In this work I investigate the possibility of Type II supernovae being the origin for positrons producing observed annihilation radiation observed toward the Galactic center. It was my contention that the decay of $\sp{56}$Co coupled with falling densities would allow for the production and extended existence of positrons in the supernova outflow. Supernova 1987A has prompted many people to construct models of supernova outflow. I use the results of two existing models as the initial conditions in my models. I have created both an analytic and a computer model for the survival of positrons. These models show that while Type II supernovae fall short of the needed production of surviving positrons, the lower densities existing in Type I supernovae may be a more promising source.

This thesis is a theoretical study about the Earth's tail plasma sheet with regard to two aspects: the equilibrium structure for the anisotropic pressure, and the ideal-MHD ballooning stability. By adopting a stretched magnetotail model where ion motions are generally nonadiabatic, and assuming that the anisotropy resides only in the electron pressure tensor, it is shown that the magnetic field lines with $\rm p\sb\perp > p\sb\Vert$ are less stretched than the isotropic cases. As the parallel pressure p$\sb\Vert$ exceeds the perpendicular pressure p$\sb\perp$ approaching the conventional marginal firehose limit, $\rm p\sb\Vert = p{\sb\perp}$ + B$\sp2$/$\mu\sb0$, the magnetic field lines are more and more stretched. It is also shown that the current density is highly enhanced at the same limit, a situation that might be subject to a microscopic instability. However, we also emphasize that such an enhancement in the current density is heavily localized near the z = 0 plane, and thus it is unclear if such a microscopic instability can significantly alter the global configuration of the tail. It is further argued, in terms of the radius of the field curvature versus the particle's gyroradius, that the conventional adiabatic description of electrons may become questionable, very close to the conventional marginal firehose limit. To study the ideal-MHD ballooning mode, we first adopt a hard ionospheric boundary condition where the perturbation is required to vanish at the ionospheric foot points. For such a hard boundary condition, an "untypical" magnetic field configuration is found to be unstable to a ballooning mode that is antisymmetric about the equatorial plane while most of the "typical" tail plasma-sheet configurations are stable against the ideal-MHD ballooning mode. The unstable magnetic field model, however, does not look like the average observation-based model, but rather resembles some of the characteristics of the steady-state magnetic field models by Hau (1989, 1991). In addition, a physical argument is presented to show that the hard ionospheric boundary condition is the most appropriate simple boundary condition for the Earth's plasma sheet.

This thesis aims to compute the mean age of interstellar dust particles using a sputtering and recycling model originally formulated by Liffman and Clayton. The ages of the particles are evaluated after a period of 6 $\times$ 10$\sp9$ years, deemed here to be the age of our Galaxy, when the Solar System formed. I find a correlation between resulting particle sizes and their mean age. The consequence of this correlation is that if interstellar particles can be sorted dynamically by size, then the conglomerations of these size populations as dust grains will be composed of matter that is of differing ages. This age variation in grains of differing sizes will produce isotopic variations, or anomalies, due to the time dependent nature of secondary versus primary nucleosynthesis. The most important example of such an observed anomaly is the 5% enrichment of $\sp{16}$O in inclusions of Al$\sb2$O$\sb3$ in meteorite structures relative to the abundance of $\sp{16}$O in the solar gas. (Abstract shortened with permission of author.)

In order to understand and explain the low-energy plasma structures observed by the PLS experiment on Voyager 2 in the Uranian inner magnetosphere, an analytic and self-consistent model of a time-dependent solar-wind driven convection system at Uranus has been developed in the corotating coordinate system. Many important results of this model agree with the observations very well. Because of the unusual orientation of the planetary rotation and magnetic dipole axes, magnetic merging on the dayside magnetopause varies as a function of planetary spin, in response to the changing orientation of the planetary magnetic field relative to the upstream interplanetary magnetic field, which is assumed to have a fixed direction for many planetary rotations. Therefore the magnitude of the solar-wind driven convection electric field varies sinusoidally in time with the 17.2 hr planetary spin period, even though the field direction is fixed in the corotating frame in a direction analogous to the dawn-to-dusk direction in the Earth's magnetosphere. By assuming conservation of the first adiabatic invariant we find that the "hot" (few keV) protons observed by the PLS experiment in the inner magnetosphere may be convected Sunward from a pick-up source provided by electron impact ionization of the neutral torus of the outermost satellite Oberon. Under the time-dependent convection field this hot plasma forms a ring-current shielding layer in the region L = 5 $\sim$ 7, similar to an Alfven layer because the hot plasma convection timescale ($\sim$20 days) is much larger than the 17.2 hr period of variation of the convection field. Inside of the shielding layer the time-averaged electric field is much smaller than the time average of the imposed field. The sinusoidal oscillation of the imposed electric field, however, is not significantly shielded by the shielding layer because the shielding timescale ($\sim$30 hr) is longer than the 17.2 hr oscillation period. A fraction of the hot plasma is therefore able to penetrate the shielding layer to form a trapped ring-current population. This trapped ring-current population is sufficiently long-lived to undergo charge-exchange and inelastic collisions with the widely distributed neutral hydrogen corona, resulting in the energy degradation of the "hot" component and the simultaneous appearance of the "intermediate" (few 100 eV) and "warm" (few 10 eV) components evident in the PLS results in the region between L = 5 and L = 7. The region 2 Birkeland current system, in our model, is concentrated near the region of the ring-current shielding layer. By analogy with the Earth's magnetosphere, the lower boundary of the Uranian aurora is predicted by mapping the location of the shielding layer in the magnetic equatorial plane along the magnetospheric magnetic field lines onto the Uranian ionosphere.

We investigate a new interaction of globular clusters with galactic field stars. By dynamical friction, high-velocity field stars passing through individual globular clusters are decelerated. This frictional interaction contributes to cluster heating, and, in conjunction with disk shocking and other mechanisms, it helps regulate the evolution of globular clusters. Moreover, penetrating field stars with low relative velocities can even be captured by globular clusters. Our calculated rate of captures suggest that there is a substantial population of stars having an origin external to the globulars in which they now reside. Intriguing candidates for this "immigrant" population include some blue straggler stars and short-period pulsars.

In the large magnetic fields associated with highly magnetized neutron stars, the Compton cross section exhibits a resonance at the local cyclotron energy. In this work I describe applications of magnetic Compton scattering to models of both rotation powered and accretion powered pulsars. Radio pulsars are generally considered to be rapidly rotating, highly magnetized neutron stars. The rapid rotation coupled with the large magnetic field induces large electric fields that can accelerate electrons in the neutron star magnetosphere to large energies. I have produced a Monte Carlo code to model $\gamma$-ray emission from rotation powered pulsars utilizing pair cascades initiated by Comptonized photons. Previous polar cap $\gamma$-ray emission models have relied on pair cascades initiated by curvature radiation photons. This Monte Carlo model can reproduce the double peak pulse profiles often observed from rotation powered pulsars and explain the trend for harder spectra from slower pulsars. X-ray Pulsars are thought to be highly magnetized neutron stars accreting matter from an ordinary stellar companion. The accreting matter is channeled onto the neutron star polar cap by the magnetic field. This material produces a hot spot on the neutron star surface that emits X-rays. I have investigated the effects of radiation pressure due to these X-rays on the accreting material. The radiation pressure is greatly enhanced by the resonance in the magnetic Compton cross section. Because the electron cyclotron energy varies with distance from the neutron star, the energy dependent X-ray spectrum maps to a spatially varying radiation force. This force can exceed the force of gravity over a limited region of the X-ray pulsar magnetosphere. I postulate that when this occurs matter can be elevated above the neutron star surface outside the accretion column. This material will act as an energy dependent "lamp shade" that will produce pulse profiles that vary with photon energy. This model is capable of reproducing the energy dependent pulse profiles observed from the X-ray pulsars 4U 1626-67, 4U 1538-52, 4U 1907+09, and Vela X-1.

Narrow band filter images of the inner Orion nebula were taken by the the Hubble Space Telescope WF/PC in 1990 and 1991. Two data sets were reduced and astrometric solutions were obtained for the two fields with good accuracy. The three major emission line filters for the 1991 data were calibrated into absolute energy units. The continuum contribution was removed from the images, and the $\rm H\alpha$ contribution was removed from the (NII) images. Calibration constants were found to differ from predicted prelaunch values, and possible explanations were investigated. VLA and HST images of two compact sources near the center of the Orion Nebula were compared. The VLA and HST images are similar when the two are scaled to the same units. VLA images show a higher concentration of surface brightness, reflecting its better resolution; whereas, the HST images, due to greater sensitivity of the HST, showed extended cometary form. The positions of three Herbig-Haro objects on HST images were compared with ground based observations at various wavelengths. The basic agreement between these positional observations can be used to analyze the structure of these sources, because of the different excitation mechanism for each source.

An optical study of the HII region populations evident in three galaxies, M101, M51 and NGC 4449, has been made. Using narrow-band filters, emission line imagery has been taken using a CCD focal-reducing camera, at wavelengths covering the emission from H $\alpha,$ H $\beta,$ (O III) $\lambda$5007 and (S II) $\lambda\lambda$6716+6731. Using several identification techniques to spatially select the HII regions, emission line properties have been derived for 625 HII regions in M101, 465 in M51 and 163 in NGC 4449, making this the most complete study of its kind to date. Several trends have been discovered concerning the properties of the HII regions with radial position within their galaxy. M101 exhibits a large gradient in excitation, and oxygen abundance, as well as a gradient in the line-of-sight reddening. No positional variation in the derived ionization parameter for each HII region was found. Local variations in the effective collapse density for neutral gas have been detected for both M101 and M51. No such analysis was possible for NGC 4449 due to a lack of available data. M51 shows systematic emission variations only in the brightest cores of its largest HII regions, an effect attributed to a larger influence of the local ISM on the properties of the fainter, and more obscured, HII regions. M51 exhibits a spiral pattern that does not follow a single mathematical description, departing most dramatically at the corotation radius. A variation in the evolutionary time from peak local compression to peak star formation with radius has been detected for one of the arms in the galaxy, but not the other. NGC 4449 displays no systematic variations in the derived emission properties of its HII region population. This is attributed to a star formation mechanism that is independent of the radial ordinate, contrasting with the spiral density wave mechanism dominant in spiral galaxies. Unprecedented deep CCD imagery of this galaxy is presented, revealing the complicated structure of ionized filaments between the HII regions. The emission properties of these filaments are studied.

The Comet Rendezvous and Asteroid Flyby Mission (CRAF) will include, as one of its complement of thirteen scientific instruments, a plasma electron analyzer capable of providing 3-dimensional measurements of the energy and angular distribution of electrons in the solar wind, asteroidal and cometary environments. After initial instrument selection, mission planners at JPL suggested that an instrument capable of performing angular scanning electronically rather than mechanically be investigated. This thesis describes the development of the redesigned CRAF plasma electron detector, consisting of an electronic scanning component, called the "elevation analyzer", and an energy analyzing component based on the Soft Particle Spectrometer (SPS). The numerical simulation of each component's operation--consisting of ray-tracing particles through the electrostatic field of each analyzer and collecting statistics on those particles successfully transmitted--is used to determine the energy and angular response functions of each component and the design dimensions that optimize these responses.

Gamma-ray bursts (GRBs) remain one of the most inexplicable astrophysical phenomena observed today. While counterparts at other wavelengths would provide the best clues as to the nature of GRBs, none have been observed. To supplement studies on GRB distribution and population statistics, temporal morphologies, and spectral line searches, we focus on the analysis of GRB continuum spectral evolution. Previous spectral evolution studies have shown a variety of patterns: most individual pulses show a hard-to-soft evolution, but studies of both the SIGNE and BATSE GRB databases reveal several other patterns, including hardness-intensity tracking, soft-to-hard, static, and chaotic spectral evolution. This type of analysis attempts to identify spectral evolution signatures that can discriminate between different physical scenarios or different GRB subpopulations based on temporal profile, duration, intensity, or spatial distribution. Contrary to most studies that use only one model and one parameter to characterize spectral evolution, several models are used here. Statistically equivalent models are shown to give consistent physical results. I verify the variety of spectral evolution patterns present in GRBs, and investigate how the actual shape of the spectrum evolves, following multi-parameter spectral fits in time. Different spectral evolution patterns exist simultaneously in multiple parameters. Hardness-intensity correlations in pulse and over burst decay phases are quantitatively examined: correlation is often significant, but the relation between hardness and intensity is non-unique. Hardness-intensity lag-times are found to correlate to the rise-time of the hardness profile. Comparisons of double-pulse GRBs reveals a variety of results, including the implication that late-emitting pulses are less affected by early emission.