Magnetohydrodynamic dynamo in disc-like astrophysical bodies has been considered for some time. Important astrophysical objects like accretion discs, protostellar and protoplanetary nebulae and galaxies are thought to regenerate a magnetic field through a dynamo mechanism. Although there is a well developed theory for describing the regeneration of magnetic field in these objects, there are not any specific methods how to calculate such magnetic fields in general case. In this work, after a description of the dynamo theory, the specific method for solving the nonspherical dynamo is introduced. The unique feature of this method is accommodation of variable magnetic diffusivity in order to model the shape of a disc-like body. The detailed construction of the method is presented, as well as description of mathematical and numerical methods used for obtaining the solution. The method of checking the model with respect to well established spherical models is also presented. Finally, some examples are calculated and discussion is given on the behaviour of calculated magnetic field and possible astrophysical implications.

A number of effects of low to medium strength (< 2000 gauss photospheric) magnetic fields on otherwise normal stars are proposed and examined. We consider magnetic perturbations to the standard stellar structure and evolutionary calculations in the core, the deep envelope, and the extreme outer envelope in intermediate to high mass stars. In the stellar core the gas pressure probably far exceeds the (B²/8π) magnetic field pressure term so that the only effect of such a field may come from its inhibiting convection in the core. We present isochrones of both convective and radiative core models of 2-5 M(⊙). In the deep envelope, we may expect to see mixing of partially nuclear processed material driven by rising and falling magnetic flux tubes. The effects of this mixing will be brought to the surface during the deep convection phase of the star's tenure as a red giant. We use this model to predict a signature for magnetic mixing based on the CNO isotope and abundance ratios. In the outer envelope the gas pressure is low enough that we might expect to see a perturbation of the stellar structure due to the magnetic field pressure itself. We calculate this perturbation under several physical models for intermediate and high mass stars and determine that sufficient magnetic field energy may be available in the outer envelope to expand a star by about 20% over its unperturbed radius. Finally we consider the evidence for the existence of non-magnetic neutron stars, concluding that while no non-magnetic neutron stars have ever been positively identified, we have no evidence that prevents the existence of at least as many non-magnetic as magnetic neutron stars.

Thesis (M. Phil.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves 92-94).

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves [83]-90).

A thesis submitted to the Department of Physics, University of the Witwatersrand in fulfilment of the academic requirements of the degree of Doctor of Philosophy. Johannesburg, June 17, 2016. / The nature of dark matter remains one of the most significant problems in modern physics. One candidate hypothesis with strong theoretical motivation is a WIMP (Weakly Interacting Massive Particle) in the form of the supersymmetric neutralino. Although this lacks experimental evidence, supersymmetry may manifest in collider experiments and also has observable consequences in the emission of annihilation products from cosmic structures which would play host to large neutralino densities. Historically, interest has been focussed upon the direct detection of WIMPs, as well as indirect observation through -rays produced by pair-annihilation processes. In this work we present a detailed argument for a multi-frequency observational strategy. We motivate this by studying the redshift evolution of radio synchrotron radiation emitted by WIMP annihilation-product electrons. In so doing, we demonstrate the potential power of the SKA to probe the neutralino parameter space, being able to produce constraints several orders of magnitude better than current limits and distinguish between differing neutralino masses and annihilation channels. Furthermore, we motivate the SKA as a machine to study dark matter by discussing its ability to characterise cosmic magnetic fields through rotation measures and polarimetry, as well as resolve between non-thermal emissions produced by dark matter and those resulting from strictly baryonic processes, like star formation. These technical capabilities obviate the central uncertainties in the study of synchrotron radiation. Additionally, we highlight optimal laboratories for neutralino detection via the SKA, based on structures that might produce dark matter emissions with lower background synchrotron radiation. In particular we highlight galaxies and galaxy clusters at redshifts z 1 and local dwarf spheroidal galaxies. Finally, we demonstrate the potential of X-rays, through the ASTRO-H mission, to compliment the SKA in a multi-frequency search. We then demonstrate our multi-frequency approach in the analysis of several prominent claims of signals compatible with dark matter-induced emissions. Showing that models designed to account for the AMS-2/Fermi/PAMELA antiparticle and galactic centre -ray excesses are incompatible with existing data as well as demonstrating that a dark matter interpretation of the Reticulum-2 -ray excess is untenable for a wide range of dark matter particle masses. / LG2017

Abstract not available

Not available / text

No description available.

Thesis (M. Phil.)--University of Hong Kong, 1993.