"June 8, 1961." / Bibliography: p. 19. / Army Signal Corps Contract DA36-039-sc-78108. Dept. of the Army Task 3-99-20-001 and Project 3-99-00-000.

The solar tachocline is a thin transition layer between the solar radiative zone rotating uniformly and the solar convection zone, which has a mainly latitudinal differential rotation profile. This layer has a thickness of less than $0.05R_{sun}$ and is subject to extreme radial as well as latitudinal shears. Helioseismological estimates put this layer at roughly $0.7R_{sun}$. The tachocline mostly resides in the sub-adiabatic, non-turbulent radiative interior, except for a small overlap with the convection zone on the top. Many proposed dynamo mechanisms involve strong toroidal magnetic fields in this transition region. The exact mechanisms behind the formation of such a thin layer is still disputed. A very plausible mechanism is the one involving a weak, relic poloidal magnetic field trapped inside the radiative zone, which is responsible for expelling differential rotation outwards. This was first proposed by citet{RK97}. The present work develops this idea with numerical simulations including additional effects like meridional circulation. It is shown that a relic field of 1~Gauss or smaller would be sufficient to explain the observed thickness of the tachocline. The stability of the solar tachocline is addressed as the next part of the problem. It is shown that the tachocline is stable up to a differential rotation of 52% in the absence of magnetic fields. This is a new finding as compared to the earlier two dimensional models which estimated the solar differential rotation (about 28%) to be marginally stable or even unstable. The changed stability limit is attributed to the changed stability criterion of the 3-dimensional model which also involves radial gradients of the angular velocity. In the presence of toroidal magnetic field belts, the lowest non-axisymmetric mode is shown to be the most unstable one for the radiative part of the tachocline. It is estimated that the tachocline would become unstable for toroidal fields exceeding about 100~Gauss. With both formation and stability questions satisfactorily addressed, this work presents the most comprehensive analysis of the physical processes in the solar tachocline to date. / Die Sonne besteht aus verschiedenen Zonen, die durch die Art des Energietransports von innen nach aussen unterschieden werden. Der innere Teil heißt Strahlungs-, der äußere Teil Konvektionszone. Die Grenzschicht zwischen den beiden Regionen liegt bei etwa 70% des Sonnenradius. Beide Zonen rotieren in ausgezeichneter Weise um die Sonnenachse. In der konvektiven Zone ändert sich die Rotationsrate mit dem Breitengrad und ist nur schwach von der radialen Position abhängig. Dies wird als latitudinale differentielle Rotation bezeichnet. Im Gegensatz dazu rotiert ein Großteil der Strahlungszone gleichförmig. Der Übergang von gleichförmiger Rotation im Inneren zu differentieller Rotation außen geschieht innerhalb einer sehr dünnen Schicht, die ungefähr mit der Grenzschicht zwischen den beiden Zonen übereinstimmt. Diese Schicht hat eine Ausdehnung von etwa 5% des Sonnenradius und wird als “Tachokline” bezeichnet. Die Existenz der Tachokline wurde vor etwa zwei Jahrzehnten bestätigt. Seit ihrer Entdeckung wurden verschiedenste Modelle vorgeschlagen, um die Existenz einer solchen Schicht zu erklären. Diese Arbeit wendet das bislang beliebteste und erfolgreichste dieser Modelle an, das zuerst von Rüdiger & Kitchatinov (1997) vorgeschlagen wurde. Darin wird angenommen, dass während ihrer Entstehung ein schwaches Magnetfeld im Inneren der Sonne eingeschlossen wurde. Ein solches Feld verdrängt die differentielle Rotation erfolgreich in den äußeren Randbereich der Strahlungszone und erzeugt so die Tachokline. Die Theorie nimmt weiter an, dass die Tachokline aktiv mit der darunterliegenden strahlungsdominierten Zone verbunden ist, gemäß der Beobachtung, dass ein Großteil der Tachokline unterhalb des Fußes der Konvektionszone liegt. Diese Arbeit legt verbesserte numerische Simulationen vor, die dem früheren Modell zwei neue physikalische Effekte hinzufügt: schwache radiale und horizontale Strömungen (“meridionale Strömungen” genannt) und Temperaturgradienten. Es wird gezeigt, dass ein eingeschlossenes Feld von weniger als einem Gauß ausreichend wäre, die beobachtete Dicke der Tachokline zu erkären. In einem weiteren Schritt wird versucht zu ergründen, ob die Tachokline eine stabile Schicht innerhalb der Sonne ist. Es wird gezeigt, dass sie, in Abwesenheit eines Magnetfeldes, stabil bleibt, solange die Winkelgeschwindigkeit am Pol nicht unter 52% derer am Äquator fällt. Da sekundäre Strömungen hauptsächlich horizontal verlaufen, haben Temperaturgradienten wenig Einfluss auf die Stabilität der Tachokline. In Gegenwart eines Magnetfeldes wird die Grenzschicht für Felder stärker als 100 Gauß instabil. Indem sowohl Fragen zur Entstehung als auch zur Stabilität zufriedenstellend angesprochen werden, stellt diese Arbeit die derzeit umfassendste Analyse der physikalischen Vorgänge in der Tachokline dar.

Results from several studies are presented which detail explorations of the physical and spectral properties of low luminosity active galactic nuclei. An initial Sagittarius A* general relativistic magnetohydrodynamic simulation and Monte Carlo radiation transport model suggests accretion rate changes as the dominant flaring method. A similar study on M87 introduces new methods to the Monte Carlo model for increased consistency in highly energetic sources. Again, accretion rate variation seems most appropriate to explain spectral transients. To more closely resolve the methods of particle energization in active galactic nuclei accretion disks, a series of localized shearing box simulations explores the effect of numerical resolution on the development of current sheets. A particular focus on numerically describing converged current sheet formation will provide new methods for consideration of turbulence in accretion disks.

This study uses the MHD (Magnetohydrodynamics) model to simulate unsteady two-dimensional transport variables in helium plasma under low pressure between two infinite planar electrodes suddenly biased by a negative voltage. Plasma has been widely used in etching, ion implantation, light source, and encountered in nuclear fusion, etc. Studying transport processes of plasmas therefore is important. By account for momentum exchange collisions, electric fields and magnetic fields the computed results in this work quantitatively show density, velocity, electric potential, temperature, viscosity, thermal conductivity of the ions and electrons across the sheath to the surfaces suddenly biased by a dc negative voltage.

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This thesis deals with the dependence of the linear spatial growth rate of the cross-field ion acoustic instability on various plasma parameters. A kinetic theory model, with elastic and inelastic ion-neutral collisions included, is presented and used to conduct a numerical survey of the instability. The growth rate is computed as a function of distance into the plasma, taking into account the attenuation of the ion beam by charge exchange collisions. Further calculations show the variation in growth rate as a function of the following quantities: electron and ion beam temperature, electron density, beam velocity, background ion temperature, magnetic field, the angle between magnetic field direction and wave vector and the finite width of the plasma. The instability was observed in a double plasma device where an ion beam was passed through a background of stationary magnetized electrons. The magnetic field was sufficiently weak to allow approximately rectilinear ion motion. The growth rate of the wave was studied using interferometer techniques. It was identified by the dispersion relation as the cross-field ion acoustic wave propagating as the slow mode of the beam. It was found that the background ions play an important role in determining the phase velocity. Experimental data of the growth rate dependence on wave number, beam velocity and magnetic field strength were found to be well described by the theoretical model. The growth rate dependence of magnetic field direction on plasma width was furthermore found to be in qualitative agreement with the model. / Thesis (M.Sc.)-University of Natal, Durban, 1990.

A magnetohydrodynamic (MHD) cavity-resonance model is developed to study the ultra low frequency (ULF) response in the magnetosphere to an external compressional impulse. It is assumed that the magnitude of the impulse is small enough such that non-linear terms remain negligible. The MHD differential equations are derived in a cold, non-uniform plasma imbedded in a cylindrical ambient field geometry and are solved using numerical finite difference integration methods. The crucial feature of the model is that it allows for the investigation of the response within the magnetospheric cavity to an impulse that has both temporal and spatial form. There is strong observational evidence that low-latitude Pi 2 pulsations have, or are associated with, a global propagation mechanism. Evidence alluding to the global nature of low-latitude Pi 2 is the characteristically low azimuthal (or axial) wavenumbers, (Irnl ;S 1 ). Further evidence of the global nature of Pi 2 is the lack of arrival time difference between globally separate events, as well as the similarity in the spectral content of globally separate events. As an application, the cavity-resonance model is applied to investigate the Pi 2 pulsation event. The cavity-resonance waves are excited by an impulsive perturbation at the magnetopause which is centred about the midnight meridian. The excitation signal is chosen representing the causal Pi 2 mechanism thought to be associated with the sudden, short circuiting of the cross-tail current to the auroral oval. Various aspects of the cavity-resonance wave modes are investigated and the appropriateness of this type of modelling for -the study of Pi 2 is evaluated. Numerical integration and well as Fourier and Laplace methods are used to investigate the transmission of the impulsive signal through the magnetosphere. Coupling between the isotropic (cavity) and the transverse Alfven (resonance) mode is studied. The effect of the plasmapause is considered. Longitudinal variations of polarization as well as the latitudinal phase variations of the perturbed fields are computed. Computational results are compared with observational features of the Pi 2 event. / Thesis (Ph.D.)-University of Natal, Durban, 1993.

Cataclysmic variable stars (CVs) are interacting binary systems. One of the stars (referred to as the primary) is a white dwarf, the other (referred to as the secondary) is usually a late main sequence star such as a red dwarf. Due to the closeness of the two stars, the white dwarf accretes gasses from the secondary. If the white dwarf does not possess a strong magnetic field, these gasses go into orbit, forming an accretion disk around the primary. If the white dwarf does possess a strong magnetic field, the gasses cannot form an accretion disk because they are entrained by the magnetic field lines. Cataclysmic variable stars in which the magnetic field is strong enough to prevent the formation of the accretion disk are called AM Herculis-type systems, after their prototype. In this study, the time-resolved spectroscopy of two AM Herculis-type binary systems, QQ Vul and EF Eri, are analyzed. In addition, Doppler Tomography, an analysis technique previously applied primarily to cataclysmic variable stars with accretion disks, is applied to these systems. / Department of Physics and Astronomy

Thesis (Ph. D.)--University of Washington, 1989. / Vita. Includes bibliographical references (leaves [136]-137).

Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 124-131). Available also in a digital version from Dissertation Abstracts.