Computer Modelling and Polarimetric Studies of the Magnetic Field in the Crab Nebula Plasma

[摘要] In this thesis a study of polarimetric observations of the Crab nebula is presented. We also describe the development of a computer model which predicts the polarization pattern for a dipole magnetic field structure. The Crab nebula is a plasma and it's plasma behaviour is relevant to the development of a computer model of the nebular magnetic field. Therefore, we also examine in this thesis the basic plasma physics required for study of the Crab nebula. Chapter 1 is an introduction to this thesis outlining the raison d'etre of this project and presenting the relevant background information about the Crab nebula. Brief descriptions of the physical properties and history of the nebula are given, followed by a more detailed review of past theoretical works which have a bearing on this thesis. In chapter 2 we outline the theory of polarization of electromagnetic waves. The polarization parameters used in this thesis, the Stokes intensities and Stokes parameters, are introduced. The expressions for degree and angle of polarization derived from the Stokes intensities or parameters are formulated and we discuss these quantities and the effects of depolarization with relevance to this work. From the review in chapter 1 we find that it has long been known that the mechanism by which the observed radiation from the Crab nebula is produced is the synchrotron mechanism. Chapter 3 presents the theory of synchrotron radiation. In the latter sections of chapter 3 we examine the theory of polarization of synchrotron radiation and arrive at the equations required to calculate the Stokes intensities of synchrotron radiation for any given electron density distribution and magnetic field structure. In chapter 4 we consider the plasma aspects of the Crab nebula. A consideration of the macroscopic magnetohydrodynamic equations, which are a generalised form of Ohm's law, the Maxwell curl equations for the electric and magnetic fields, the equation of continuity, the equation of motion and the adiabatic equation for conservation of energy for the plasma, lead us to the concept of "frozen-in" magnetic fields. This has great relevance to the behaviour of the Crab nebula and we discuss recent papers in which attempts to solve the frozen-in field equation have been presented. We also include a discussion and a brief treatment of the frozen-in field equation which was made as a result of the study of recent work in this field. Chapters 5 and 6 make up the body of original work in this thesis. Chapter 5 presents the methods and results of analysis of the polarimetric data of Woltjer and McLean, Aspin and Reitsema. These observations are separated by a period of 26 years and so we examined the data for changes in polarization angle, and thus magnetic field direction, which might have occurred during this time. We also searched the data for centres of polarization or intensity patterns by a method of symmetry testing. Chapter 6 describes the development and presents the results of the computer generated model of a dipole magnetic field. We begin by considering special cases of the orientation of the dipole axis to the observer's line of sight and then move on to a model which allows any angle of inclination of the observer to the dipole axis. The final version of the model incorporates a component of magnetic field in the o-direction of the dipole coordinate system. This model produced some interesting results. The results of the data analysis and computer modelling are discussed in the relevant chapters, but chapter 7 contains some concluding remarks and an overview of the thesis. All computer programs written during the course of this project were written in Fortran and are included in the appendices at the back of the thesis.

[发布机构] University:University of Glasgow

[关键词] Astrophysics, Plasma physics