Tracking the motion of a shock along a channel in the low solar corona

Abstract

Context. Shock waves are excited by coronal mass ejections (CMEs) and large-scale extreme-ultraviolet (EUV) wave fronts and can result in low-frequency radio emission under certain coronal conditions. Aims. In this work, we investigate a moving source of low-frequency radio emission as a CME and an associated EUV wave front move along a channel of a lower density, magnetic field, and Alfvén speed in the solar corona. Methods. Observations from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, the Nançay Radio Heliograph (NRH), and the Irish Low Frequency Array (I-LOFAR) were analysed. Differential emission measure maps were generated to determine densities and Alfvén maps, and the kinematics of the EUV wave front was tracked using CorPITA. The radio sources’ positions and velocity were calculated from NRH images and I-LOFAR dynamic spectra. Results. The EUV wave expanded radially with a uniform velocity of ∼500 km s−1. However, the radio source was observed to be deflected and appeared to move along a channel of a lower Alfvén speed, abruptly slowing from 1700 km s−1 to 250 km s−1 as it entered a quiet-Sun region. A shock wave with an apparent radial velocity of > 420 km s−1 was determined from the drift rate of the associated Type II radio burst. Conclusions. The apparent motion of the radio source may have resulted from a wave front moving along a coronal wave guide or by different points along the wave front emitting at locations with favourable conditions for shock formation.