Halpha Impact Polarization as a Solar and Astrophysical Particle Diagnostic

Fletcher, Lyndsay (1993) Halpha Impact Polarization as a Solar and Astrophysical Particle Diagnostic. PhD thesis, University of Glasgow.

Abstract

This thesis is concerned chiefly with the development and application of three different models for the generation of impact polarization in the Halpha line, during solar flares. It also includes work on the possible production of impact polarization ion the stellar source SS433. In chapter 1 the observational background to the solar work is described, followed by a general introduction to the current ideas about the solar chromosphere in which the Halpha excitation is thought to occur. A summary is also given of the processes involved in solar flares and the various emissions resulting. The atomic physics needed for the calculations in the following chapters is described in the second part of the introductory chapter. Chapter 2 outlines the methods of calculation of background emission and the general numerical techniques which will be used in future chapters. It then discusses the generation of impact polarization by beams of intermediate energy protons impacting the chromosphere. The main result of this chapter is that the proton beam model is found to put unrealistically large demands on the total energy budget for the flare and, contrary to the conclusions of previous work, is not considered capable of explaining the observations. Chapter 3 is concerned with the excitation of polarized radiation by high energy electron beams scattering in the collisionally thick chromosphere whilst having their motion affected also by a converging chromospheric magnetic field. It is found, in general to be a successful model, in terms of explaining the polarization magnitude and orientation, and also in accounting for the sporadic appearances of accompanying HXR signatures. Included in this chapter is a discussion of the application of the method of solving Fokker-Planck equations using stochastic simulations. Chapter 4 moves away from excitation by beams associated with the transfer of energy from the corona to the chromosphere and considers the possibility that the response of the atmosphere to impulsive phase energy input is the generation of fragmented, relatively low energy upflows of ionised material which through their interaction with surrounding neutral material excite impact polarized radiation. This model is successful at explaining the direction, the area and the timescales of the polarization observations, and the polarization fraction generated is, with conservative choices of atmospheric and flow parameters, of the same order of magnitude as that observed. Chapter 5 moves away from impact polarization as a purely solar particle diagnostic and discusses the generation of impact polarized Halpha adiation in the optical bullets of SS433, as a result of the supposed interaction of the bullets with the ionised wind from a stellar companion. Here it is found that under a wide range of bullet physical conditions, a considerable polarization fraction is to be expected if the wind interaction model is correct. Chapter 6 is a final short chapter discussing the shortcomings of the work so far carried out in the chromospheric flare investigations, and proposing ways in which it could be extended and improved. In particular it is suggested that the effects of radiative transfer of polarized radiation, and of the magnetic fields in the chromosphere be studied in an attempt to explain temporal and spatial variations in the observed polarization fraction. Comment is made on the extension of the Halpha impact polarization diagnostic to other astrophysical sources.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: John C Brown
• Keywords: Astrophysics
• Date of Award: 1993
• Depositing User: Enlighten Team
• Unique ID: glathesis:1993-75290
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 21:19
• Last Modified: 19 Nov 2019 21:19
• URI: https://theses.gla.ac.uk/id/eprint/75290