Thesis (Ph. D.)--Ohio State University, 2004. / Document formatted into pages; contains xvi, 158 p. Includes bibliographical references. Abstract available online via OhioLINK's ETD Center; full text release delayed at author's request until 2005 Sep. 20.

A numerical model has been developed for atomic hydrogen densities in the Titan hydrogen torus. The effects of occasional collisions were included in order to accurately simulate physical conditions inferred from the Voyager 1 and 2 Ultraviolet Spectrometer (UVS) results of Broadfoot et al. (1981) and Sandel et al. (1982). The model employed Lagrangian perturbation of orbital elements of hydrogen atoms launched from Titan and Monte-Carlo simulation of collisions and loss mechanisms. The torus is found to be azimuthally symmetric with the density sharply peaked at Titan's orbit, and decreasing rapidly in the outward and perpendicular directions and more gradually inward from 17 to 5 R(s). The energetic hydrogen atoms from Saturn's upper atmosphere, first predicted by Shemansky and Smith (1982), were also investigated. Collisions of these Saturnian atoms with the torus population do not contribute to the torus density, and will lead to a net loss of torus atoms if their launch speeds from Saturn extend above 40 km/sec. The Saturnian atoms produce a corona which was modelled using the theory of Chamberlain (1963). Based on the energetic hydrogen production rate given by Shemansky and Smith (1986), the coronal density at Saturn's exobase is taken to be 200 to 300 cm⁻³, decreasing to 3 or 4 cm⁻³ at 20 R(s). Without the coronal population, the torus model does not reproduce the Voyager 2 UVS Lyman α intensities because the hydrogen atoms are too closely confined toward Titan's orbital plane. The observations can be reproduced by a model that includes the corona and has central plane maxima of 62 cm⁻³ at Titan's orbit and 318 cm⁻³ at Saturn's exobase. The effect of Titan's exospheric temperature (T(E)) on torus structure is seen in the column abundances perpendicular to the central plane at radii of 5 to 15 R(s). Spacecraft observations of these column abundances should allow verification of T(E) to within about 100°K. Similar observations of other species expected to be present in the torus, such as H₂, N, and N₂, would indicate their approximate launch speeds from Titan and thus the relative importance of thermal and non-thermal loss mechanisms.

In 1979, the IRIS infrared spectrometers on the two Voyager spacecraft obtained over 1000 disk-resolved thermal emission spectra of Europa, Ganymede, and Callisto, Jupiter's three large icy satellites. This dissertation describes the first detailed analysis of this data set. Ganymede and Callisto subsolar temperatures are 10°K and 5°K respectively below equilibrium values. Equatorial nighttime temperatures are between 100°K and 75°K, Callisto and Europa being colder than Ganymede. The diurnal temperature profiles can be matched by 2-layer surfaces that are also consistent with the eclipse cooling observed from earth, though previous eclipse models underestimated thermal inertias by about 50%. Substrate thermal inertias in the 2-layer models are a factor of several lower than for solid ice. These are 'cold spots' on Ganymede and Callisto that are not high-albedo regions, which may indicate large thermal inertia anomalies. All spectra show a slope of increasing brightness temperature with decreasing wavelength, indicating local temperature contrasts of 10-50°K. Callisto spectra steepen dramatically towards the terminator, a trend largely matched with a laterally-homogeneous model surface having lunar-like roughness, though some lateral variation in albedo and/or thermal inertia may also be required. Subsolar Ganymede spectra are steeper than those on Callisto, but there is no steepening towards the terminator, indicating a much smoother surface than Callisto's. The spectrum slopes on Ganymede may indicate large lateral variations in albedo and thermal inertia. A surface with similar areal coverage of dark, very low thermal inertia material, and bright material with thermal inertia a factor of 2-3 below solid ice, fits the diurnal and eclipse curves, and (less accurately) the IRIS spectrum slopes. Europa spectra have very small slopes, indicating a smooth and homogeneous surface. Modelling of surface water ice migration gives a possible explanation for the inferred lateral inhomogeneities on Ganymede. Dirty ice surfaces at Jupiter are subject to segregation into high-albedo ice-rich cold spots and ice-free regions covered in lag deposits, on decade timescales. Ion sputtering and micrometeorite bombardment are generally insufficient to prevent the segregation. The reflectance spectra of Ganymede and Callisto may be consistent with this type of segregated surface.

An observational program was designed for systematic spatial and temporal monitoring of the Jovian atmosphere at several wavelengths chosen for their different absorptive properties. The weak broadband (5Å/pixel) CH₄ absorptions (6190 and 7270Å) probe the deep (2-4 bars) cloud layer while the stronger band at 8900Å probes the upper 400-600 mbars. The high resolution (~50mÅ/pixel) 3-0 H₂ quadrupole wavelengths probe to about 1-2 bars. The gradual increase in the measured equivalent widths of the H₂ quadrupole lines from the east to west limb is most likely indicative of a diurnal change in the vertical cloud structure. Such a variation is consistent with the properties of a convective layer driven by internal heat, with solar heat deposited at the top. The CH₄ data from the same time period was modelled for the south tropical zone. Since these absorptions are sensitive to several atmospheric layers it is difficult to separate the effects of the various cloud parameters on the [(I/F)(band)]/[(I/F)(cont)] values. There are no obvious limb to limb variations in these bands but several cloud parameters may be changing simultaneously, introducing compensating affects on the [(I/F)(band)]/[(I/F)(cont)] values. The two limbs may not, therefore, appear significantly different even if they are representative of substantially different cloud structures. The June 1983 H₂ data has been modelled at seven different latitudes and cloud structure differences are indicated. The average models representing the belt regions require somewhat thinner optical depths for the upper ammonia cloud (τ(cl) = 3-4.5) than the zones (τ(cl) = 5.5-6.5) or the equatorial region (τ(cl) = 6.5-7). These data also provide some constraints on the thermodynamic state of the hydrogen. A model atmosphere with only "normal" hydrogen (ortho-H₂ to para-H₂ of 3:1) is not able to fit both of the 3-0 lines simultaneously. Model atmospheres with all of the hydrogen in a state of equilibrium fit the two lines much better. Models with small amounts of disequilibrium hydrogen in the upper atmosphere also provide reasonable average fits to our H₂ data and cannot be easily distinguished from those that incorporate only equilibrium hydrogen at all levels or from those which incorporate "normal" in the top 300 mbars of the Jovian atmosphere.

Thesis (Ph.D.)--Boston University / PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / The Earth's magnetic cusps are the regions with the most direct transfer of energy, mass, and momentum from the flowing solar wind to the Earth's magnetosphere. Spacecraft observations in the cusp have revealed a high energy component to the thermal particle distribution. This has raised the question as to whether significant plasma heating may also be occurring in this region. Since the cusp is magnetically connected to a number of other regions in geospace, plasma heating in this region could be a significant contributor to magnetospheric dynamics. The goal of this thesis is to answer the question, what is the source of the energetic particle population in the cusp? Since the initial observations measuring the energetic component were made, the source of the energetic population has been open to conjecture. A number of sources have been proposed: (1) the terrestrial bow shock, (2) the Earth's high-latitude trapping region, and (3) heating of plasma locally in the cusp. Depending on which source is the dominant provider of the energetic particles, the particle population will exhibit different properties. Particle flow direction, intensity, spectral characteristics, and species/charge state are all properties that can change depending on the dominant source. In-situ measurements by the ISEE, Polar, and Cluster spacecraft are used to derive the particle properties. These properties are compared with predictions for each of the proposed sources to determine which is most consistent with the observations. Case studies show that, under different conditions, the high-latitude trapping region and local heating can both be the dominant source of the energetic particle population up to energies of hundreds of keV. Results from a large scale statistical study, however, are more consistent with local heating indicating that this is the dominant source the majority of the time. / 2031-01-01

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The Penn-State/NCAR MM5 mesoscale model was adapted for mesoscale simulations of the Martian atmosphere (the OSU MMM5). The NASA Ames Mars GCM provides initial and boundary conditions. High-resolution maps for albedo, thermal inertia and topography were developed from Mars Global Surveyor (MGS) data; these baseline maps are processed to appropriate resolutions for use in the GCM and the mesoscale model. The OSU MMM5 is validated in Chapter 2 by comparing with surface meteorology observed at the Viking Lander 1 (VL1) and Mars Pathfinder (MPF) landing sites. How the diurnal cycle of surface pressure (the surface pressure tide) is affected by boundaries, domain/nest choices and the resolution of surface properties (topography, albedo and thermal inertia) is examined. Chapter 2 additionally shows the influence of regional slope flows in the diurnal surface pressure cycle for certain locations on Mars. Building on the methods of Chapter 2, Chapter 3 describes the northern midsummer polar circulation and the circulations (both large and small scale) that influence it. Improvements to the model for these studies include: the topographical gradient is now considered when computing surface insolation, and the thermal inertia maps and model initialization are improved for high latitudes; this yields a realistic simulation of surface temperatures for the North Pole Residual Cap (NPRC) and the surrounding region. The midsummer polar circulation is vigorous, with abundant and dynamically important transient eddies. The preferred locations of transients varies significantly during this study, between L[subscript s]=l20 and L[subscript s]=l50. At L[subscript s]=l20 transient circulations are seen primarily along the NPRC margin, consistently producing strong flow over the residual cap (~l5 m/s). By L[subscript s]=135, transient eddies form a "storm track" between the northern slopes of Tharsis and the NPRC. By L[subscript s]=150, the circulation is becoming strong and winter-like. These transient eddies may be important in the Martian annual water cycle; many of the observed circulations are poorly (or not) simulated in present day Mars GCMs. Increased resolution and polar stereographic domains provide improvement over GCMs for high latitude studies of atmospheric circulations. These results are in agreement with recent observations. Future work includes model refinements and water vapor transport studies. / Graduation date: 2005

According to both spectroscopic measurements and interior models, Jupiter, Saturn, Uranus and Neptune possess gaseous envelopes that are enriched in heavy elements compared to the Sun. Straightforward application of the dominant theories of gas giant formation - core accretion and gravitational instability - fail to provide the observed enrichment, suggesting that the surplus heavy elements were somehow dumped onto the planets after the envelopes were already in existence. Previous work has shown that if giant planets rapidly reached their cur rent configuration and radii, they do not accrete the remaining planetesimals efficiently enough to explain their observed heavy-element surplus. We ex plore the likely scenario that the effective accretion cross-sections of the giants were enhanced by the presence of the massive circumplanetary disks out of which their regular satellite systems formed. Perhaps surprisingly, we find that a simple model with protosatellite disks around Jupiter and Saturn can meet known constraints without tuning any parameters. Fur thermore, we show that the heavy-element budgets in Jupiter and Saturn can be matched slightly better if Saturn’s envelope (and disk) are formed roughly 0.1 — 10 Myr after that of Jupiter. We also show that giant planets forming in an initially-compact con figuration can acquire the observed enrichments if they are surrounded by similar protosatellite disks. Protosatellite disks efficiently increase the capture cross-section, and thus the metallicity, of the giant planets. Detailed models of planet formation must therefore account for the presence of such disks during the early stages of solar system formation.

Thesis (Ph. D.)--University of Washington, 1991. / Vita. Includes bibliographical references (leaves [419]-429).