Thermal and Nonthermal Radiation Signatures in Solar Flares: Models and Observations

Matthews, Sarah A (1996) Thermal and Nonthermal Radiation Signatures in Solar Flares: Models and Observations. PhD thesis, University of Glasgow.

Abstract

The work of this thesis has been concerned with various aspects of the solar flare phenomenon, and covers a wavelength range from the optical to gamma-ray, in varying degrees of depth. The approach taken has been a combination of modelling of physical processes and the application of these models to solar observations from the Solar Maximum Mission (SMM), and in particular the Yohkoh mission. Attempts have also been made, in Chapters 3 and 5, to relate nonthermal high energy phenomena such as gamma-rays and hard X-rays to the lower energy thermal radiation signatures of soft X-ray and optical emission. Chapter 1 provides an overview to the solar flare phenomenon, with particular emphasis on the observations. Where appropriate, the mechanisms responsible for the production of radiation signatures from flares has been described, and a brief and by no means complete, description of the possible mechanisms for flare energy release and transport are described. For those radiation signatures which are considered in further detail in later chapters the emphasis is on describing recent observations, particularly those from Yohkoh. A brief description of future up and coming satellite missions is also given. Chapter 2 furnishes an introduction to the Yohkoh satellite and its instrumentation. Much of the work in later chapters involves the analysis of data from Yohkoh and it is necessary to have a basic understanding of how this data was collected, and its limitations to enable results based on it to be placed in a proper perspective. Chapter 3 discusses the hypothesis that stochastic acceleration by Alfven wave turbulence is responsible both for the acceleration of gamma-ray producing protons, and for the observed nonthermal line broadening in coronal soft X-ray lines. A general background to the subjects of nonthermal line broadening, particle acceleration and gamma-ray line production is presented and the model is described together with the results of its application to data from four flares observed by SMM and one flare observed by Yohkoh. This model is particularly interesting on the basis of its self-consistent approach to the problem of relating the thermal and nonthermal aspects of solar flares. Chapter 4 investigates the posssibility that decreasing levels of Hydrogen ionization in the deep chromosphere of solar flares could cause the return current associated with a thick target electron beam to become unstable to the generation of ion-acoustic wave turbulence. To begin with the background behind the need for a nearly co-spatial return current to exist is described, along with some observational evidence for the existence of electron beams in solar flares. A simple model is described and the results of its application to semi-empirical model solar atmospheres is described. We find that the possibility for such an instability to develop exists and that it can lead to the break up of the thick target electron beam and significant anomalous energy deposition at the level in the solar atmosphere where this occurs. Chapter 5 follows on from Chapter 4 in considering that the generation of ion-acoustic wave turbulence via a return current instability can provide sufficient heating to power the white-light flare. To begin with, the white-light flare phenomenon is described in some detail, with emphasis on the various existing theories for its production. Then the analysis of the hard X-ray and white-light emission from four white-light flares observed by the Yohkoh satellite is described. The limitations of the results in respect to the available data are also described. It is found that there is a definite correlation between the hard X-ray and white-light flare and that the hypothesis of the white-light flare being powered by anomalous ohmic heating cannot be ruled out. Chapter 6 draws together the results of the previous chapters, providing a summary of the work achieved in this thesis. It also points out some of the limitations of the approaches adopted and outlines possible improvements and new directions.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: John Brown
• Keywords: Astronomy
• Date of Award: 1996
• Depositing User: Enlighten Team
• Unique ID: glathesis:1996-74997
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 27 Sep 2019 14:41
• Last Modified: 27 Sep 2019 14:41
• URI: https://theses.gla.ac.uk/id/eprint/74997