# The Low-Frequency Solar Corona in Circular Polarization

## TL;DR

This study presents pioneering low-frequency spectropolarimetric imaging of the solar corona, revealing diverse circular polarization features, their correlation with magnetic fields, and introducing an algorithm to correct instrumental artifacts.

## Contribution

First spectropolarimetric images of the low-frequency solar corona are presented, along with an algorithm to mitigate calibration artifacts and a survey of polarization features during solar maximum.

## Key findings

- Detected ~700 polarized sources with polarization fractions up to 100%.
- Observed 'bullseye' polarization structures in coronal holes.
- Found correlation between polarization structures and magnetic field models at low frequencies.

## Abstract

We present spectropolarimetric imaging observations of the solar corona at low frequencies (80 - 240 MHz) using the Murchison Widefield Array (MWA). These images are the first of their kind, and we introduce an algorithm to mitigate an instrumental artefact by which the total intensity signal contaminates the polarimetric images due to calibration errors. We then survey the range of circular polarization (Stokes V) features detected in over 100 observing runs near solar maximum during quiescent periods. First, we detect around 700 compact polarized sources across our dataset with polarization fractions ranging from less than 0.5% to nearly 100%. These sources exhibit a positive correlation between polarization fraction and total intensity, and we interpret them as a continuum of plasma emission noise storm (Type I burst) continua sources associated with active regions. Second, we report a characteristic "bullseye" structure observed for many low-latitude coronal holes in which a central polarized component is surrounded by a ring of the opposite sense. The central component does not match the sign expected from thermal bremsstrahlung emission, and we speculate that propagation effects or an alternative emission mechanism may be responsible. Third, we show that the large-scale polarimetric structure at our lowest frequencies is reasonably well-correlated with the line-of-sight (LOS) magnetic field component inferred from a global potential field source surface (PFSS) model. The boundaries between opposite circular polarization signs are generally aligned with polarity inversion lines in the model at a height roughly corresponding to that of the radio limb. This is not true at our highest frequencies, however, where the LOS magnetic field direction and polarization sign are often not straightforwardly correlated.