CME-driven Shock and Type II Solar Radio Burst Band Splitting

Coronal mass ejections (CMEs) are believed to be effective in producing shocks in the solar corona and interplanetary space. One of the important signatures of shocks and shock acceleration are Type II solar radio bursts that drift with the shock speed and produce bands of fundamental and higher harmonic plasma radio emission. An intriguing aspect of Type II radio bursts is the occasional split of a harmonic band into thinner lanes, known as band splitting. Here we report a detailed imaging and spectroscopic observation of a CME-driven shock producing band splitting in a Type II burst. Using LOFAR, we examine the spatial and temporal relation of the Type II burst to the associated CME event, use source imaging to calculate the apparent coronal density, and demonstrate how source imaging can be used to estimate projection effects. We consider two widely accepted band-splitting models that make opposing predictions regarding the locations of the true emission sources with respect to the shock front. Our observations suggest that the locations of the upper and lower subband sources are spatially separated by ∼0.2 ± 0.05 R _⊙. However, we quantitatively show, for the first time, that such separation is consistent with radio-wave scattering of plasma radio emission from a single region, implying that the split-band Type II sources could originate from nearly cospatial locations. Considering the effects of scattering, the observations provide supporting evidence for the model that interprets the band splitting as emission originating in the upstream and downstream regions of the shock front, two virtually cospatial areas.