Spatially resolved observations of a split-band coronal type II radio burst

Context. The origin of coronal type II radio bursts and the nature of their band splitting are still not fully understood, though a number of scenarios have been proposed to explain them. This is largely due to the lack of detailed spatially resolved observations of type II burst sources and of their relations to magnetoplasma structure dynamics in parental active regions. Aims. To make progress in solving this problem on the basis of one extremely well observed solar eruptive event. Methods. The relative dynamics of multithermal eruptive plasmas, observed in detail by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, and of harmonic type II burst sources, observed by the Nançay Radioheliograph at ten frequencies from 445 to 151 MHz, was studied for the 3 November 2010 event arising from an active region behind the east solar limb. Special attention was given to the band splitting of the burst. Analysis was supplemented by investigation of coronal hard X-ray (HXR) sources observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager. Results. We found that the flare impulsive phase was accompanied by the formation of a double coronal HXR source, whose upper part coincided with the hot (T ≈ 10 MK) eruptive plasma blob. The leading edge (LE) of the eruptive plasmas (T ≈ 1-2 MK) moved upward from the flare region with a speed of v ≈ 900-1400 km s ^-1. The type II burst source initially appeared just above the LE apex and moved with the same speed and in the same direction. After ≈ 20 s, it started to move about twice as fast, but still in the same direction. At any given moment, the low-frequency component (LFC) source of the splitted type II burst was situated above the high-frequency component (HFC) source, which in turn was situated above the LE. We also found that at a given frequency the HFC source was located slightly closer to the photosphere than the LFC source. Conclusions. Based on the set of established observational facts, we conclude that the shock wave, which could be responsible for the observed type II radio burst, was initially driven by the multi-temperature eruptive plasmas, but later transformed to a freely propagating blast shock wave. The preferable interpretation of the type II burst splitting is that its LFC was emitted from the upstream region of the shock, whereas the HFC was emitted from the downstream region. The shock wave in this case could be subcritical.