Zhang, Jinge; (2024) Exploring the Solar Middle Corona through Low Frequency Radio Bursts. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Solar radio bursts are signatures of electron beams traveling through various magnetic flux tube structures in the solar corona. Type III bursts move along ’open’ magnetic fields into interplanetary space, while J and U bursts travel along closed loops. This thesis delves into these bursts to dissect the kinetic properties of accelerated electron beams and the physical attributes of the middle coronal plasma and magnetic field. Three distinct research projects are introduced in this thesis. Firstly, using the LOw-Frequency-ARray (LOFAR) and the Parker Solar Probe (PSP), 54 type III and J-bursts were identified during a significant solar radio noise storm on 10 April 2019. These observations revealed a critical similarity in exciter velocities between the burst types, suggesting a shared electron injection acceleration region across both open and closed magnetic structures during the event. Furthermore, a new method was introduced for inferring the plasma density model and physical parameters of large coronal loops using J burst spectra. Secondly, with the LOFAR interferometric solar imaging data from 5 June 2020, a large coronal loop extending to 2.3 solar radii was captured as a bright Uburst was observed. This marked the first simultaneous imaging of both the upward and downward legs of a large loop extending into the middle corona. The analysis allowed for unprecedented comparisons of beam velocities and loop physical parameters between the two legs of this expansive loop, revealing electron beam deceleration and shedding light on the historically ill-defined large loops. Lastly, by utilizing microwave and radio measurements during a solar burst on 3 June 2021, the travel time of the electron beam from the microwave emission to the radio emission region was estimated. By testing various numerical coronal density models and comparing them with the time gap between peak fluxes at two different wavelengths, a coronal plasma density model for the event was extrapolated. This offers a novel multi-wavelength approach for diagnosing corona plasma conditions. Firstly, using the LOw-Frequency-ARray (LOFAR) and the Parker Solar Probe (PSP), 54 type III and J-bursts were identified during a significant solar radio noise storm on 10 April 2019. These observations revealed a critical similarity in exciter velocities between the burst types, suggesting a shared electron injection acceleration region across both open and closed magnetic structures during the event. Furthermore, a new method was introduced for inferring the plasma density model and physical parameters of large coronal loops using J burst spectra. Secondly, with the LOFAR interferometric solar imaging data from 5 June 2020, a large coronal loop extending to 2.3 solar radii was captured as a bright U-burst was observed. This marked the first simultaneous imaging of both the upward and downward legs of a large loop extending into the middle corona. The analysis allowed for unprecedented comparisons of beam velocities and loop physical parameters between the two legs of this expansive loop, revealing electron beam deceleration and shedding light on the historically ill-defined large loops. Lastly, by utilizing microwave and radio measurements during a solar burst on 3 June 2021, the travel time of the electron beam from the microwave emission to the radio emission region was estimated. By testing various numerical coronal density models and comparing them with the time gap between peak fluxes at two different wavelengths, a coronal plasma density model for the event was extrapolated. This offers a novel multi-wavelength approach for diagnosing corona plasma conditions.

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