Fine structure of a solar type II radio burst observed by LOFAR

Abstract:

Solar eruptive events, such as coronal mass ejections (CMEs) and flares are frequently associated with shock waves. The longest known and frequently studied signatures of coronal shocks are metric type II radio bursts that can provide information on the shock wave driver and the ambient plasma conditions. Type II radio bursts are frequently employed in tracking of the shock waves all the way from the Sun to Earth (metric to kilometric wavelengths).

We present the outstanding, high frequency/time resolution LOFAR (LOw Frequency ARray) observations of a metric type II radio burst. LOFAR observations show a strong fragmentation of the type II emission, in both frequency and time domain, during the whole duration of the event.

We also report a very unusual splitting of already split type II bands, which we call band-split of the band-split. This type of fragmentation of the shock associated radio emission is reported for the first time. The richness of fine structure, observed in both fundamental and harmonic band of type II burst is unprecedented. The type II fine structures are morphologically similar to the fine structures sometimes observed in the type IV continuum. We classified the type II fine structures into three main categories: simple narrowband, broadband and complex fine structures. The characteristics of the bursts were listed and discussed. LOFAR observations of fragmented shock-associated radio emission bring new insight into the physics of the radio emission associated with the coronal shock waves, and also new challenges for the theory of the electron acceleration at the coronal shock waves.