Present models of the Venus solar wind interaction do not allow for changes in the orientation of the field as you approach the planet. Analysis of high resolution magnetic field data from the Pioneer Venus Orbiter spacecraft has revealed two distinct field rotations that are observed to occur in conjunction with the dayside ionosphere and ionopause. These rotations are a result of the velocity shear at the ionopause and indicate an alignment of the magnetic field with the day to night ionospheric plasma flow. From these results a new configuration of the dayside field draping has been determined. In addition, the field diagnostics discovered here can be used to probe the ionosphere of Mars which may otherwise go unobserved due to a lack of ion instrumentation onboard the Mars Observer spacecraft.

Venus represents the prototype for a class of objects whose interaction with the solar wind is characterized by the dominance of an ionospheric obstacle to the magnetized plasma. Though the interaction region between the bow shock and the ionopause boundary of Venus has been extensively studied and successfully modeled, the ionosphere itself, especially on the night side, has only been the subject of piecemeal models. These models either restrict themselves to two dimensions, or treat only one ionospheric phenomenon at a time. However, it is possible to combine the information from these models of the ionosphere into a coherent three dimensional model of the large-scale fields of the Venerean ionosphere. The model, which makes use of magnetic potentials to insure the proper continuity relations of field across boundaries and to insure the magnetic field is globally divergenceless, is developed by breaking the field into altitude-independent toroidal, poloidal, and flow-parallel components. These components are fit to terminator characteristics that can be specified by a very few number of parameters, and to an approximate adherence to Newtonian pressure balance at the dayside ionopause. Finally, the altitude profiles of the field are inserted into the model as the potentials are renormalized and fit to a more exacting ionopause boundary condition on the dayside determined by a gasdynamic treatment of the magnetosheath. In addition, methods of applying the model to similar objects are discussed.

The dayside magnetosphere contains a region where the field strength has a local maximum. This region, located just inside the magnetopause around the equatorial plane and between the cusps, has a width of 2--3 Re. When a drift shell with a sufficiently small mirror field intersects this region, it will bifurcate into two branches near local noon, each branch going across one cusp and joining together at the symmetrical local time. The particle then drifts around the Earth over a single branch until it comes back to local noon. In the neighborhood of the bifurcation points, the bounce period tends to infinity, and thus the adiabaticity of the bounce motion is broken there, but not elsewhere. This breaking causes a small but finite jump Delta I in the second invariant. Repeated crossings lead to a random walk in second invariant space, and thus to radial diffusion. We use theory and simulations to determine the magnitude of DeltaI. Our study is limited to static magnetic fields, but it can be extended to general fields. Our results indicate that DeltaI is sensitively dependent on bounce phase at bifurcation, and it can grow significantly for some initial conditions. When the initial second invariant I0 is much larger than the mirror gyroradius rhom, we use separatrix crossing theory. The average of DeltaI over bounce phases is zero, and the rms DeltaI is of the order of rho m. When I0 is comparable to rhom , the equation of bounce motion is approximated as the second Painleve equation, whose asymptotic solutions are used to determine Delta I. In this limit, the rms DeltaI is still O (rhom); however, the average is nonzero, in the form exp(- I0/rhom). Drift-shell bifurcation leads to significant radial diffusion. For MeV electrons, the diffusion coefficient can be several R e per day. Also, because of bifurcation, some quasitrapped particles can remain in the magnetosphere for a finite number of drifts before they leave permanently. Such behavior leads to metastable particles, a new kind of trapping. These results can be useful for radiation-belt modeling efforts.

This thesis employs low-altitude satellite data taken in the northern and southern dayside magnetospheric cusps in order to determine if the magnetopause current layer has a continuous and identifiable footprint at low altitudes. The magnetopause current layer, at the outermost edge of the magnetosphere, is the site of interconnection between the Earth's geomagnetic field and the Interplanetary Magnetic Field of the magnetosheath. It is an active region in which the normal MHD assumptions cannot hold. Data from eight near-polar orbiting spacecraft are compared with predictions of a small wedge-shaped cusp at the dayside boundary of the polar cap. Precipitating particle data as well as high-energy particles, fields, and wave data are shown which are consistent with predicted features. A kinetic raytracing model and statistical survey of Astrid-2 and DMSP satellite data were also undertaken as part of this work and show this feature to be persistent and dependent on the IMF angle at the magnetopause, as expected. The study of this feature may lend new insight into the dynamics of the cusp and magnetospheric particle entry.

The first major step has been completed in a long range project to merge the Fedder-Lyon Global 3D magnetohydrodynamic code and the Rice Convection Model (RCM) of the Earth's magnetosphere. Using MHD results as initial and boundary conditions, RCM runs were carried out for three different values of the energy invariant $\lambda$ of the plasma-sheet ions: $\lambda$ = negligibly small as in ideal MHD, $\lambda$ estimated from global MHD results, and $\lambda$ estimated from observations. In the first two runs, the RCM produced thin, well-defined patterns of region-2 magnetic-field-aligned currents shielding the inner magnetosphere from the convection electric field. These results differed substantially from the MHD result, indicating inaccuracy in the MHD code's numerical method when applied to the inner magnetosphere. The third run produced weak shielding and non-classic current patterns, which provide insight into the effect of plasma-sheet temperature on shielding.

A statistical study of ten years of solar wind particle and magnetic field observations, ionospheric convection measurements, and geomagnetic index data is combined with a case study of the interaction of the 18-21 October 1995 magnetic cloud event to illuminate several aspects of solar-terrestrial coupling. Models of polar cap responses to the solar wind are presented and compared to the observations from the case study. The sudden southward turning of the IMF during the event approximated a step function input to the coupled magnetosphere-ionosphere system. The resulting polar cap size, expansion rate, and polar cap potential are unusually large. This allows a straightforward analysis of effects which have traditionally been difficult to assess. During the event, the polar cap expanded by up to 5$\sp\circ$MLAT/hour, which is roughly the fastest rate of polar cap expansion observed by DMSP in a decade of continuous in-situ measurements. The rapid expansion is used to compare flow observations, estimates of the polar cap potential, and the induced emf which corresponds to the polar cap expansion by Faraday's Law. The analysis also resolves earlier indications that the hypothesized saturation of the polar cap potential drop exists, and confirms the numerical and functional predictions of Hill et al (1976). The implications for high time resolution models of the total polar cap potential are discussed. The statistical analysis includes an expanded set of empirical proxies which relate commonly used magnetospheric parameters. An analysis of the solar wind and ionospheric data also confirms the predictions of Hill (1985) regarding the rate of magnetic flux loss along the length of the magnetotail. In addition, while the ratio of open flux to polar cap potential is often approximated as a constant, the analysis reveals a functional dependence of the ratio which has implications for the length scale of the magnetotail. The ionospheric data used came from six low altitude Defense Meteorological Satellites (DMSP), while WIND and the Interplanetary Monitoring Platform (IMP 8) solar wind monitoring satellites provided solar wind plasma and field data. The data set spans the period from 1987 through 1996.

Physical knowledge of the laminar-turbulent transition process, prediction of the transition location, as well as the ability to control transition are essential in many engineering applications. However, control of the laminar-turbulent transition depends critically on various environmental sources and their ability to excite the instability waves in the flow, which are responsible for the laminar-turbulent transition. The process by which external disturbances are converted into instability waves is called receptivity. The research described in this thesis focuses on the receptivity of two- and three-dimensional boundary layers. The main objective of this research is to formulate, validate and apply adjoint analysis in order to predict receptivity. Adjoint analysis is a powerful approach for investigating the receptivity of different flows for arbitrary environmental sources. In this work, Adjoint Navier-Stokes (ANS) equations are formulated based on the sensitivity approach, and adjoint predictions are validated against Linearized Navier-Stokes (LNS) calculations. Further, Adjoint Parabolized Stability Equations (APSE) are derived as an approximation of ANS equations and compared against the ANS results. Our studies indicate that the APSE method should be constructed as an approximation to the ANS equations, not as the formal adjoint of the PSE. When implemented in this manner, we show that APSE is a viable method for receptivity prediction, even in highly nonparallel flows. The APSE is first applied to predict receptivity of weakly nonparallel two-dimensional boundary layer flows for a variety of parameters. We find that these flows are generally more receptivity to oblique disturbances although two-dimensional disturbances are less stable. We also find that favorable pressure gradient boundary layers are more receptive then adverse pressure gradient boundary layers, although adverse pressure gradients are destabilizing. The APSE are then applied to highly nonparallel three-dimensional boundary layers where we find that for the inviscidly unstable crossflow instability, stability effects typically dominate receptivity effects. Comparison of receptivity for stationary and unsteady crossflow instabilities shows that receptivity to both localized momentum sources and streamwise wall-velocity excitations is larger for unsteady modes, but that receptivity to wall-normal excitations is larger for stationary modes. Finally, we consider receptivity for the swept parabolic cylinder and observe that convex surface curvature tends to enhance receptivity to wall roughness, in agreement with prior studies. Utilizing the efficiency of adjoint methods, we also consider other excitations for the swept parabolic cylinder and show that convex surface curvature also enhances receptivity to streamwise momentum sources, but that receptivity to both normal and tangential velocity disturbances is slightly reduced.

The goal of laser astrophysics is to provide a means by which aspects of specific astrophysical phenomena can be reproduced in the laboratory. Although the hydrodynamic instabilities of early supernova remnants have already been studied using this method, the role of significant radiative losses in shock propagation (for example, in late-term remnants) has only been imperfectly modeled. This thesis introduces an improved self-similar analytic approach to radiative blast-wave evolution where the total amount of energy loss remains constant in proportion to the energy flux entering the shock front. The approximation is solved for the cases in which both energy loss from the shock front and heating of the shock (due to the presence of ionization precursors) are significant. Because this solution is independent of the exact method of cooling, it is appropriate for both the laboratory and astrophysical regimes. In addition, this thesis applies the analytic approximation to laboratory-produced radiative blast waves as well as to numerical models of these experimental blast waves. These results will allow for better design of laser-based experiments with further applications to astrophysical phenomena, as well as for an increase in the understanding of the challenges involved in scaling radiative phenomena between laboratory experiments and astrophysical theory.

The magnetospheric cusp is the principal site of solar wind plasma entry into the magnetosphere, and plasma entry through this region constitutes an important source of plasma in the Earth's magnetosphere. The goal of this dissertation is to understand the dynamics and location of the plasma injection process and the subsequent transport of this plasma throughout the magnetosphere by numerically modeling the cusp in terms of the "zeroth-order" physical processes. A quantitative model of ion injection and number density in the magnetospheric cusp is developed, incorporating mutually consistent electric and magnetic fields. This work extends the method of Onsager et al., who calculated precipitating particle fluxes from quantitative models of magnetosheath flow and ion acceleration at the magnetopause. We have simulated cusp ion energy-latitude spectrograms at mid-altitude. Both the large-scale energy-latitude dispersion and the embedded small-scale energy-pitch-angle V signatures are clearly evident in these simulated spectrograms. Our results show that a much finer V microsignature is obtained when the ion injection source is restricted to a small region. However, the cutoff of the plasma injection at the magnetosheath sonic line also yields relatively narrow V's, even without restricting the injection region to a small locus on the magnetopause. This effect is most noticeable in winter conditions. To explain the frequently observed multiple cusp ion injections that appear to overlap on the same field lines, we present two independent approaches. Our simulations have successfully reproduced the meso-scale cusp ion overlapping structure by firstly incorporating temporal effects of separate bursts of reconnection which last 1.4 min and are 3.6 and 4.6 mins apart; and secondly by introducing a time-dependent magnetosheath plasma density variation along the magnetopause to our cusp model, even with assuming steady interconnection. Our cusp injection model which returns precipitating particle flux also allows us to calculate the number density profile in the cusp. Our result along the noon-meridian cusp demonstrates that the density gradient is sharper on the equatorward edge than the poleward edge, and that the equatorward edge of the density structure shifts to higher latitude at lower altitude.

Advances in high-performance computing and Large-Eddy Simulation (LES) have made it possible to obtain accurate solutions of complex, turbulent flows at moderate Reynolds numbers. With these advances, computational modeling of turbulent flows in order to develop, evaluate, and optimize active control strategies is feasible. In this thesis, we present approaches to numerical modeling of opposition control and optimal control of turbulent flows with attention to algorithms that utilize the dynamic subgrid-scale LES model. Approximately 25% drag reduction is achieved by opposition control using LES, which is in good agreement with previous DNS results at a low Reynolds number of Retau = 180. Based on this success, we have used our LES approach to extend opposition control to a high Reynolds number flow of Retau = 590. With the sensing location at y+ ≈ 15, which is the best sensing plane for Retau = 180, only 21% drag reduction can be achieved, suggesting that opposition control is less effective at higher Reynolds numbers. An optimal control scheme based on LES has been implemented successfully using an instantaneous control approach with a nonlinear conjugate gradient algorithm used to update the control. The flow sensitivity is computed from the adjoint LES equations which are presented herein, and LES results are compared to prior DNS results for optimal control under similar conditions at Retau = 100. These comparisons indicate that optimal control based on LES can relaminarize low Reynolds number turbulent channel flow similar to results obtained using DNS but with significantly lower computational expense. Optimal control is also explored for turbulent channel flow at Retau = 180. At this Reynolds number, our optimal control approach is not as effective as the results at Retau = 100, the flow eventually enters a statistically stable state with approximately 40% drag reduction. Some possible ways to improve the control effectiveness are discussed, including the use of the discrete adjoint equations. Results are also presented for a novel hybrid LES/DNS scheme in which the optimization iterations are performed using LES while the flow is advanced in time using DNS for flow at Retau = 100. These hybrid simulations retain the computational efficiency of LES and the accuracy of DNS. Results from hybrid simulations clearly demonstrate that the controls computed based on LES optimization are also viable in the context of DNS. In all cases, the agreement between LES, DNS, and hybrid LES/DNS indicates that reliable turbulence control strategies can be efficiently developed based on LES models. We conclude that LES can be used as a reduced order model for optimal control of turbulence and this conclusions is shown to hold for even low resolution LES. The control mechanisms for drag reduction using opposition control and optimal control are discussed. Opposition control creates a virtual wall that affects all scales of turbulent motions near the physical wall. In contrast, optimal control creates a virtual wall for the large scale roll mode of the turbulent flow. Since this virtual wall affects directly the scales of motion responsible for increased drag in a turbulent flow, the optimal control is able to achieve larger drag reductions.