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published_or_final_version / Physics / Master / Master of Philosophy

The propagation and stability of waves in relativistic astrophysical plasmas is presented. Our investigation, using a relativistic two-fluid model, is different from previous relativistic fluid studies in that the plasma is treated fully relativistically, both in temperature and in directed speed. Much of this study is devoted to relativistic linear waves in pulsar pair plasmas, with a view to elucidating a possible mechanism for pulsar radio wave emission. We also study interesting nonlinear exact solutions in both relativistic and non-relativistic plasmas. Pulsar pair plasmas can support four transverse modes for parallel propagation. Two of these are electromagnetic plasma modes, which at high temperature become light waves. The remaining two are Alfvénic modes, split into a fast and a slow mode. The slow mode, always sub-luminous, is cyclotron (Alfvén) two-stream unstable at large wavelengths. We find that temperature effects, within the fluid model used, do not suppress the instability in the limit of large (finite) magnetic field. The fast Alfvén mode can be super-luminous only at large wavelengths; however, it is always sub-luminous at high temperatures. In this incompressible approximation, only the ordinary mode is present for perpendicular propagation. We discuss the implications of the unstable mode for radio emission mechanisms. For typical values, the instability is quite fast, and the waves can grow to sizable levels, such that, the magnetic modulation could act as a wiggler. The pulsar primary beam interacting with this wiggler, could drive a free electron laser (FEL) effect, yielding coherent radiation. Investigation of the FEL in this setting and demonstrating that the frequency spectral range, and luminosities, predicted by this mechanism is well within the observed range of radio frequency (and luminosity) emissions, is one of the principal results of this dissertation. It is tempting to speculate, then, that an FEL-like radiation effect could be responsible for the highly coherent radio wave emissions from pulsars. In the study of nonlinear exact solutions we have generalized the results to the incompressible Hall Magnetohydrodynamics (HMHD). We find that for cases when the plasma is weakly magnetized the frequencies of the modes decrease as the wave amplitude (effective mass) increases. For very strongly magnetized plasmas the light-like modes tend to be asymptotically linear; the frequency is unaffected by wave amplitude. / text

Scalar and vector electromagnetic boundary-value theories of surface scattering have been applied to a microrough sample of fused silica coated with aluminum. Surface profiles were made with a mechanical and interferometric profilometer. The resulting values of rms surface height and autocorrelation length were used to calculate the scattering for various angles, wavelengths, and polarization conditions. Bidirectional reflectance distribution function (BRDF) measurements associated with scatter value were made and their corresponding power spectra were generated. Both were used to compare with the theoretical values. Special attention was also paid to the various methods of determining optical parameters with the BRDF data and the corresponding power spectra.

Approved for public release; distribution is unlimited / Under meteorological conditions associated with extreme cold air outbreaks (CAO) off the U.S. East Coast, large ocean waves sometimes develop along the North Wall of the Gulf Stream. These wave events produce wave heights above those expected given the short fetch and moderate winds. The highest waves are often very localized, which suggests localized forcing by the atmosphere. In this study, results from four cases are examined to characterize the role of high resolution, mesoscale wind forcing in generating localized regions of large ocean waves during events with large air-sea temperature differences. A known "true" atmosphere is simulated through the use of the Navy's Coupled Oceanographic and Atmospheric Mesoscale Prediction System (COAMPS). Model surface wind output from COAMPS is used to generate a wave field using Wavewatch Three (WW3), which is then compared to buoy observations and ship reports. Results of these cases show the mesoscale wind forcing of ocean waves during CAO and the importance of mesoscale atmospheric modeling in localized generation of ocean wind waves. Additionally, empirical wave forecast techniques are compared to WW3 model output for these cases to further reinforce the mesoscale atmospheric forcing during rapid growth of wind wave events in fetch limited environments. / Lieutenant Commander, United States Navy

M.Sc., Faculty of Science, University of the Witwatersrand, 2011 / Rayleigh wave dispersion curves are used to invert for the group velocity maps of the southwestern block of the Congo craton. The group velocity maps were then inverted to obtain the three dimensional shear-wave velocity of the lithosphere beneath the region. In the process, the adjacent Kalahari craton and Damara mobile belt were also mapped to help constrain the southernmost edge of the Congo craton. To obtain the surface wave group velocity tomography, event-station dispersion curves of Rayleigh waves were measured using the multiple filter analysis method. Then the dispersion curves were inverted using the conjugate gradient least-square (CGLSQR) inversion method. To check the reliability of the result, a checkerboard test was performed. The 2-dimensional group velocities and 3-dimensonal shear-wave velocities were found to be faster beneath the southwestern block of the Congo craton and the Kalahari craton and slower in the Damara mobile belt. The group velocity map at 20s period shows that basins are 0 to 3% slower than PREM model. For longer period (50s to 120s), the Central and East African Rift system are ~ 5 % faster, cratons are 5 to 8% faster, and the adjacent mobile belts are 0 to 4% faster than the PREM model. The Afar depression is the slowest, up to 6% slower than the continental PREM model at all periods. The shear-wave velocity maps reveal that (1) the Afar area is the slowest (up to 8% slower than the IASP91 model), (2) the cratons are faster (up to 6% faster than IASP91) than the surrounding mobile belts (up to 2% faster than IASP91). The East African Rifts system is also slow (up to 5%). The Damara mobile belt constitutes a clear separation terrain between the Congo craton and the Kalahari craton. This result is consistent with previous studies by Pasyanos and Nyblade (2007), and Priestly et al. (2006, 2008), who also found faster shear-wave velocities beneath the Kalahari, Congo and Tanzania cratons. The relatively slow seismic velocities (-1 to 2% compared to IASP91) in the Proterozoic Damara mobile belt between the southwestern block of the Congo craton and the Kalahari craton are explained by the view that the Proterozoic lithosphere has hotter rock materials than the SW block of the Congo craton and the Kalahari craton. Our model of faster lithosphere beneath the SW block of the Congo and the Kalahari craton is also consistent with the model of strongly depleted (in basaltic components) lithosphere beneath these craton; compared to less depleted lithosphere beneath the DMB.

This work introduces formulae of Fresnel type related to reflection and transmission of a plane electromagnetic wave from a moving boundary separating two isotropic dielectrics. The dielectrics themselves remain immovable, so, the ensuing material formation represents an example of an activated dynamic material assembled from the LC-arrays serving as the discrete versions of each dielectric.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982. / Supervised by Erik Mollo-Christensen. Vita. Includes bibliographical references (leaves 321-326).

Thesis (M.C.E.)--University of Delaware, 2005. / Principal faculty advisor: James T. Kirby, Dept. of Civil & Environmental Engineering. Includes bibliographical references.

Thesis--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution. / Includes bibliographical references (leaves 220-233).