First detailed polarimetric study of a type-II solar radio burst with the Murchison Widefield Array

Type-II solar radio bursts are plasma emissions generated by magnetohydrodynamic shocks that are predominantly driven by coronal mass ejections (CMEs), the most potent drivers of space weather. These narrow-bandwidth (~few MHz) emissions show slow drift towards lower frequencies in the dynamic spectrum and appear at the fundamental and harmonic of the local plasma frequency. The evolution and geo-effectiveness of CMEs are governed by their magnetic fields and interaction with the coronal magnetized plasma. Therefore, understanding the CME-entrained magnetic fields and the ambient medium is important. Polarimetric properties of type-II bursts provide promising diagnostics for measuring and understanding the magnetic field strength, and topology at the CME-shocks. In the literature, their polarization properties have been reported to vary from being unpolarized to very strongly circularly polarized, and no linear polarization has ever been reported. The vast majority of these studies rely on dynamic spectra which can only provide spatially integrated information. Instruments like Murchison Widefield Array (MWA) and robust spectropolarimetric snapshot solar imaging pipeline, P-AIRCARS, (Kansabanik et al., 2022, 2023) have enabled high-fidelity and high-dynamic range solar radio imaging with good temporal, spectral and reasonable angular resolution. Benefiting from these, we have used the MWA to carry out detailed imaging polarimetric characterization of a type-II solar radio burst. We detect a weak circular polarization of ~ 4% during the type II. We also report the first imaging detection of low levels of linearly polarized type-II emission. This robust but surprising detection goes against the conventional wisdom that differential coronal Faraday rotation should wipe out any linear polarization signatures in coronal emissions. We characterize the polarimetric structures in both circular and linear polarizations, meticulously investigating their temporal and spectral evolutions. Our study marks the start of the use of polarimetric imaging observations to further the understanding of type-II radio bursts and coronal propagation.