Probing the Solar Corona at High Temporal and Spatial Resolution with the Low Frequency Array

Abstract

The solar corona is the outermost layer of the Sun's atmosphere. Advancements in radio astronomy over the last 50 years have revealed a number of radio phenomena which occur in the corona each with different temporal and spectral characteristics. Current generation interferometers such as the LOw Frequency ARray (LOFAR) give an unprecedented insight into the fine spatial, spectral and temporal structure of these radio bursts. Of particular interest are what are known as type III radio bursts, which are indicative of electrons propagating along open magnetic field lines in the solar corona. Observations of type III bursts allow for the remote sensing of the plasma at various heights in the corona due to the relation between emission at the plasma frequency and electron density. High spatial resolution observations of radio bursts give insight into the role of radio wave scattering on the observed source sizes while high temporal and spectral resolution observations can be used to determine the power density of electron density fluctuations in the corona.

Key results of this thesis come from observations of solar radio emission at the highest temporal, spectral and spatial resolutions to date. Firstly, the REAL-time Transient Acquisition backend (REALTA) was developed and installed at the Irish LOFAR station (I-LOFAR) to record the raw voltages from the station at $5.12 \mu$s temporal resolution. This is among the most advanced data acquisition clusters installed at an international LOFAR station to date. First light observations from REALTA are shown, including a variety of solar radio bursts showcasing fine temporal and spectral structure. The installation of REALTA allows for observations of solar radio bursts at some of the highest temporal resolutions to date and is a key resource in investigating the fine temporal structure of solar radio emission.

Secondly, a new technique was implemented to directly measure the size of radio bursts from their interferometric visibilities. This is the first time that such a technique has been used to study a type IIIb radio burst. Spectroscopic analysis of the fine frequency structure of a type IIIb burst is used to calculate an expected a source size of $3.18$~arcsec. The full width at half maximum height (FWHM) along the major and minor axes of the burst at 34.76~MHz is found to be $18.8$~$\pm~0.1$~arcmin and $10.2$~$\pm~0.1$~arcmin respectively. % at a plane of sky heliocentric distance of 1.75~R$_\odot$. The new fitting technique used in this analysis removes the need for interferometric imaging and as such, these results indicate that the large size observed for the type IIIb radio burst is due to radio wave scattering in the corona. It is also determined that this effect may be over estimated by previous tied array imaging observations.% These results suggest that the level of density fluctuations in the solar corona is the major cause of the scattering of radio waves, resulting in large source sizes.

Finally, this technique was utilised to determine the size and shape of 29 type III bursts and compare them to predictions from state-of-the-art radio wave scattering simulations. It is found that these bursts have a mean size along the major and minor axis of FWHM\textsubscript{x} = 16.27~arcmin and FWHM\textsubscript{y} = 11.96~arcmin respectively. No trend of source size with respect to helioprojective longitude is found, which is in contrast to predictions from modelling of anisotropic scattering from a point source. I attribute this discrepancy to an intrinsic source size of type III bursts. These results underscore the necessity for thorough comparison between radio observations and radio wave scattering simulations before one can be used to infer information from the other.

I highlight some future work that could be built upon the research presented in this thesis to further advance the knowledge of radio wave generation and propagation in the solar corona.