Spectral and Spatial Characteristics of Solar Flare Hard X-Ray Emission: A Non-Uniformly Ionised Thick Target Approach

McArthur, Guillian K (2000) Spectral and Spatial Characteristics of Solar Flare Hard X-Ray Emission: A Non-Uniformly Ionised Thick Target Approach. PhD thesis, University of Glasgow.

Abstract

Hard X-ray emission in flares is a signature of high energy electron populations in the solar plasma. The major role these populations have in many solar flare models means that the spectral, spatial and temporal characteristics of hard X-rays will be associated with many of the diverse plasma processes ocuring during a flare. These include where and how energy release occurs, which mechanism may accelerate the particles, some methods of energy transport through the flare and eventually the radiative and atmospheric response to the distribution of the flare energy budget. The work of this thesis has been concerned with the effects introducing the non-uniform ionisation profile of the flare atmosphere has on the thick target HXR spectra and consequent interpretation of the spectra. This has involved both the theoretical modelling and observational analysis of X-ray flare emission. The data for this research was primarily obtained from the instruments on board the YOHKOH satellite. These instruments include a hard X-ray spectrometer (HXS), a grazing incidence soft X-ray telescope (SXT) and a Fourier-synthesis hard X-ray telescope (HXT). Also available is high resolution hard X-ray spectra taken by the HIREX balloon experiment. Chapter 1 gives an overview of hard X-ray observations from solar flares and describes how these observations are interpreted in the context of the varying hard X-ray production models and the implications therefore on acceleration mechanisms. This chapter also includes a brief descripition of other flare emissions, mechanisms for energy release and how these mechanisms are incorporated into the physical model of flares. Chapter 2 introduces the formulation used in this analysis for including a non-uniform ionisation profile into the the thick target model of HXR production. Using this formalism, comparisons are made of HXR yields and inferred electron spectra for the fully ionised atmosphere with a more realistic step-function atmosphere. In doing so the differing spectral characteristics of these models are desrcribed, the ultimate effect on energy and number flux budgets for these differing spectra determined and the resultant non-uniqueness of the HXR bremsstrahlung inversion for a non-uniform atmosphere discussed. Prom this point the analysis of HXR spectra moves in two directions. Firstly in Chapter 3 high resolution data from the HIREX balloon experiment is inverted using regularisation techniques. In previous analyses of this dataset, which are described in detail during this chapter, the features in the photon spectra and inferred electron spectra were thought to be suggestive of a particular mechanism of acceleration (d.c. field acceleration). In our analysis, we investigate if these features in the inferred electron spectra can instead be reproduced by choosing an appropriate atmospheric struture, and therefore cannot be considered signatures of any particular acceleration mechanism. In chapter 4 a second approach is taken to utilise HXR spectra as a diagnostic of the flare atmosphere. By assuming that the observed spectral features are caused entirely by the effects on non-uniform ionisation, (i.e. the electron population is featureless, so any break is caused by the propagation effect) and incorporating the response function of the spectrometer we attempt to relate HXR spectra to particular coronal column density. In chapter 5 this approach is used for HXR spectral observations taken by the hard X-ray spectrometer onboard YOHKOH. Based on these spectral observations estimates of the amount of material between the acceleration site and the top of the chromosphere are made. These values can be compared to estimates of the column density for the SXR loops using SXT and also those given by time of flight analysis of the hard X-ray bursts to determine if the estimates are physically realistic. Finally, in chapter 6, results of the previous chapters are drawn together, providing a summary of the work achieved in this thesis. At the same time improvements to the analysis are discussed as are the possibilites for this analysis with the expected festival of data available soon from HESSI.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: John Brown
• Keywords: Astronomy
• Date of Award: 2000
• Depositing User: Enlighten Team
• Unique ID: glathesis:2000-75909
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Dec 2019 09:15
• Last Modified: 19 Dec 2019 09:15
• URI: https://theses.gla.ac.uk/id/eprint/75909