The solar wind interaction with the non-magnetic Venerean ionospheric obstacle is unique. Ionospheric models of this interaction have primarily been in two dimensions that do not allow for changes in the orientation of the solar wind magnetic field near the obstacle. Analysis of high resolution magnetic field data from the Pioneer Venus Orbiter spacecraft has revealed field rotations that are observed to occur in conjunction with the dayside ionopause. These rotations are a result of the velocity shear at the ionopause and indicate the alignment of the magnetic field with the radial day to night flow of ionospheric plasma. A new configuration of the dayside magnetic field draping has been derived from these results. In addition, a new current system to account for this changing field orientation has been determined and is discussed in relation to current systems derived from previous models. These new aspects of the dayside solar wind interaction at Venus can be applied to other similar solar system objects. Assuming Mars also represents a non-magnetic obstacle to the flow, as past experimental observations indicate, the field diagnostics discovered here make it possible to probe the structure of the Martian ionosphere using magnetometer data in the absence of ion mass spectrometer data. These results will play a major role in predictive modeling and data analysis for future Mars missions.

Lightning on Venus has been suggested by many researchers. The evidence for this lightning has been claimed to be impulsive electric field signals which have been detected by a number of spacecraft. However, others have proposed that the observed signals are a result of local ionospheric plasma instabilities and do not originate in the lower atmosphere. An investigation of 100 Hz electric field signals detected within the ionosphere of Venus has been conducted. The study encompassed both previously published 100 Hz signals which have been identified as being a result of lightning in the lower atmosphere and data which was newly selected for this research. It is shown that the 100 Hz signals result most often from the lower hybrid plasma wave mode. The remaining signals are from other plasma instabilities due to magnetic field and/or ion density gradients.

Solar wind interactions with planets that possess neither an intrinsic magnetic field nor a significant ionosphere have not been well studied. We have constructed a model to simulate the interaction between a hot magnetized plasma and a planet containing only a neutral atmosphere. Examination of the boundary conditions that yield a physically valid solution shows that the interaction is similar to the solar wind interaction with Venus. We show that most (97%) of the incident flow is deflected around the atmosphere, and the small fraction that enters interacts in such a way as to transfer the flowing momentum through the neutral atmosphere to the planetary body. This demonstrates that the true barrier to a flowing plasma, such as the solar wind, is neither its ionosphere nor its intrinsic magnetic field, but rather the planetary body itself.

The Voyager 2 encounter with Neptune and Triton in August of 1989 provided clues to an intriguing problem. Instruments onboard the spacecraft showed a large ionosphere at Triton. Subsequent studies have tried to explain the production of such high levels of ionization but have ignored the possible plasma dynamics originating from the interaction between Neptune's magnetosphere and Triton. This study applies knowledge gained from studying the solar wind-Venus interaction to this case. In doing so we find that observations made by Voyager 2 can be explained by downward convection of magnetospheric plasma into Triton's atmosphere. Furthermore, we find that the flowing momentum is transferred to the moon just below the exobase, calculated here to be approximately 750 km. From this point down the atmosphere is not in hydrostatic equilibrium, and cannot be until below the ionization peak. Finally we show that when the momentum gets transferred to the moon the flow must shut off. This is accomplished when both the convective velocity and magnetic field go to zero. By showing the magnetic field vanishes at an altitude of roughly 650 km, we conclude the accepted mechanism by which the ionosphere is produced to be invalid. This mechanism was identified early-on to be impact ionization from hot, or superthermal, electrons originating in Neptune's magnetosphere. These precipitating hot electrons are thus shown to operate independently of the magnetic field below the exobase. This is a result not previously discovered, and one which implies that the plasma interaction between Neptune's magnetosphere and Triton cannot be ignored.

Cross sections for electron impact ionization of molecules and rare gases are necessary for the modeling of phenomena related to the earth's upper atmosphere and to plasmas. The construction of an apparatus to improve the accuracy of measurements of such cross sections is reported. The ions are produced in a static gas target crossed by an electron beam and are counted with a new ion detector technology, which insures a increased accuracy of the ion flux measurement and of the target length. The target gas density is measured using an ionization gauge calibrated against a high accuracy capacitive transducer. This apparatus also features a pulsed mode of operation, which allows a product analysis through time of flight measurement.

A modified steady state flow/field model is applied to the direct interaction of the solar wind with the Martian ionosphere. The original flow/field model (Cloutier et al., 1987) is a one-dimensional, self-consistent derivation of differentials in vertical velocity, magnetic field, and ion densities from the coupled MHD equations. While successful in reproducing features of the ionosphere of Venus (Cloutier et al., 1987; McGary, 1987) and of Mars (Stewart, 1989), the flow/field model required an independently specified heating term (Q). The requirement of this term implies the presence of an energy source not accounted for in conventional calculations. This source was previously simulated with the inclusion of Q, but an unrecognized momentum or pressure term may also provide the coupling with the solar wind without the need of the free parameter Q. An in-depth analysis of Pioneer Venus data in relation to the total conservation of momentum of the system led to the discovery that the total momentum was in most cases not entirely accounted for, and that this "missing" term was correlated with solar wind dynamic pressure. By including this missing pressure, a new set of differential equations, which were also extended to include horizontal velocity terms, was derived. Extrapolation of the missing pressure to Mars gave results that faithfully reproduced the ionospheric features associated with previous flow/field models while maintaining agreement with Viking 1 and 2 observations. Finally, we suggest that the source of P$\sb{\rm missing}$ could be a population of suprathermal particles within the ionosphere. The missing pressures in the Viking simulations are consistent with measured suprathermal pressures at Mars (Hanson and Mantas, 1988).

The Pioneer Venus Orbiter Electric Field Detector (PVO OEFD) detected numerous impulsive electric field events in its 100 Hz channel when in nightside ionospheric troughs. What the source of the signals is has been under debate for over 10 years. Some researchers claim that lightning is generating whistler waves which are then being detected, while other researchers have supported the view that the signals are caused by a local generation of a plasma instability due to the conditions inside the troughs. We believe that the evidence collected by the PVO clearly disproves a lightning source and, in fact, points to an electrostatic wave as being the producer of the transients. We studied several wave modes in an effort to determine the source of the signals. We looked at the two-stream instability, the ion-acoustic instability, the gentle-bump instability, the lower-hybrid-drift instability and the Alfven wave mode. We found that not one of the wave modes studied could account for all of the signals. It would appear that there is not a large enough current within the troughs to support the two-stream instability. While the ion-acoustic instability may well be present within some troughs, the conditions needed to produce 100 Hz waves may not be universal trough conditions. Thus, this wave mode is not a likely generator of all the signals. The gentle-bump instability cannot produce 100 Hz waves and any Alfven waves will be damped. The lower-hybrid-drift instability seems to be the most likely candidate of the wave modes studied for producing the transient signals. The frequencies of the lower-hybrid-waves, however, appear to be too low to be detected by the OEFD. We believe that either the frequencies that we calculated are too small due to factors not taken into account, or there is a cascade of energy from the lower-hybrid waves to waves with frequencies that can be detected by the OEFD, or there is a combination of the two.

Before the IMAGE mission, electron densities in the high latitude, high altitude region of the magnetosphere were measured exclusively by in situ means. The Radio Plasma Imager instrument onboard IMAGE is capable of remotely observing electron densities between 0.01 and 100,000 e-/cm-3 from distances of several Earth radii or more. This allows a global view of the high latitude region that has a far greater accuracy than was previously possible. Soundings of the terrestrial magnetic cusp provide the first remote observations of the dynamics and poleward density profile of this feature continuously over a 60-minute interval. During steady quiet-time solar wind and interplanetary magnetic field conditions, the cusp is shown to be stable in both position and density structure with only slight variations in both. Peak electron densities within the cusp during this time are found to be somewhat higher than predicted. New procedures for deriving electron densities from radio sounding measurements are developed. The addition of curve fitting algorithms significantly increases the amount of useable data. Incorporating forward modeling techniques greatly reduces the computational time over traditional inversion methods. These methods are described in detail. A large number high latitude observations of ducted right-hand extraordinary mode waves made over the course of one year of the IMAGE mission are used to create a three dimensional model of the electron density profile of the terrestrial polar cap region. The dependence of electron density in the polar cap on average geocentric distance (d) is found to vary as d-6.6. This is a significantly steeper gradient than cited in earlier works such as Persoon et al., although the introduction of an asymptotic term provides for basic agreement in the limited region of their joint validity. Latitudinal and longitudinal variations are found to be insignificant. Both the mean profile power law index of the electron density profile and, to a stronger degree, its variance show dependence with the DST index.

A setup for the realization of dual-phase experiments using xenon as the active medium in a radiation detector has been built. The setup consists of a gas purification system capable of achieving a purity of the gas in the ppb level and a chamber system consisting of an ionization chamber containing the sensitive elements and a cooling component used to reach cryogenic temperatures inside the chamber in the range of liquid xenon temperature. The main goal of the dual-phase experiments is the operation of gas electron multipliers (GEM) in a cryogenic environment similar to the conditions found in experiments aimed to detect the most promising candidate for dark matter, i.e. the lightest supersymmetric particle known as neutralino or WIMPS.

A computer model has been built to simulate the dynamic evolution of relativistic electrons in the outer radiation belt. The model calculates changes in electron flux due to three mechanisms: (1) fully-adiabatic response of electrons to variations in the magnetic field, (2) time-dependent radial diffusion, parameterized by overall magnetospheric activity, and (3) penetration of new particles into the model via a time-dependent outer boundary condition. Data from Los Alamos geosynchronous satellites, the CRRESELE statistical electron flux model, the Kp index, and the Toffoletto-Hill-Ding magnetic field model are all used to provide realistic, time-dependent inputs to the model. To evaluate the model, a simulation of the radiation belts during the November 3--12, 1993 magnetic storm was generated. Comparison of results to Global Positioning System (GPS) radiation dosimeter data indicates that the model can accurately predict storm-time flux variations for electrons with energies less than 600 keV. Modeled fluxes for higher energy electrons show insufficient enhancement during the recovery phase of the storm, suggesting the existence of an acceleration mechanism other than fully adiabatic variations and radial diffusion.