No description available.

Incoherent scatter radars are powerful ground based instruments for ionospheric measurements. By analysis of the Doppler shifted backscatter spectrum, containing the signature of electrostatic plasma waves, plasma bulk properties are estimated. At high latitudes the backscattered radar power is occasionally enhanced several orders of magnitude above the thermal backscatter level. These enhancements occur during geomagnetic disturbed conditions and are referred to as naturally enhanced ion acoustic echoes (NEIALs). NEIALs are linked to auroral activity with optical auroral emission observed in the vicinity of the radar measurement volume simultaneously to NEIALs. The backscatter enhancements are thought to be caused by wave activity above thermal level due to instability. A number of theories have been put forward including streaming instabilities and Langmuir turbulence to explain NEIAL observations. NEIALs occur in two classes distinct by their Doppler features. Observations of the first type, which has been studied more extensively, are generally modelled well by the Langmuir turbulence model. The difficulty in trying to understand the driving mechanism of the instability is the limited spatial resolution of the radar measurements. Observations of the second type, reported on more recently, have been interpreted as evidence for naturally occurring strong Langmuir turbulence by means of their Doppler features. Aperture synthesis is a technique to increase the spatial resolution of the radar measurements to below beam width of the single receiver antennas. The technique is employed to investigate the structure of NEIALs in the plane perpendicular to the magnetic field at sub-degree scale corresponding to hundreds of meters to a few kilometres at ionospheric altitudes. Calibration of the radar interferometer is necessary and a calibration technique is presented in paper I. Interferometry observations of a NEIAL event with receivers deployed at the EISCAT incoherent scatter radar on Svalbard are presented in paper II. The size of the enhanced backscatter region is found to be limited to 900 x 500m in the plane perpendicular to the geomagnetic field. These observations constitute the first unambiguous measurements giving evidence for the limited size of the enhanced backscatter region. In paper III observations of strong Langmuir turbulence signatures are presented. The apparent turbulent region in these observations is limited to two narrow altitude regions, 2km extent, and electron density irregularities caused by the turbulence are thought to reach down to decimeter scale length. The turbulence observations were obtained during energetic electron precipitation thereby differing from other observations during which a low energy component in the electron precipitation is reported. In paper IV a statistical study of strong Langmuir turbulence radar signatures is presented. The study reveals differing local time distributions for these signatures from type I NEIALs indicating di_ering driving conditions for the two types of NEIALs. It is found that strong Langmuir turbulence signatures are predominantly observed in the pre-midnight sector where auroral break-up aurora prevails. / <p>QC 20150303</p>

Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 252-257). Available also in a digital version from Dissertation Abstracts.

At high winds, turbulence in the ocean surface mixed layer is dominated by organized coherent structures in the form of counterrotating helical vortices known as Langmuir cells. While the dynamics of the ocean surface layer has been studied rather extensively at lower wind speeds, the detailed physics at higher winds has remained largely inaccessible because of limited sea-going operations and difficulty conducting in situ measurements at high sea states. In the present thesis new measurement techniques, based on acoustical remote sensing, are described. A freely drifting imaging sonar was employed, which allowed us to follow time-evolving features for an extended period of time. This imaging sonar extends the acoustical approach beyond fixed orientation sonars and covers a full 360° circle on the surface. The full circle capability turns out to be a key addition to the measurements: it allowed quantitative evaluation of the directional properties of Langmuir circulation surface structure. These new methods allow us to sample near-surface circulation and bubble distributions even in extreme conditions, and contribute to our understanding of small scale dynamics in the wind driven surface layer. Using vertical velocity measurements in the convergent regions of Langmuir circulation and a model scaling, we infer the effective viscosity relevant to cell generation. Matching velocity- and temperature-inferred turbulent viscosities we estimate the depth scale over which the wind-wave forcing is of most importance. The velocity-inferred viscosity compares favorably with the mean model viscosity values evaluated at approximately two significant wave heights below the surface. Combining the effective viscosity calculated at different depths with the observed Stokes drift and friction velocity we estimate Langmuir numbers La between 0.015 and 0.1. We observe evolving cell patterns at larger La (between 0.02 and 0.05), which indicates that higher viscosity values than previously assumed in the models may be relevant for Langmuir circulation dynamics. Acoustical observations of the orientation of surface bubble clouds and the directional wave field during several deployments provided an opportunity for comparison of the directional properties of Langmuir circulation with a model that takes into account effects associated with misalignment of the Stokes drift and wind forcing. Model results imply that the growth rate is maximal overall when wind and waves are aligned. For a given angle between the Stokes drift and the wind (the misalignment angle) the direction of the cell axis for maximal growth lies between the Stokes drift and the wind and is mainly determined by (i) the misalignment angle and (ii) the ratio of the Stokes drift shear and mean Eulerian shear. Our ocean observations showed Langmuir cells responding to the changes in wind direction within 15 to 20 min. On two occasions, when the wind changed direction and waves lagged behind, the cells were observed to form in an intermediate direction (between wind and waves) consistent with model predictions. Observations of the near-surface circulation and thermal structure during a storm motivate analysis in terms of the Froude number derived from the measured vertical density gradient, the turbulent diffusivity which is inferred from the measured temperature distributions, and velocity and spatial structure of the circulation. The results demonstrate inhibition of Langmuir circulation by the presence of warm surface water at the beginning of a storm and provide a test of model description of the balance between wind-driven stirring and buoyant resistance. To better understand our measurements and the limitations of the approach, based on the acoustical backscatter, a technique for scatter location estimation is proposed. By comparing velocity magnitudes, independently measured with side-looking and upward-looking sonars, we estimate an effective scattering depth. These results show that the backscatter measured with side-looking sonars originates not right at the surface but at some depth below. / Graduate

No description available.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Teymour Boutros-Ghali. / Ph.D.

Plasma turbulence on a uniform density gradient with unfavorable magnetic curvature is investigated extensively in the Helimak device. The turbulence is strong with density and electrostatic potential fluctuation levels in excess of 40%. Measurements of the dispersion relation, k[subscript z], and k[subscript parallel lines] identify the the fluctuations with drift-waves, which propagate in the poloidal direction at the diamagnetic drift velocity and have a small, but nite parallel wavenumber. A non-zero phase shift between the density and potential fluctuations gives rise to turbulent cross-eld particle transport, which is measured using spectral techniques. In addition, the electrostatic drift-wave fluctuations have a small magnetic component that is driven by the turbulent parallel current [scientific symbols]. An examination of nonlinear processes associated with the plasma turbulence uncovers high levels of intermittency near the plasma edge and long-time persistence of the density fluctuations on the order of the parallel confinement time. An analysis of the bispectrum conrms the existence local and nonlocal three-wave interactions between unstable drift-waves although the turbulent saturation of the density fluctuation spectrum is likely due nonlinear processes acting in the time domain. / text

Abstract available in PDF file. / Thesis (Ph.D.)-University of Natal, 1979.

The study of low-frequency turbulence in magnetized plasmas is a difficult problem due to both the enormous range of scales involved and the variety of physics encompassed over this range. Much of the progress that has been made in turbulence theory is based upon a result from incompressible magnetohydrodynamics (MHD), in which energy is only transferred from large scales to small via the collision of Alfv ́n waves propagating oppositely along the mean magnetic field. Improvements in laboratory devices and satellite measurements have demonstrated that, while theories based on this premise are useful over inertial ranges, describing turbulence at scales that approach particle gyroscales requires new theory. In this thesis, we examine the limits of incompressible MHD theory in describing collisions between pairs of Alfvén waves. This interaction represents the fundamental unit of plasma turbulence. To study this interaction, we develop an analytic theory describing the nonlinear evolution of interacting Alfv ́n waves and compare this theory to simulations performed using the gyrokinetic code AstroGK. Gyrokinetics captures a much richer set of physics than that described by incompressible MHD, and is well-suited to describing Alfvénic turbulence around the ion gyroscale. We demonstrate that AstroGK is well suited to the study of physical Alfvén waves by reproducing laboratory Alfvén dispersion data collected using the LAPD. Additionally, we have developed an initialization alogrithm for use with AstroGK that allows exact Alfvén eigenmodes to be initialized with user specified amplitudes and phases. We demonstrate that our analytic theory based upon incompressible MHD gives excellent agreement with gyrokinetic simulations for weakly turbulent collisions in the limit that k⊥ ρi << 1. In this limit, agreement is observed in the time evolution of nonlinear products, and in the strength of nonlinear interaction with respect to polarization and scale. We also examine the effect of wave amplitude upon the validity of our analytic solution, exploring the nature of strong turbulence. In the kinetic limit where k⊥ ρi ≥ 1 where incompressible MHD is no longer a valid description, we illustrate how the nonlinear evolution departs from our analytic expression. The analytic theory we develop provides a framework from which more sophisticated of weak and strong inertial-range turbulence theories may be developed. Characterization of the limits of this theory may provide guidance in the development of kinetic Alfvén wave turbulence.

The turbulence and fluctuation induced transport in the edge plasma of the Tokoloshe tokamak was studied using a Langmuir probe array. In this thesis three separate experiments are presented, each of which examines a particular aspect of the edge plasma. In the first experiment measurements of edge plasma parameters are presented. These include standard parameters (such as Ne, Op , Te, etc.) as well as features such as the velocity shear, T(t) during periods of both high and low Mirnov activity, Te/Te and Q. These are compared with results from other machines as well as predictions of several turbulence theories. It was found that many of the results are very similar to those obtained on other machines and that, since the operating parameter space on Tokoloshe is well within the parameter space described by drift wave theories, resistivity-driven gradient driven turbulence theories do not describe the edge turbulence. In the second experiment external windings are used to produce fields which can slow and lock magnetic islands in the toroidally rotating plasma. Edge parameters are again presented and these results compared with those from the so-called 'reference' plasmas, i.e. ones in which no locking occurred. During locking some parameters are dramatically altered, e.g.Te/Te Standard transport theory ignores the effect of Te/Te since they are usually small in reference discharges. During the locked phase, however, certain measurements used to deduce T and Q are greatly affected by increases in Te/Te. As a result, certain assumptions regarding these measurements are no longer valid. Comparison of results for different island positions (produced by different coils) indicates that the assumption of poloidal and toroidal symmetry of edge conditions is invalid. The third experiment investigates the high frequency (~60 kHz), low amplitude, magnetic oscillation which characterises the locked phase and which exhibits some small degree of correlation with the fluctuations observed on (e.g.) Of'. Since over 80% of the spectral power of Te/Te lies below 70 kHz and since Of /Te depends strongly on Te/Te , it is suggested that the magnetic mode and these large variations in Te, may be due to a similar physical process. / Thesis (Ph.D.)-University of Natal, 1993.