This thesis is an observational and theoretical study of the optical emission from nonradiative shocks in three supernova remnants: the Cygnus Loop, RCW 86 and Tycho, which together span a wide range of shock velocities (300 ≲ vS ≲ 2000 km s-1). The spectra are dominated by Balmer lines of H which have both a broad component caused by proton-neutral charge exchange and a narrow component produced by collisional excitation close to the shock front. The broad to narrow flux ratios observed in all three remnants are systematically smaller in Halpha than in Hbeta, and the narrow Balmer decrement is larger than the broad Balmer decrement. The broad component Halpha profiles of RCW 86 and Tycho are Gaussian, indicating that the postshock protons follow a Maxwellian velocity distribution. To model the data, a new numerical shock code was developed which computes the broad and narrow Balmer line emission from a nonradiative shock in partially neutral gas. The models calculate the density, temperature and size of the postshock ionization layer for arbitrary electron-proton temperature equilibrations, and use a Monte Carlo simulation to compute narrow Balmer line enhancement from Lyman line trapping. The models constrain the shock velocity and equilibration of nonradiative shocks in each remnant using the observed Halpha and Hbeta broad to narrow ratios. The models show that differences between the observed broad and narrow Balmer decrements can be explained by Lyman line trapping. The models also show that variations in electron-proton equilibration can reproduce the observed range of broad to narrow ratios. The results give 50--100% equilibration in nonradiative portions of the NE Cygnus Loop (vS ∼ 300 km s-1) and 40--50% equilibration in nonradiative portions of RCW 86 (vS ∼ 600 km s-1 ). In Tycho there are major discrepancies between the predicted and observed broad to narrow ratios, with only the Hbeta ratio matching the observations. The discrepancies may be due to additional narrow component emission that arises from a cosmic ray/fast neutral precursor. The observed Hbeta broad to narrow ratio implies ≲ 20% equilibration in Tycho. Hence, there is an inverse correlation between Mach number and equilibration for the three observed remnants. This correlation suggests there may be significant differences between collisionless shocks at low and high Mach numbers. The spectroscopic observations also led to the serendipitous discovery of a photoionization precursor in Tycho. The precursor appears as a faint, diffuse region that extends several arcminutes ahead of the Balmer-dominated shocks in Tycho. A new photoionization model shows that the diffuse emission is mostly warm, neutral gas heated to ∼12,000 K by He II lambda304 photons from the nonradiative shock. High resolution spectra indicate that the upstream gas is further heated to ∼40,000 K just before entering the shock. This additional heating may arise from a second precursor produced either by cosmic rays or fast (broad component) neutrals from behind the hot postshock gas.

This thesis details the development of the Miniaturized Electrostatic DUal top-hat Spherical Analyzer (MEDUSA) instrument, including calibration, simulation, and data processing software. MEDUSA combines two-species observations with state-of-the-art miniaturization to achieve a low-cost, compact instrument capable of making the measurements that will be required in the future. Calibration of the instrument was performed during November 1997 and July 1998. In order to better understand the characteristics of the instrument, it was also modeled in detail with a modified 2.5 dimensional raytracing code. The production processing software is designed to archive the data into the standard Instrument Data File Set (IDFS) format of the Southwest Data Display and Analysis System (SDDAS). MEDUSA has so far proven itself to be a reliable, low-cost instrument capable of making the scientific measurements required for a better understanding of the behavior of the near-Earth particle environment.

During the twentieth century, American amateur astronomers attempted to form national organizations with structures and intents similar to the British Astronomical Association (BAA), an amateur organization dedicated to the advancement of astronomy and widely admired by American amateurs and professionals alike. The Society for Practical Astronomy (1910), the American Amateur Astronomers Association (1935), and the National Astronomical Association (1945) were each intended to facilitate amateur scientific contributions in BAA-like topical sections, but each of these societies failed. Founded in 1911, the American Association of Variable Star Observers (AAVSO) and the American Meteor Society (AMS) provided an alternative for amateur astronomers who were interested in those specific topics. However, it was not until 1947, when the Association of Lunar and Planetary Observers (ALPO) formed, that another large segment of amateur astronomers found a home for their interests. A second mode of national organization succeeded at mid-century and grew to include most avocational astronomers. Founded in 1947, the Astronomical League consists of regional associations of local societies, and is oriented largely towards recreational astronomy. The League sponsors annual national and regional conventions, but contributes little to scientific programs. This study concludes that avocational astronomy cannot simply be compared with professional astronomy, and instead must be viewed on its own terms as a complex and variegated field. Although the failure of American amateurs to form a BAA-like organization was at first disappointing, the specialized associations of observers, together with a separate and larger organization devoted to recreational astronomy, have served the American astronomical community well. Professional support for both types of activity was facilitated in this mode of organization. The style in which professional support is rendered appears to be important, with strong volunteer member leadership more effective than a benevolent dictatorship by professionals. A journal in which amateur astronomers may publish observations, discuss techniques and share insights is critical for scientifically oriented associations, and provided a driving force for organizing and maintaining such associations.

Venus' nightside ionosphere may be characterized by two general magnetic field geometries. Most of the ionosphere is characterized by largely horizontal fields carried over across the terminator from the dayside. However, regions of mostly vertical fields, associated with the nightside ionospheric 'holes' and connected to regions in the ionosheath where there are strong tailward fields, are also present. A magnetic potential model has been developed to describe the interface of the ionosphere, represented as an infinite slab bounded by two magnetically impermeable planes, with the tail lobes idealized as semi-infinite solenoids terminating at the ionopause. From this model, ionospheric boundary currents are generated to confine all field lines in either the flat ionosphere or the vertical tail lobe, and the field geometries are explored. Techniques for mapping this solution to a sphere and approximating it at long ranges from the transition region are discussed, and a preliminary global nightside model is offered.

We analyze the solar wind interaction with Mars through examination of Mars Global Surveyor Magnetometer/Electron Reflectometer (MGS MAG/ER) data. We focus on data in the postshock, dayside plasma flow, especially the Magnetic Pileup Boundary (MPB). First, we present the data to characterize the features of the MPB. Next, we argue the evidence that the MPB is formed through a series of processes, beginning with electron impact ionization of planetary neutrals in the exosphere of Mars. These new ions form an unstable population of energetic ions in the post-shock flow. Ion cyclotron waves are established to scatter the ion distribution into one more stable. This removes thermal pressure from the ions. In order to maintain a constant total pressure in the flow, the magnetic field intensifies, transferring the lost ion thermal pressure into magnetic energy. Also, we develop a model of the electron impact ionization process in the martian exosphere. This model calculates the evolution of the electron distribution function as the flow encounters exospheric planetary neutrals. It reproduces the electron spectrum observed by the ER in the MPB very well. Therefore, we conclude that electron impact ionization is the process responsible for the onset of magnetic pileup in the postshock flow at Mars.

This dissertation presents one of the most comprehensive computer simulations to date of a geomagnetic storm. During the geomagnetic storm of June 4--5, 1991, five spacecraft took measurements of conditions within the inner magnetosphere, including measurements of the electric and magnetic fields and the particle distribution at several different locations. These data are used to test the theoretical understanding of magnetospheric physics by comparisons to the Rice Convection Model (RCM). The RCM is a first-principles model of the inner magnetosphere that calculates the movement of magnetospheric particles, the currents into and out of the magnetosphere, and the magnetospheric electric field patterns. Furthermore, these comparisons provide answers to some of the pressing questions in magnetospheric physics: How strong does the shielding electric field become during a magnetic storm? Can a self-consistently calculated electric field inject plasma sheet particles to within 3 Earth radii of the Earth? How much of the increase in the ring current is due to the injection of plasma sheet particles and how much is due to the energization of pre-existing particles? The comparisons between the RCM electric field results and the corresponding measurements show very good agreement between the observations and the models, and indicate that a fairly strong electric field develops during the storm. Furthermore, comparisons of RCM calculated particle fluxes and the observations indicate that a self-consistently calculated electric field is able to inject plasma sheet particles deep into the inner magnetosphere, and that this injection is the dominant cause of the increased ring current.

Starting from the 10-dimensional low-energy string effective action for the graviton and dilaton, we study cosmological implications of string theory. We find some solutions for the string equations of motion both in vacuum and with the presence of matter. / The scale factors r and R, which can be interpreted as the radii of the universe, tend to evolve in opposite directions: one radius expands and the other shrinks. / We also study the flatness problem and propose an alternative solution to this problem. / The behaviour of the radii r and R rear the Planck length $({ sim}10 sp{-33}$cm) is studied in detail. / The significance of our results lies in the fact that in the context of string theory, we may have a good chance of observing several large spatial dimensions, with other internal spatial dimensions remaining small and unobserved from a macroscopic point of view.

The Boltzmann equation for the diffusion of Hawking radiation from microscopic black holes is solved using the test particle method. Formation of a dense cloud of interacting particles analogous to the photosphere of the Sun is confirmed. We find that at least two kinds of photospheres may form: a quark-gluon plasma for black holes of mass MBH ≃ 5 x 1014 g and an electron-positron-photon plasma for MBH ≃ 2 x 1012 g. The QCD photosphere extends from the black hole horizon for a distance of 0.2--4.0 fm for 109 g ≃ MBH ≃5 x 1014 g, at which point quarks and gluons with average energy of order ΛQCD hadronize. The QED photosphere starts at a distance of approximately 700 black hole radii and dissipates at about 400 fin, where the average energy of the emitted electrons, positrons and photons is inversely proportional to the black hole temperature. Possible consequences of these photospheres are discussed.

The cosmological constant is historically reviewed from its introduction in classical and relativistic cosmology through its modern quantum guise where it appears as a vacuum energy density. Limits on the empirical value are in glaring contradiction to the expectations of field theoretical calculations. / Motivated by the natural connection between dilatation invariance and the extinction of the vacuum energy density, a phenomenological realization of a global scale symmetry is constructed. A complete treatment of such a realization in the context of a supergravitational toy model is calculated to one loop using an effective potential formalism. Particular attention is paid to the quantization of both supersymmetric and general coordinate gauges and to the concomitant ghost structure since traditional treatments have introduced non-local operators in the ghost Lagrangian and generating functional. Contributions to the effective potentid from the gravity sector are thus determined that contradict the literature. A particular class of tree-level scalar potentials that includes the 'no-scale' case is studied in the that space limit. While it is found that scale invariance can be maintained at the one-loop level and the cosmological constant made to vanish for all potentials in the class this is directly attributable to supersymmetry. A richer form of the Kahler potential or an enlarged particle content may facilitate the breaking of supersymmetry. / Phenomenological consequences of supergravity are investigated through a one-loop calculation of the electromagnetic form factor of the gravitino. Should such a form factor exist a signature of the gravitino might be found in processes with unlabeled products such as $e sp+e sp- to nothing.$ It is found that the form factor vanishes to this order, the Lorentz structures generated being too impoverished to withstand a constraining set of polarization conditions. / Finally the wormhole solution to the cosmological constant problem is examined in a semiclassical approximation. The notion that scalar field worm-holes must have associated conserved charges is questioned and a model of massive scalar field wormholes is delineated and proven to provide a counterexample. As the model allows baby universes nucleated with a certain eigenvalue of the scalar field momentum to classically evolve to a different value, competing semiclassical paths contribute to the same transition amplitude. Numerical simulations demonstrate that the novel semiclassical paths available to massive solutions cannot be overlooked in approximating the tunneling amplitude.

We describe a general formalism for computing scattering rates of weak probes in macroscopic systems, based on a density matrix formalism. We show that weak probes in general scatter off fluctuations in the medium. In the limit that the neutrino wavelength is much larger than the lengthscale of the fluctuations, we show that the scattering rate can be calculated from knowledge of the equation of state of the medium through which the neutrinos travel. Using radial profiles of a post-bounce, shocked supernova core and a well-established equation of state for nuclear matter we compute these scattering rates for various times in the vicinity of the shock. We find that, behind the shock, these correlative effects can enhance neutrino scattering rates by factors of 8 compared to standard calculations which ignore interactions in the nuclear medium. These results may have implications for how efficiently neutrinos can restart a stalled shock, although firm conclusions regarding the ultimate effects of such an enhancement await full hydrodynamic simulations, which are not performed here.