Two experimental campaigns designed to study high-latitude auroral arcs have been conducted in Sonde Stromfjord, Greenland. The Polar Acceleration Regions and Convection Study (Polar ARCS) on February 26, 1987, consisted of a coordinated set of ground-based and sounding rocket measurements of a weak, sun-aligned arc within the duskside polar cap, while the Rodeo I and II experiments, conducted during December, 1988 and October, 1989, involved uniquely coordinated optical and radar measurements of high-latitude arcs occurring at the poleward boundary of the auroral oval. Analysis of the large-scale Polar ARCS data indicate anti-sunward convection in the region between the duskside auroral oval and the sun-aligned arc. This convection signature is consistent with either a model in which the sun-aligned arcs formed on open field lines over the polar cap or on closed field lines threading an expanded low-latitude boundary layer, but not a model in which the polar cap arc field lines map to an expanded plasma sheet. Electron measurements indicate that the rocket passed through three narrow ($\le$20 km) regions of low-energy ($\le$100 eV) electron precipitation. An electrodynamic analysis has shown the electric and magnetic field perturbations in these regions to be well correlated and associated with small-scale upward and downward field-aligned currents of 1-2 $\mu$A/m$\sp2.$ The Rodeo measurements have been used to examine the aeronomic and electrodynamic characteristics of two optically stable arcs occurring at different magnetic local times and exhibiting different relationships to the polar cap/convection reversal boundary. The first case study is associated with a reversal from antisunward to sunward flow and also the boundary between open and closed field lines. In contrast, the second case study involved an arc with a much greater average precipitation energy and a significant cross-arc flow, evidenced by the radar measurements as well as the convective motion of a polar cap patch directly across the arc. Owing to the relative motion between the F-layer plasma and the arc precipitation, this arc is interpreted as forming across the nightside merging gap on field lines which map to a region of stable reconnection in the tail.

This paper continues the work of Ansher et al. (1992) by identifying and examining density structures in Jupiter's magnetosphere. The 110 hours of data used are from a 4-second temporal resolution density data set derived from plasma wave instruments on board both Voyager 1 and Voyager 2. One hundred five structures are identified. They are believed to be the same type of structures as seen by Ansher et al. (1992) and are found to have sharp density gradients at the boundaries, average scale sizes of about one Jovian radius, and typical density variations between 50% and 200% of the background. Many structures show good correlation with the magnetic field data. In addition, the existence of the density structures has little if any dependence on radial distance from Jupiter, System III longitude, or magnetic latitude. Comparison with four plasma transport models indicates that the observed structures resemble flux tubes of varying plasma content compared to the background density. These findings are in agreement with those of Ansher et al. (1992).

The interaction between the solar wind and a rotating planet causes the field lines in the planetary magnetotail to twist into a helix. Using a simplified magnetotail model, we examine hydromagnetic waves propagating down the magnetopause for such a field configuration and derive the dispersion relation of the waves. It turns out that only under certain special circumstances can the hydromagnetic waves be stable. In a thin magnetopause boundary layer, the helical wave is found to be always stable and its wave frequency depends weakly on the plasma and the field within the layer. The current system of the boundary layer is found to be modulated by the wave and the modulation is proportional to the velocity perturbation of the plasma. The wave influence on the spiral angle is examined briefly for some special cases for which we find the variation of the angle increases monotonically with increasing radial distance.

A model is constructed to describe the magnetic field, the global current system, the electric field and the potential in the solar wind interaction with Mars assuming that Mars has no intrinsic magnetic field. It, therefore, incorporates the physics learned from the Pioneer Venus Orbiter, which observed the interaction of Venus, an unmagnetized planet, with the solar wind for 14 years. Integrating recent knowledge of the global current system at Venus (Law 1995) into the Flow/Field model of Cloutier et al (1987) and expanding the model to represent three dimensions, we adapt the Flow/Field model for application to Mars. We investigate the 3-D current system to learn the physics of the interaction. Then, the model is applied to test simple geometries in order to validate it. Future applications are discussed.

The Rice Convection Model has been modified and applied to the study of the Jovian magnetospheric system, which is interchange unstable. The basic interchange instability of the Io plasma torus is opposed by pressure gradients in the energetic particles outside the torus. Many simulations have been performed for cases where the overall system is inter-change unstable under the ideal-MHD assumption E + v $\times$ B = 0. For such cases, the torus breaks up predominantly into long fingers unless the initial condition strongly favors some other mode. The ends of the fingers tend to be rounded, and they are connected to the main torus by tails that thin rapidly with time if the torus runs out of plasma. Our calculations place an upper limit of $\sim$1R$\sb{\rm J}$ on the average distance between fingers. For an initially asymmetric large-scale torus, fingers generally form on a time scale shorter than the one on which the heavy side of the torus falls outwards. However, the fingers form predominantly on the heavy side. Galileo may observe such finger features outside the Io torus, at L $\approx$ 7 to 15. Additionally, in this thesis, drift-wave theory has been used to investigate the effect of energetic (KeV or MeV) particles on the Io torus plasma transport. It is shown that the MHD stability criterion, where the interchange motion would be completely stabilized if the energy density of the hot stabilizing plasma is greater than or dual to 3/4 of that of the cold unstable plasma, no longer holds owing to the gradient/curvature drift of the energetic particles. This differential-drift effect, which is a departure from the ideal-MHD and frozen-in flux, may play a significant role in plasma transport in the Jovian magnetosphere.

A steady state flow/field model is applied to the direct interaction of the solar wind with the Martian ionosphere. We have coupled observational data with the kinetic theory plasma equations within a single self-consistent framework. We do not imply that the interaction is purely ionospheric, but rather we show that a self-consistent, direct ionospheric interaction is feasible. The model demonstrates how the dynamic solar wind pressure is transferred to the ultimate obstacle, the planet itself, without the requirement of an intrinsic Martian magnetic field. The results of the model are in agreement with Viking observations, and hence make possible predictions of quantities as yet unmeasured within the Martian ionosphere. We predict that the magnetic field configuration of the Martian ionosphere may be similar to the "Venusian dip" structure observed by Pioneer Venus. (Abstract shortened with permission of author.)

A model is presented which simulates the behavior of superthermal ions previously reported in the dayside ionosphere of Venus. The model considers effects of E $\times$ B and gradient drifts, charge exchange and collisions with the ambient neutral atmosphere and the possible effects of a wave-particle (anomalous) scattering process. Results indicate that scattering processes are required if superthermal ions are the explanation for the observed "missing pressure" component in the dayside Venus ionosphere. The scattering scale length required to match the "missing pressure" distribution is similar to the scale length previously predicted for growth of a lower hybrid beam instability.

Due to their formation at about the same time in the same region of the early solar nebula, it is reasonable to assume that the primitive atmospheres of Earth, Mars and Venus were similar and that present-day differences have arisen as a result of their differing masses and incident solar fluxes. Using a radiative-convective model, we determine maximum and minimum carbon dioxide levels for the early atmospheres which are consistent with this assumption and with climatic conditions thought to have existed on the three terrestrial planets 4.0 billion years ago. Rather than employ the cloud-free atmosphere approach of earlier studies, we include an interactive water vapor transport and cloud formation scheme in the model. Due to uncertainties about the direction of cloud cover feedback, we treat cloud cover as fixed. For most cases examined, we set the cloud cover at 50%, but the effect of varying cloud cover is also explored.

The solar wind interacts with the non-magnetic planet Venus by processes within the mantle region, located between the upstream shock and the ionosphere. In this region exospheric neutral atoms from Venus interact with electrons and ions in the moving plasma and modify its flow, resulting in a region of sharp ion density gradients at the boundary between the ionosphere and the mantle called the ionopause. The effect of mass-loading may be simulated by modifying the mass, momentum, and energy conservation equations to include source and loss terms and electromagnetic forces. Using the assumption that the plasma behaves like a fluid, we construct a model to simplify the physics in the mantle. With this model it is possible to generate an oxygen "ledge" in the ion density similar to the observed ionopause. The calculations also show an enhancement in the magnetic field strength above the ionopause as that observed at Venus.

A model is presented that simulates the physics of the Venus mantle plasma. A modified magnetohydrodynamic (MHD) fluid picture is assumed where the post-shock solar wind plasma is mass-loaded by photoionizations and other atomic interactions with exospheric neutral atoms. By assuming Newtonian pressure profiles and draped magnetic field geometry in the mantle, the three-dimensional steady-state flow problem is reduced to a one-dimensional calculation along the stagnation (subsolar) flowline. In addition, the validity of the model assumptions and questions about the plasma thermodynamics are addressed. When the resulting model is run using various solar wind conditions, the computed magnetic field features correspond with those measured by the Pioneer Venus spacecraft. The model reproduces the observed region of sharp ion density gradients, known as the ionopause, that separates the mantle plasma from the denser ionospheric plasma below. The straightforward application of the model reproduces the shape and location of the low-altitude ionopause cases well, but for solar wind conditions that correlate with high-altitude ionopauses, the computed ionopauses tend to be lower than those observed. The addition of anomalous heating terms to the model, however, raises the computed ionopause to locations consistent with the medium altitude cases. The inability of the model to adequately describe the high altitude cases may indicate that they are transient events and thus cannot be simulated in steady state. While the source of the anomalous heating is not specified, the presence of hyperthermal ions or plasma-wave interactions are suggested as possible heating mechanisms.