Exploring the circular polarisation of low-frequency solar radio bursts with LOFAR

TL;DR

This study uses LOFAR to analyze the circular polarisation of low-frequency solar radio bursts, revealing frequency-dependent polarisation trends and diverse origins of type III bursts, advancing understanding of solar radio emission mechanisms.

Contribution

First detailed analysis of circular polarisation in low-frequency solar radio bursts with LOFAR, including the first Stokes V images at these frequencies, highlighting diverse burst origins.

Findings

Degree of circular polarisation increases with frequency for fundamental emission.

Type III bursts show different senses and locations of polarisation, indicating multiple origins.

First low-frequency Stokes V images of the Sun with LOFAR in tied-array mode.

Abstract

The Sun is an active star that often produces numerous bursts of electromagnetic radiation at radio wavelengths. Low frequency radio bursts have recently been brought back to light with the advancement of novel radio interferometers. However, their polarisation properties have not yet been explored in detail, especially with the Low Frequency Array (LOFAR), due to difficulties in calibrating the data and accounting for instrumental leakage. Here, using a unique method to correct the polarisation observations, we explore the circular polarisation of different sub-types of solar type III radio bursts and a type I noise storm observed with LOFAR, which occurred during March-April 2019. We analysed six individual radio bursts from two different dates. We present the first Stokes V low frequency images of the Sun with LOFAR in tied-array mode observations. We find that the degree of circular polarisation for each of the selected bursts increases with frequency for fundamental emission, while this trend is either not clear or absent for harmonic emission. The type III bursts studied, that are part of a long--lasting type III storm, can have different senses of circular polarisation, occur at different locations and have different propagation directions. This indicates that the type III bursts forming a classical type III storm do not necessarily have a common origin but instead they indicate the existence of multiple, possibly unrelated, acceleration processes originating from solar minimum active regions.