Spectropolarimetric Radio Imaging of Faint Gyrosynchrotron Emission from a CME : A Possible Indication of the Insufficiency of Homogeneous Models

TL;DR

This study reports the first detection of faint gyrosynchrotron emission from a CME at large distances using high-dynamic-range spectropolarimetric imaging, revealing limitations of homogeneous models and suggesting the need for inhomogeneous modeling.

Contribution

It introduces the first joint use of Stokes I and V spectra to constrain CME gyrosynchrotron models, highlighting the inadequacy of homogeneous models for accurate magnetic field estimation.

Findings

Detected gyrosynchrotron emission up to 8.3 solar radii

Successfully used Stokes V for polarization analysis

Homogeneous models fail to fit both intensity and polarization

Abstract

The geo-effectiveness of coronal mass ejections (CMEs) is determined primarily by their magnetic fields. Modeling of Gyrosynchrotron (GS) emission is a promising remote sensing technique to measure the CME magnetic field at coronal heights. However, faint GS emission from CME flux ropes is hard to detect in the presence of bright solar emission from the solar corona. With high dynamic-range spectropolarimetric meter wavelength solar images provided by the Murchison Widefield Array, we have detected faint GS emission from a CME out to $\sim 8.3\ R_\odot$, the largest heliocentric distance reported to date. High-fidelity polarimetric calibration also allowed us to robustly detect circularly polarized emission from GS emission. For the first time in literature, Stokes V detection has jointly been used with Stokes I spectra to constrain GS models. One expects that the inclusion of polarimetric measurement will provide tighter constraints on GS model parameters. Instead, we found that homogeneous GS models, which have been used in all prior works, are unable to model both the total intensity and circular polarized emission simultaneously. This strongly suggests the need for using inhomogeneous GS models to robustly estimate the CME magnetic field and plasma parameters.