Imaging spectroscopy of a spectral bump in a type II radio burst

Methods. 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.

Results. 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(-1), and this corresponds to an estimated speed of 295 km s(-1).

The projected velocity of the radio source obtained from imaging is 380 km s(-1) 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.

Conclusions. The spectral bump shows an 8 MHz deviation at 60 MHz, which corresponds to a 25% 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.