A wide-band high-frequency type-II solar radio burst

Aims. Type-II radio bursts are typically observed below ∼400 MHz and are characterized by the narrowband, slowly drifting fundamental and harmonic structures. Here we report an unusual high-frequency wide-band type-II burst with a starting frequency as high as 670 MHz and an instantaneous bandwidth as wide as ∼300 MHz. Methods. We used radio imaging from the Nançay Radio Heliograph, spectroscopic data from ORFEES, extreme-ultraviolet (EUV) observations from Solar Dynamics Observatory, and white-light observations from LASCO to determine the nature and origin of the observed radio burst as well as its propagation in the corona. Results. The estimated average spectral drift is ∼2.18 MHz s−1, its mean duration at each frequency is ∼3 min, and the maximum brightness temperature can exceed 1011 to 1012 K. According to the simultaneous EUV and radio imaging data, the radio sources are distributed over a relatively broad region centered on a dip in the nose front of the shock-like EUV wave structure. The dip is likely caused by the strong interaction of the eruption with the overlying closed dense loops that are enclosed by the large-scale streamer structure, indicating that the type-II burst originates from coronal mass ejection shocks interacting with dense, closed-loop structures. Conclusions. The observations suggest that the unusual wide-band high-frequency type-II radio burst originates from a dense streamer region in the corona; this is further evidenced by an EUV shock-like structure that steepens very close to the solar surface, at ∼1.23 R⊙, and the fact that the type-II radio source coincides with the shock dip. The wide-band feature is due to the source stemming from a region with significant density variations and not due to the intensity variations across the shock structure.