Low Frequency Observations of the Solar Corona using LOFAR

Abstract

The Sun is a highly dynamic, active star, dictated by the evolution of its magnetic fields, which transition from periods of maximum activity to periods of minimum activity over an 11-year cycle. During solar maximum the Sun often produces a variety of large-scale, energetic events such as solar flares, erupting prominences, jets and coronal mass ejections (CMEs). The absence of solar activity is referred to as the quiet Sun. There is much to be learned by observing these seemingly quiet periods of minimal activity. The studies presented in this thesis take advantage of times of minimal solar activity to answer two of solar physics’ largest questions, (1) How does scattering affect observed radio emission from the Sun? (2) Are micro- and nanoflares contributing to the heating of the Sun’s outer atmosphere, the solar corona? This thesis made use of observations from the LOw Frequency AR-ray (LOFAR), a low frequency (10–270 MHz) radio interferometer spanning the continent of Europe. LOFAR allows for the study of the low frequency radio Sun with unprecedented spectral, spatial and temporal resolution. Ireland joined the LOFAR network in 2016, which granted us observing time and access to archival LOFAR data as seen in this thesis. The first part of this thesis focuses on the construction, testing and first results from the Irish LOFAR station, I-LOFAR. It describes the pivotal role I played in the setting up of I-LOFAR. I was heavily involved in the project in its planning and organisation stage in 2016, in its construction and testing in 2017, and presently I perform station maintenance and observations. Working hands-on with such a modern, cutting-edge telescope has given me an invaluable insight into radio observations and analysis. Before the start of this thesis research, solar interferometric imaging with LOFAR was extremely rare. This is due to it being such an involved process with expert knowledge required. In this thesis, two separate pipelines were developed which made use of two different imaging software (CASA and WSCLEAN) to perform calibration and interferometric imaging of solar data. In addition, this thesis developed novel, unique imaging techniques to improve upon the results provided by standard interferometric imaging. The second project of this thesis is a LOFAR imaging study of the solar corona during a solar eclipse on 2015 March 20. A technique called multi-frequency synthesis (MFS) was employed to image the solar eclipse between 120–180 MHz. In addition, a lunar de-occultation technique was developed to achieve higher spatial resolution (∼0.6') than that obtainable using standard interferometric imaging (∼2.4'). This high resolution imaging technique provided a means of studying the contribution of scattering to apparent source size broadening. It was found that the de-occultation technique revealed a more structured quiet corona that is not resolved from standard imaging. Recent studies have used imaged source sizes, paired with ray tracing simulations to quantify the effect of radio wave scattering on low frequency radio waves. The results presented in this project suggest that standard imaging techniques may be overestimating the extent of scattering in the radio quiet Sun. The third project of this thesis involved the imaging of the quiet Sun using LOFAR on 2019 March 30. Again, multi-frequency synthesis (MFS) was used to achieve increased sensitivity. A novel technique was developed that extracted the standard deviation of each pixel’s brightness over time. This technique removed the quiet Sun background and revealed small scale variations in the brightness temperature over time that could be attributed to microflare activity. This imaging technique allowed for the identification of an extremely faint type III burst (0.2 times the brightness of the quiet Sun). The origin of the burst was determined as being a coronal bright point within a polar coronal hole. This unique set of observations highlight the faint radio emission associated with quiet Sun solar activity that may be contributing to coronal heating. Finally, this thesis is concluded with some preliminary future work results that can build on the research carried in this thesis.