No description available.

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.

No description available.

Optical polarimetry results are presented for four spiral galaxies: NGC 5194 (M51), NGC 1068, NGC 4565 and NGC 4594 (Ml04). M51 and NGC 1068 show spiral polar isation patterns which are interpreted as indicating a spiral magnetic field in each case. NGC 4565 and M104 show polar isations in their dust lanes which are parallel to their galactic planes, and which are interpreted in terms of a magnetic field in the plane of each. It is hypothesised that the observed magnetic fields may be linked to galactic shocks. A discussion of the origin of galactic magnetic fields concludes that there is no evidence which necessitates a primordial magnetic field.

This thesis is concerned with the effect of static and oscillatory magnetic fields on the yield of radical recombination reactions and the proposal that such effects may constitute a possible mechanism for the interaction of environmental electromagnetic fields with biological systems. A brief overview of research pertaining to the biological effects of environmental electromagnetic fields is presented. Next, the concept of the radical pair is introduced and the theory of its behaviour in solution is examined in order to illustrate the mechanism by which magnetic fields can affect its probability of separation. Three different experimental systems involving the attack of free-radicals on DNA are presented. The extent of DNA damage is assayed in the presence and absence of a static magnetic field. These systems involve the killing of yeast cells as observed by the growth of subsequent colonies, direct observation of strand breaks to DNA, in vitro, by gel electrophoresis and the direct observation of intra- nuclear DNA damage by microgel analysis. In all systems, magnetic field effects are observed but are difficult to reproduce consistently. The design of novel apparatus for the observation of resonant radiofrequency effects is described. The application of a 30-40 MHz oscillating magnetic field is found to alter the yield of exciplex fluorescence in the photoreaction of anthracene-d₁₀ and 1,3-dicyanobenzene. The effect is interpreted in terms of a change in the efficiency of singlet andleftrightarrow; triplet interconversion in the {anthracene cation - DCB anion} radical pair when the oscillating field is in resonance with hyperfine splittings in the DCB anion radical.

There is increasing public concern over the possible harmful effects of extra-low-frequency (ELF) electromagnetic (em) fields. A variety of harmful effects have been linked with em field exposure, for example depression and suicide, carcinogenesis, birth defects, and spontaneous abortion. The epidemiological evidence, especially the more recent studies, suggests that there may be a small increased risk of childhood cancers in those living near overhead power lines. However laboratory evidence to date does not support this link. An epidemiological correlation does not necessarily imply a causal relationship: to confirm such a relationship, biological and laboratory evidence is required. This work studies the magnitude of human exposure to ELF magnetic fields from overhead power lines and visual display units (VDUs) and also investigates possible health effects at the microscopic level through cell culture experiments. From the epidemiological evidence, an effect threshold of approximately 300 nT was estimated. Detailed magnetic field distributions were measured round a variety of visual display units and only a very few emitted magnetic fields even close to the 300 nT threshold. Following an in-depth case study of a VDU operator who suffered from a combination of symptoms including eyestrain and headaches, a study of the health of nearly 200 office workers and VDU users was set up. The aims of the study were to investigate the range and severity of symptoms suffered by VDU users, to investigate possible causes and to study response to VDU work at an individual level. It was found that those working between 10 and 30 hours per week at a VDU were more likely to suffer symptoms of ill health. In contrast, those working more than 30 hours per week at a VDU were not more likely to suffer symptoms when compared with a control group of those working less than 10 hours per week at a VDU. In particular, increased reporting of eye problems and fatigue were most strongly associated with VDU work.

A thesis submitted to the faculty of science, University of the Witwatersrand, Johannesburg, In fulfilment of the requirements for the Degree of Doctor of Philosophy Johannesburg 1990 / The processing of near-earth satellite magnetometer data towards extracting an improved lithosphere anomaly map is addressed. The two aspects of the data reduction route where data quality has been compromised most by contemporary methods is researched, namely;[Abbreviated Abstract. Open document to view full version] / GR2017

The magnetic (B) field is ubiquitous throughout the Milky Way. Several fundamental questions about the B-field in the cool, star-forming interstellar medium (ISM) remain unanswered. In this dissertation, near-infrared (NIR) polarimetric observations are used to study the large-scale Galactic B-field in the cool ISM in a spiral arm and to determine the role of B-fields in the formation of Infrared Dark Clouds (IRDCs). NIR polarimetry of 31 star clusters, located in and around the Perseus spiral arm, were obtained to determine the orientation of the plane-of-sky B-field in the outer Galaxy, and whether the presence of a spiral arm influenced B-field properties. Cluster distances, which provide upper limits to the B-field probed by observations, were estimated by developing a maximum likelihood method to fit theoretical stellar isochrones to stars in cluster color-magnitude diagrams (CMDs). Using the distance estimates, the cluster locations relative to the Perseus arm were found. The cluster polarization percentages and orientations were compared between clusters foreground to the arm and clusters inside or behind the arm. The cluster polarization orientations are predominantly parallel to the Galactic plane. Clusters inside and behind the arm have larger polarization percentages, likely a result of more polarizing material along the line of sight. The cluster polarization data were also compared to optical, inner Galaxy NIR, and Planck submm polarimetry data, and showed agreement with all three data sets. The polarimetric properties of one IRDC, G28.23, were determined using deep NIR observations. The polarization orientations relative to the cloud major axis were found to change directions with distance from the cloud axis. The B-field strength was estimated to be 10 to 100uG. Despite these large inferred B-field strengths, the B-field was found not to be the dominant force in the formation of the IRDC, though the B-field morphology was influenced by the cloud. Using NIR observations, the B-field of 27 IRDCs were studied. The relative polarization orientations with respect to the cloud major axes were found. No preferential relative orientation was found, implying that the B-field did not greatly influence the formation of this sample of IRDCs.

Graduation date: 2003

No description available.