LOFAR tied-array imaging and spectroscopy of solar radio bursts

Abstract

The Sun is the source of the most powerful explosions in the solar system such as solar ares and coronal mass ejections (CMEs). These explosions can occur up to several times per day and they release up to 1032 ergs (1025 J) of energy. Radio emission at frequencies ranging from a few kHz to a few hundred GHz is often associated with these energetic phenomena. At low radio frequencies (<200 MHz), the Sun has only been studied at selective frequency bands due to instrumental and imaging limitations of previous radio telescopes. With the recent commissioning of the Low Frequency Array (LOFAR), the low frequency Sun can now be studied with unprecedented spectral, spatial and temporal resolution. The low frequency radio spectrum and the possibility of imaging of radio bursts at low frequencies was explored with LOFAR, which has only been done before on limited occasions with older radio telescopes. A new observational method was applied to solar observations, tied-array beams, to analyse the spectral and spatial characteristics of Type III radio bursts. Type III radio bursts are expected to be a good diagnostic of the electron density in the solar corona as they are emitted by the plasma emission mechanism. However, due to the highly dynamic corona, the locations of Type III radio bursts found in this analysis (1-4 R.) occurred at altitudes in excess of existing 1D electron density models of the solar corona. Some of these bursts were found at very high altitudes and had non-radial trajectories which could not be taken into account by existing density models. The non-radial high altitude Type IIIs were found to be associated with the expanding flank of a CME deflecting surrounding coronal streamers. This study showed that the highly dynamic corona has a large impact on the propagation of electron beams which needs to be studied in more detail as existing electron density models cannot predict the location of these beams. The second part of this thesis focuses on the possibility of imaging fine temporal and spectral structures in dynamic spectra, making use of the high spectral and temporal resolution of the LOFAR tied-array beams. This was investigated after the promising results obtained when imaging Type III radio bursts in the first part of the thesis. This work advanced the study of solar S bursts that have been observed at low frequencies on only a number of occasions. LOFAR observations were used to study the spectral and spatial characteristics of a multitude of S bursts, as well as their origin and possible emission mechanisms. Since S bursts have short time scales of < 1 s, standard interferometric imaging was not suitable for their detection. Tied array images with a high cadence of 50 ms were used to image S bursts for the first time and relate them to the coronal magnetic field. In addition, the S burst properties were constrained by a statistical study of over 3000 S bursts observed during an 8-hour observation campaign with LOFAR. The unusual nature of solar S bursts such as short durations and higher degree of circular polarisations that differ significantly to Type III radio bursts, motivated the study on the possible emission mechanisms for coherent radio emission in the corona. Plasma emission is believed to be the dominant coherent emission mechanism for most solar radio bursts. The electron-cyclotron maser (ECM) emission has only been proposed to explain the coherent emission of more complex, highly polarised bursts that occur mostly at decimeter wavelengths. The conditions of ECM emission in the solar corona were investigated using two-dimensional data-constrained magnetic field and electron density maps of the corona. These maps were used to estimate the ratio of the plasma frequency to the electron-cyclotron frequency which dictates if ECM is possible. The ECM condition was satisfied in the solar corona but only at small heights (< 1:1 R. from the centre) within a large active region with a complex magnetic structure. In addition, high Alfven speeds were also found in these regions (> 0:02 c) that have not been reported before. ECM could be a possible emission mechanism for high-frequency radio and microwave bursts that have previously been observed at these heights. Data constrained models have not been applied to an active region before and it was shown that ECM is a likely coherent emission mechanism on the Sun. Finally, small scale solar energetic events such as solar jets were studied to determine the nature and origin of accelerated electrons that generate solar radio bursts. A bright Type III-like radio burst was observed by LOFAR and it was found to be associated with a solar jet. Multi-wavelength data were combined to study the solar jet and the magnetic field configuration in the vicinity of the jet as well as the path followed by the accelerated electrons produced at the same time as the jet. Newly emerged positive magnetic flux in the negative polarity region of an area of bipolar plage appeared to be the trigger of the jet. Radio imaging showed that the Type III-like burst originated in a region above the newly emerged magnetic field and then followed long, closed magnetic field lines to the top of the loop at a height of ~360 Mm. Magnetic reconnection between the overlying coronal field lines and the newly emerged positive field lines was most likely the cause of the solar jet and accelerated electrons that produced the radio burst. This work provides new observational evidence in which a radio burst related to a solar jet occurs due to magnetic flux emergence on the Sun, outside of an active region.