Solar radio emissions.
Creators: name: Yip, Wan Kwan
Publisher: name: University Of Tasmania
Year: 2023

description: All radiation processes described can take place in the solar corona, depending on different physical characters of the radiating electrons and the coronal plasma, such as electron energy, electron pitch angle and the magnitude of the ratio f-2p/f-2H of the medium in the region where the radiators reside. On the other hand, the observed spectral features of a solar radio emission are determined by both the mechanism responsible for the emission and the propagation conditions of electromagnetic waves in the solar corona. Thus, the observed dynamic spectra of the solar radio emissions reveal essentially the physical conditions of the solar corona during the emission periods. The emissions of narrow bandwidth type III bursts and drift pair bursts have been proposed as the consequence of plasma radiation caused by electron streams in the isotropic corona, which is understood to refer to electromagnetic radiation extracted from Cerenkov plasma waves through scattering or wave-mode coupling (Ginzburg and Zheleznyakov, 1958; Roberts, 1958; Wild et al., 1963; Zheleznyakov, 1965). However, many observed spectral characteristics of the type III bursts and its related U bursts, the drift pairs and the newly discovered hook bursts clearly indicate that it is necessary that the sunspot magnetic field be taken into account in the theories. So far the conditions of plasma radiation by electron streams in a magnetoactive plasma have not been investigated in detail and consequently, many characteristics of these emission events remain unexplained. In this thesis, we begin by formulating the theory of plasma radiation in a magnetoactive plasma (Chapter II and Chapter V). The theory is then applied to the interpretation of the polarized type III burst and U burst emission event and the drift pair and hook burst emission event in Chapter VI and Chapter VII respectively. The origins of various components of the complicated type IV emission have been interpreted as the results of synchrotron radiation by relativistic electrons trapped in the sunspot magnetic field configurations (Boischot and Denisse, 1957; Takakura and Kaip 1961; Kai, 1964; etc.). However, without taking into account the effects of the medium on the radiation process, previous theories encounter various difficulties and fail to account for the outstanding polarization characters of the type IV emission. In order to set up a satisfactory theory of type IVA emission, we study, for the x-mode and the o-mode, the power spectra from single electrons, the process of amplification of cyclotron radiation in a stream-plasma system and the escape conditions in detail in Chapter III and Chapter VIII. Cyclotron radiation of electromagnetic waves in the x-mode and the o-mode and the Cerenkov excitation of plasma waves by helical electron streams moving at the base of the solar corona is proposed as the origin of the type IVA emission. It is found that most of the important characteristics of the emission can be well accounted for by the theory. In Chapter IX, coherent synchrotron radiation from a system of relativistic electrons is studied taking into account the effects of the ambient plasma. The origins of some broad band solar continuum emissions are also discussed. In the final chapter, a conclusion of the thesis is given and further researches are suggested.