Abstract

Observations of solar, type II radio bursts provide a unique opportunity to analyze the nonthermal electrons accelerated by coronal shocks and diagnose the plasma density distribution in the corona. However, there are very few high-frequency resolution interferometric observations of type II radio bursts that are capable of tracking these electrons. Recently, more spatially resolved high-resolution observations of type II radio bursts have been recorded with the Low-Frequency Array (LOFAR). Using these observations, we aim to track the location of a type II radio burst that experienced a sudden spectral bump. We present the first radio imaging observations of a type II burst with a spectral bump. We measured the variation in source location and frequency drift of the burst and deducted the density distribution along its propagation direction. We have identified a type II burst that experiences a sudden spectral bump in its frequency-time profile. The overall frequency drift rate is 0.06 MHz/s, and this corresponds to an estimated speed of 295 km/s. The projected velocity of the radio source obtained from imaging is 380 km/s toward the east. At the spectral bump, a deviation in the source locations of the type II split bands is observed. The band separation increases significantly in the north--south direction. The spectral bump shows an 8 MHz deviation at 60 MHz, which corresponds to a 25<!PCT!> decrease in the plasma density. The estimated crossing distance during the spectrum bump was 29 Mm, suggesting that this density variation occurs in a confined area. This indicates that the shock most likely encountered the upper extent of a coronal hole.

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