LOFAR observations of radio burst source sizes and scattering in the solar corona

TL;DR

This study uses LOFAR radio observations to measure the true size and position of a solar radio burst source, revealing insights into coronal turbulence and scattering effects at low frequencies.

Contribution

It introduces a direct visibility fitting method with LOFAR data to accurately determine radio source sizes, bypassing image deconvolution issues and providing new constraints on coronal turbulence.

Findings

Measured source sizes at 34.76 MHz are approximately 19' and 10' along major and minor axes.

Results indicate density fluctuations cause significant radio wave scattering in the solar corona.

The turbulence level may be lower than previously estimated from tied-array observations.

Abstract

Low frequency radio wave scattering and refraction can have a dramatic effect on the observed size and position of radio sources in the solar corona. The scattering and refraction is thought to be due to fluctuations in electron density caused by turbulence. Hence, determining the true radio source size can provide information on the turbulence in coronal plasma. However, the lack of high spatial resolution radio interferometric observations at low frequencies, such as with the LOw Frequency ARray (LOFAR), has made it difficult to determine the true radio source size and level of radio wave scattering. Here we directly fit the visibilities of a LOFAR observation of a Type IIIb radio burst with an elliptical Gaussian to determine its source size and position. This circumvents the need to image the source and then de-convolve LOFAR's point spread function, which can introduce spurious effects to the source size and shape. For a burst at 34.76 MHz, we find full width at half maximum (FWHM) heights along the major and minor axes to be $18.8^\prime$ $\pm~0.1^\prime$ and $10.2^\prime$ $\pm~0.1^\prime$, respectively, at a plane of sky heliocentric distance of 1.75 R$_\odot$. Our results suggest that the level of density fluctuations in the solar corona is the main cause of the scattering of radio waves, resulting in large source sizes. However, the magnitude of $\varepsilon$ may be smaller than what has been previously derived in observations of radio wave scattering in tied-array images.