Fine Structure of a Solar Type II Radio Burst Observed by LOFAR

Shock waves in the solar corona are closely associated with coronal mass ejections and flares. The longest-known and frequently studied signatures of coronal shock waves are metric type II radio bursts, which provide information on the shock driver and ambient plasma conditions. We report on outstanding high frequency/time resolution LOw Frequency ARray (LOFAR) observations of a metric type II radio burst. The LOFAR observations show a strong fragmentation of the type II emission, in both the frequency and time domains, during the whole duration of the event. A very unusual splitting of an already-split type II band, which we call the band-split of the band-split, was observed for the first time. The richness of fine structure, observed in both the fundamental and harmonic bands of the type II emission, is unprecedented. Fine structures, morphologically similar to those seen superposed on a type IV continuum, were observed for the first time within a type II burst. We classify the fine structures into three categories: simple narrowband, broadband, and complex fine structures, and discuss their properties. LOFAR observations of fragmented shock-associated radio emission have the potential of bringing new insight into the physics of coronal shock waves, and also new challenges for the theory of electron acceleration by shocks.