Type II radio burst. The compression is caused by the interaction between an expanding structure (coronal mass ejection/shock) and the HS against the CH boundary. Observations in white light show a sharp compressive feature that propagates radially outward, while STEREO-A/EUVI images show loop oscillations at the same position angle, indicating that the interaction occurs across a range of heights. The loop oscillations cease when the compressive front loses its sharp boundary. This transition indicates a reduction of the density compression at the front and the cessation of the radio emission.