The role of the superthermal ions in the dayside postshock ionospheric flow field at Venus is investigated in a 2-D model by integrating the equation of motion of a superthermal population of O$\sp+$ test particles and statistically examining their contribution to the altitude variation of number density and horizontal current distribution. The model reveals superthermal ions $\vec {\rm E} \times \vec {\rm B}$ drifting downward and gradient drifting horizontally with attendant currents. Regions of "wrapped up" plasma known as fluxropes and identified previously as resulting from Kelvin-Helmholtz instability in the ionosphere are found to significantly scatter ions and limit the horizontal currents. Monte-Carlo simulation of charge exchange and subsequent neutral-on-neutral collision processes produce a substantial vertical flux of ballistic neutrals with hyperbolic trajectories possibly representing a heretofore unrecognized atmospheric loss mechanism at Venus. Modeled altitude profiles of superthermal density superficially agree with empirical analysis by Stewart (1991) for a "missing" pressure term in the momentum balance requirements. (Abstract shortened with permission of author.)

A quantitative comparison has been made between observed low-energy ($\sim$1 keV $-$ 30 keV) particle fluxes in the inner magnetosphere (r $<$ 6.6 Earth radii) and those calculated by the Rice Magnetospheric Specification and Forecast Model (MSFM). The MSFM, which was delivered on February 28, 1994, is an operational computer model of the terrestrial magnetospheric environment which is designed to facilitate US Air Force spacecraft operations. The model will be used as a diagnostic tool for spacecraft charging and other anomalies in a real time operational setting as well in post-event analysis. The MSFM specifies electron and ion (H$\sp+$ and O$\sp+$) fluxes of energies between 100 eV and 100 keV in the distance range of 2-10 R$\sb{\rm E}$ on a two-dimensional grid. Comparison of CRRES observations from the August 26-27, 1990 magnetic storm with output from the progenitor of the MSFM, the Magnetospheric Specification Model (MSM), showed that the modeled electron fluxes were being depleted by the MSM loss algorithm before they could convect into the inner magnetosphere, thus creating an unacceptably large discrepancy between the output and the data. Based on the CRRES observations, adjustments were made to the loss algorithm which greatly enhanced the accuracy of the modeled fluxes in the inner magnetosphere, and these adjustments were incorporated into the MSFM. The MSFM was then subjected to an extensive test program in which the model was run for six unique magnetic storm intervals, and the accuracy of the modeled electron and ion fluxes at geosynchronous orbit and in the inner magnetosphere were quantified by calculation of standard deviations (RMS errors) between the extensive set of observational data assembled for each interval, and the model output. Overall, the accuracy of the MSFM electron and ion fluxes in the inner magnetosphere is quite good; the model moves roughly the right number of particles to roughly the right location. However, due to a fundamental lack of knowledge about the low-energy plasma environment in the inner magnetosphere, some inadequacies still exist in the MSFM loss algorithm which should be corrected when feasible to further improve its accuracy.

The gross structures of the magnetospheres of the outer planets are decided by processes quite different from those predominant in that of the earth. The terrestrial plasmapause, the boundary beyond which plasma motion is principally determined by magnetospheric interaction with the solar wind, is typically inside geosynchronous orbit. Within the plasmasphere, rotational effects are present, but gravity exceeds the centrifugal force of corotation. In contrast, the Jovian plasmasphere extends to a distance at least twenty times farther than synchronous orbit, affording a large region where rotational effects are expected to he clearly manifest (Brice and Ioannidis, 1970). The goal of this thesis is to develop an appropriate theoretical model for treating the problem of plasma transport in a corotation dominated plasmasphere. The model presented here is intended to describe the radial transport of relatively cold plasma having an azimuthally uniform distribution in a dipolar magnetic field. The approach is conceptually similar to that of the radial diffusion model in that small scale motions are examined to infer global consequences, but the physical understanding of those small scale motions is quite different. In particular, discrete flux tubes of small cross section are assumed to move over distances large compared to their widths. The present model also differs from the corotating convection model by introducing a mechanism whereby the conservation of flux tube content along flowlines is violated. However, it is quite possible that a global convection pattern co-exists with the motions described here, leading to longitudinal asymmetries in the plasma distribution.

Among available quantitative magnetospheric models, the Tsyganenko models represent empirical modeling, while the Toffoletto and Hill model is theoretical analysis. By mapping flux tubes between the ionosphere and the magnetosphere, those two models are analyzed and compared. Both the implied and actual field-aligned currents (FAC) are calculated in both models. The implied FAC, required to maintain the model field in magnetostatic equilibrium and given by the Vasyliunas equation, has the same order of magnitude as observational data, even in non-equilibrium models. The actual FAC, given by Ampere's law, is much smaller than the implied FAC, which means no large extraneous FAC is included in those models. Several new approaches imply that none of these models are in magnetostatic equilibrium, while the discrepancies are considerably larger for the Tsyganenko models. The magnetic effects of a preliminary FAC model have been evaluated in the Toffoletto and Hill theoretical model.

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.