Today, the earth's instrinsic magnetic field prevents the streaming solar wind plasma from directly interacting with the terrestrial atmosphere. Periodically the geomagnetic field reverses direction, and during the transition period, the earth's magnetic barrier is thought to disappear. In the past, studies about the atmospheric-solar wind interaction dynamics were hindered by a scarcity of observational data. However, since December 1978, the Pioneer-Venus Orbiter has been providing daily observations of the atmospheric dynamics produced by the direct solar wind interaction with the atmosphere of Venus (the only planet known to lack an intrinsic magnetic field). This thesis develops the first three dimensional magnetohydrodynamic (MHD) theory of this interaction. Within the ionosphere of Venus analytic solutions to the MHD equations are possible because of a favorable geometry between the induced ionospheric magnetic fields and the ionospheric plasma motions. It is shown that variations in the solar wind speed and interplanetary magnetic vector direction cause variations in the dayside ionospheric plasma flows and the induced magnetic field configuration, and that these changes can account for the variety of magnetic structures observed by Pioneer-Venus. Portions of the Venus ionosphere are shown to be susceptible to the Kelvin-Helmholtz shear instability. The unusual shape of the computed region of stability is shown to be an important key to understanding the highly variable Pioneer-Venus observations. Model calculations are compared to observations for a number of selected orbits, and the model is shown to match the observations in fine detail.

Horizontal ionospheric conduction currents are driven by magnetic field-aligned currents generated in the Earth's magnetosphere. Traditional ideas about field-aligned currents imply that most of the higher latitude set of field-aligned (region-1 Birkeland) current flows on field lines that connect to antisunward-flowing magnetospheric plasma. This conclusion disagrees with recent satellite data, and that disagreement provides a major motivation for this thesis. This research includes a stability analysis that is based on current conservation, which implies that field-aligned current is balanced by the divergence of ionospheric current. This stability analysis demonstrates that there may be a sector, within the plasma sheet in the nightside magnetosphere, where plasma pressures are reduced relative to the surrounding regions. In order to simulate this depleted region, several "computer experiments" have been performed, enforcing gradients in plasma content at the tailward boundary of the calculation. These "computer experiments" use an adapted form of the Rice Convection Model which has previously been used to calculate Birkeland currents (as well as other magnetospheric currents) in the inner magnetosphere by utilizing time dependent data from specific geophysical events. The present work extends the model further out in the magnetosphere. Results of these "computer experiments" applied to the 19 September 1976, substorm event show that it is possible to generate high latitude magnetic field-aligned currents (region-1) connecting to regions of sunward plasma motion in the magnetosphere, indicating a significant departure from classical notions. The new model agrees better with observations than the earlier Rice model, with regard to the latitudinal distribution of the lower latitude (region-2 Birkeland) field-aligned currents, with a general increase in latitudinal extent. The peak magnitudes of the generated currents relative to region-2 current strength are (TURN)50% on the dusk side, and (TURN)100% on the dawn side. However, approximately 50% of the current may exist outside the modeling region. This generating mechanism may very well be the most important region-1 current source in the regions of (TURN)18:00 LT to 21:00 LT and (TURN)3:00 LT to 6:00 LT which causes flow of region-1 current on sunward convecting flux tubes.

Particle precipitation around the earth's polar regions may be the footprint of various energizing phenomena in the magnetosphere. Satellite-observed electron fluxes whose peak energy increases then decreases are called inverted V's. The Atmosphere Explorer-D Low Energy Electron (LEE) data for January 11, 1976 indicates that the precipitating ions have been accelerated. In this event the spectrograms of the ion flux shows the change of the peak energy with time characteristic of an inverted V. The electron population is decelerated as the ion population is accelerated, consistent with a downward electric field. The Birkeland current at an inverted V may be calculated in two ways: from the divergence of the electric field or from the observed particle fluxes. We found that the two methods agree on the location of Birkeland current throughout the event, but the magnitudes are not the same. This is not surprising, since the component of (DEL)((')(SIGMA)(.)(')E) perpendicular to the trajectory can not be determined. The electric potential along the spacecraft's trajectory (790-650 km altitude) was calculated from the measured electric fields. The sum of the parallel potential drop (inferred from the ion distribution function) and the ionospheric potential gives the potential profile at the magnetosphere. The parallel electric field thus partially decoupled the ionospheric flow from the magnetospheric flow. The electric field pattern in the magnetosphere-ionosphere system demands field-aligned currents. When the thermal current is insufficient, a field-aligned potential drop can accelerate particles to satisfy the requirements. The thermal electron current from the ionosphere is much greater than that from the magnetosphere. Thus, it is more common to observe the signatures of an upward electric field: the electron "inverted V". In the ion inverted V observed during AE-D orbit 1141, the postulated parallel potential has reduced the required parallel current. This high potential had to develop because the required amount of downward current would have quickly evacuated the ionospheric electrons available to supply the original requirement of a downward current.

The beam-plasma instability is one of the most basic and general situations in plasma physics. Under suitable conditions, it gives rise to strong electrostatic waves, which interact with the plasma electrons and ions. If the neutral density is high enough, a charge is ignited, the Beam-Plasma Discharge. In this thesis, the onset of the strong-beam-plasma instability is studied, and scaling laws of the discharge ignition are obtained for different configurations. The mechanism responsible for the discharge, i.e., the energization of plasma electrons by the plasma waves, is also studied. The model proposed here is consistent with the experimental data, is free from the contradictions present in other models, and explains the observed discharge signatures (wave, light and particle characteristics). The agreement with the experimental scaling law is remarkably good.

An answer is suggested to the question of why plasma and magnetic energy accumulate in the Earth's magnetotail to be released in sporadic events, namely substorms. It is shown that the idea of steady convection is inconsistent with the idea of slow, approximately lossless, plasma convection in a long, closed-field-line region that extends into a long magnetotail, such as occurs during Earthward convection in the Earth's plasma sheet. This inconsistency is argued generally and demonstrated specifically using several quantitative models of the Earth's magnetospheric magnetic field. These results suggest that plasma-sheet convection is necessarily time dependent. If flux tubes are to convect adiabatically Earthward, the confining magnetic pressure in the tail lobes must increase with time, and the magnetotail must evolve into a more stretched configuration. Eventually, the magnetosphere must find some way to release plasma from inner-plasma-sheet flux tubes. This suggests an obvious role for the magnetospheric substorm in the convection process. To probe this process further, a two-dimensional, self-consistent, quasi-static convection model has been developed. This model self-consistently includes a dipole field and can reasonably account for the effects of inner-magnetospheric shielding. Starting with a variety of initial configurations, forcing plasma-sheet flux tubes to convect Earthward results in stretching of inner-plasma-sheet flux tubes, the development of a local minimum in the normal magnetic field component in the near-Earth plasma sheet, and an increasing lobe magnetic field. This behavior occurs generally, independent of the specific magnetopause or far-tail boundary conditions. These results suggest that the magnetospheric substorm is the inevitable outcome of Earthward plasma-sheet convection, in order to release plasma from inner-plasma-sheet flux tubes and the magnetic energy in the tail lobes associated with this plasma's confinement. Also, this inner-plasma-sheet behavior indicates why substorm onset should occur in the near-Earth plasma sheet. Some recent ideas concerning why sudden compressions and northward turnings of the IMF sometimes trigger, and other times do not trigger, magnetospheric substorms are discussed in light of these results.

This thesis examines the two state structure of the solar wind and its influence upon proton temperatures. To examine this question, data from the Helios spacecraft are examined. The results from the data are interpreted in light of several theoretical models of the solar wind. In particular, it is found that the interplanetary heating of the protons observed by Helios is consistent with models that rely on extended deposition of energy and momentum in the form of Alfvenic waves. Analysis shows that between 4-10% of the time the data are consistent with two fluid models which do not include extended deposition of energy and momentum. For the rest of the data, the magnetic fluctuations are analyzed and it is found that there is dissipation of wave energy. Calculations show that the heating required by the protons can be accounted for by the apparent dissipation of Alfvenic wave energy. The relationships of temperature to velocity, to number density, and to momentum flux are also examined and are found to be consistent with a bifurcation of the solar wind based upon Alfven waves. A qualitative scenario for the generation of the two state solar wind wherein all the energy for the solar wind comes from convection in the sun is discussed.

In this thesis we derive an analytical relationship between the Earth's magnetospheric potential drop and the ionospheric conductance by adopting a simple two-dimensional model of a magnetic field draping around the tail magnetopause. Two methods are used: (1) matching currents through the tail magnetopause and the Earth's ionosphere in analogy with the Alfven wing at the Jovian satellite Io, (2) minimizing the total power that the solar wind loses to the Earth's magnetosphere. We find, in both cases, that the magnetospheric potential drop, $\Delta\phi\sb{\rm is}$, is inversely proportional to the ionospheric conductance, $\Sigma\sb{\rm is}$, and we compare this result to the revised result of the numerical model of Fedder and Lyon (1987) and to the conclusions of Hill et al. (1976) and Hill (1984). Considering the derived proportionality factor as a 'magnetosheath conductance', $\Sigma\sb{\rm msh}$, we determine that the high-latitude tail magnetospheric 'driver' acts as a current generator as opposed to a voltage generator.

In this thesis, we study the barotropic stability of the Bickley jet on the $ beta$-plane in the context of the linear and weakly nonlinear theory. / In the linear theory, the normal mode approach is used where we introduce a small wavelike perturbation to the mean parallel flow to obtain the Rayleigh-Kuo equation. Together with its boundary conditions, this equation is solved as an eigenvalue problem. As a result, new linear unstable sinuous modes are obtained within the narrow region bounded by two known neutral modes. We also locate a sinuous neutral mode which is singular and radiating, near the stability limit of $ beta=-2$. / An integral part of the thesis involves the application of the weakly nonlinear theory. The temporal evolution of the perturbation amplitude about the varicose neutral mode is studied by means of the Landau equation. Consequently, the value of the Landau constant is deduced which indicates a frequency reduction to the linear perturbation.

Single solid-stat nanopores have been fabricated in free-standing layers of amorphous silicon nitride. Using a focused electron beam and a focused ion beam, diameters ranging from about 0.7 nm to a few hundred nanometerswere achieved. Structural relaxation of nanopores fabricated by focused electron beam was observed. Depending on the initial diameter and on the length of the nanopores, two distinct phases for the relaxation process were found. An experimental setup for themeasurement of pressure-driven mass flow of helium through a single nanopore was built. The conductance of nanopores with diameters ranging from 25 nm to 315 nm was measured. A semi-phenomenological model was developed and it was shown to quantitatively describe the conductance of fluid through a short cylindrical nanopore. / Des nanopores ont été fabriqués dans de minces couches suspendues de nitrure de silicium amorphe. En utilisant un faisceau d'électrons focalisé et un faisceau d'ions focalisé, des diamètres entre 0.7 nm et 315 nm ont été obtenus. La relaxation struturelle de nanopores fabriqués par faisceau focalisé d'électrons a été observée. Dépendamment du diamètre inital du nanopore et de sa longueur, deux phases distinctes ont été identifiées. Un montage expérimental permettant la mesure de l'écoulement de masse d'hélium causé par l'application d'une différence de pression a été réalisé. La mesure de conductance de nanopores ayant un diamètre compris entre 25 nm et 315 nm a été effectuée. Un simple modèle phénoménologique permet de décrire quantitativement l'écoulement de gaz dans un court nanopore cylindrique.

The energy cascade process in turbulent flows is studied. Kolmogorov inertial range theories are critically reviewed and the multifractal characterization is discussed. Multiplicative cascade models are compared to the energy dissipation field (EDF) measured in the atmosphere. Landau's objection to the 1941 Kolmogorov theory is extended to the predictions of statistical fluid mechanics. The hypothesis $ rm Delta v( lambda L) { buildrel{d} over=} lambda sp{1/3} Delta v(L)$ is rejected with a statistical test. The moments $ rm langle( log varepsilon(L)) sp{p} rangle,$ where $ varepsilon$(L) denotes the EDF averaged over a volume of size L, are shown to be gaussian. For the EDF: Convergence tests showed that the exponents $ tau$(q) were not reliable for q $<$ 0; the correlations obey $ rm langle( mu sb{x}( delta)) sp{p}( mu sb{x+ delta}( delta)) sp{q} rangle propto delta sp{ gamma(p,q)}$ but $ gamma$ does not always equal the value obtained with a multinomial measure; a privileged scale ration r $ approx$ 1/2 is suggested by the prefactor oscillations of the correlation function. The implications of these results for the modelling of the EDF are discussed.