Nonlinear propagation of intense electromagnetic waves in quasar and pulsar plasmas

Abstract

his thesis presents the results of analytical as well as numerical investigations of radiation–plasma interaction instabilities in astrophysical plasmas. It consists of two introductory chapters, four main chapters, and one conclusion chapter. The introductory Chapter 1 provides an introduction to astrophysical situations rich with the possibilities of radiation–plasma instabilities. Chapter 2 describes the parametric plasma instabilities along with the derivation of the general dispersion relation.

The remaining four chapters are concerned with the specific cases of parametric instabilities. Each chapter begins with an introduction, derivation of analytical results followed by numerical results (figures). The effects of various parameters such as density and temperature of plasma, luminosity, bandwidth, and polarization of radiation on plasma instabilities have been studied in detail. The results, which are presented in graphical form and tables, are analyzed and discussed thoroughly. A brief conclusion at the end of each chapter sums up the results. The books and original papers referred to in the text of the thesis are listed alphabetically at the end of each chapter. Although various symbols and abbreviations are defined as and when they occur, for easy reference the list of more frequently used symbols and abbreviations are provided at the end of the thesis.

Chapter 3 explains the possible role that stimulated Raman and Compton scattering play in the continuum emission of a quasar. There are three ways in which an electromagnetic wave can undergo scattering in a plasma: (i) when the scattering of radiation occurs by a single electron, it is called Compton scattering; (ii) if it occurs by a longitudinal electron plasma mode, it is called stimulated Raman scattering; and (iii) if it occurs by a highly damped electron plasma mode, it is called stimulated Compton scattering. The non-thermal continuum of quasars is believed to be produced through the combined action of synchrotron and inverse Compton processes, which are essentially single-particle processes. Here, we investigate the role of stimulated Raman scattering and stimulated Compton scattering in the generation of continuum radiation from these compact objects. It is shown as an example that the complete spectrum of 3C 273 can be reproduced by suitably combining stimulated Compton scattering and stimulated Raman scattering. The differential contributions of these stimulated scattering processes under different values of the plasma parameters are also calculated.

In Chapter 4, the coherent plasma process such as parametric decay instability has been applied to a homogeneous and unmagnetized plasma. These instabilities cause anomalous absorption of intense electromagnetic radiation under specific conditions of energy and momentum conservation and thus cause anomalous heating of the plasma. The maximum plasma temperatures reached are functions of the luminosity of the radio radiation and plasma parameters. It is believed that these processes may be taking place in many astrophysical objects. Here, the conditions in the sources 3C 273, 3C 48, and Crab Nebula are shown to be conducive to the excitation of parametric decay instability. These processes also contribute towards the absorption of 21-cm radiation, which is otherwise mostly attributed to neutral hydrogen regions.

In Chapter 5, the change in polarization of an electromagnetic wave due to stimulated Raman scattering in a plasma is discussed. In this process, an electromagnetic wave undergoes coherent scattering off an electron plasma wave. It is found that some of the observed polarization properties, such as rapid temporal variations, sense reversal, rotation of the plane of polarization, and change of nature of polarization in the case of pulsars and quasars, could be accounted for through stimulated Raman scattering.

Chapter 6 deals with the modulational instability of a large-amplitude electromagnetic wave in an electron–positron plasma. The modulational instability arises due to the effect of relativistic mass variation of the plasma particles, harmonic generation, and the non-resonant, finite-frequency electrostatic density perturbations, all caused by the large-amplitude radiation field. The radiation from many strong sources such as quasars and pulsars has been observed to vary over a host of time-scales. It is possible that extremely rapid variations in the non-thermal continuum of quasars as well as in the non-thermal radio radiation from pulsars can be accounted for by the modulational instabilities to which the radiation may be subjected during its propagation out of the emission region

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In Chapter 7, we conclude that coherent plasma processes must be included in the study of generation, absorption, polarization, and modulation of electromagnetic radiation in high-energy sources such as quasars and pulsars.