In order to obtain a better understanding of auroral processes, statistical models have been formulated that characterize the spatial dependences of the aurora. These efforts include the Hardy probability model, the Hardy average model, and the Evans average model. Each differs in its technical approach but all three attempt to characterize electron energy fluxes at any given location in the auroral zone. In an attempt to describe the limitations of each model and perhaps make suggestions on how one can improve them, we reduced a six month sampling of DMSP-F2 electron precipitation data and averaged values over four second time intervals. Then each model generated a flux value for comparison against these satellite measurements so that Chi-square tests could be performed. Also the mean values for the whole six month period were calculated in order to determine the normalcy of the period tested. It is hoped that the knowledge gained through this endeavor will improve the accuracy of the Magnetospheric Specification Model's forecasts of the fluxes that endanger spacecraft.

We present a model describing the morphology and physical processes in NGC 6888, a 'wind-blown' nebula around the WN6 star HD192163. CCD imagery with the Palomar 1.5m revealed distinct morphological features that were further probed with spectrophotometry using the KPNO #2 telescope + Intensified Reticon Scanner. Distinctions in morphology in (O III) compared to H$\alpha$ led to a parametrization into two physical systems: (1). An inner ionized shell observable in all the emission lines, with (N II) T$\sb{\rm e}$ = 8000 $\sp\circ$K, (O III) T$\sb{\rm e}$ = 14,000 $\sp\circ$K and (S II) N$\sb{\rm e}$ = 400 cm$\sp{-3.}$, (2) The (O III) bubble and rim, visible only in (O III) and characterized by a higher (O III) T$\sb{\rm e}$ = 50,000 $\sp\circ$K. The nebula is found to have 5-10 M$\sb\bigodot$ of ionized mass. An extrapolation of (O III) $\lambda$5007 fluxes to IR emission line intensities led to revised values of neutral mass $\sim$40 M$\sb\bigodot$ for the nebula. Of this 3-6 M$\sb\bigodot$ is found to be enriched mass, 0.5 M$\sb\bigodot$ contributed by stellar wind mass loss and wind swept ISM mass is determined to be $\sim$20-60 M$\sb\bigodot$. Photoionization models demonstrate that the shell is enriched in N and He and depleted in O compared to Galactic H II regions. These results collectively indicate a scenario that is a combination of several physical processes. A slow wind ejected in an asymmetric fashion from the star is swept up by a fast stellar wind together with the ambient medium, creating the ionized shell. This interaction also creates Rayleigh Taylor instabilities which are determined to be plausible formation mechanisms for the observed knots. The hot gas ($\sim$ 10$\sp7 \sp\circ$K) penetrates the nebular material in the NW and SE as there is less ejecta pressure opposing it along the polar axis, and creates the observed (O III) bubbles in the NW and lattice in the SE.

Recent observations have discovered pervasive, low-ionization gas with unusual excitation in the halos of nearby spiral galaxies. This has led to speculation concerning the excitation mechanisms operating at high galactic latitudes, including exotic processes involving nonequilibrium physics, local grain destruction and decaying dark matter. We demonstrate that the excitation conditions of the diffuse ionized media (DIMs) in several such objects, NGC 891, the Galaxy and NGC 1068, can be understood in terms of photoionization by the dominant radiation fields in their halos, if proper consideration is given to the spatial distributions of their stellar populations, the relative importance of nuclear continuum sources and subsequent radiative transfer. Estimation of the stellar populations in NGC 891 and the Galaxy indicates that OB stars, and the central stars of planetary nebulae dominate the excitation of their DIMs. Photoionization models require local DIM electron densities of order unity to reproduce the observed emission characteristics of the high-$\vert z\vert$ gas. The DIM filling factor in NGC 891 is therefore restricted to the range 10$\sp{-3.5}\ \sbsp{\sim}{<}\ \phi\ \sbsp{\sim}{<}\ 10\sp{-1.5}$, while in the Galaxy $\phi\ \sbsp{\sim}{<}\ 10\sp{-1.2}$ is suggested. Ionization equilibrium indicates that only $\sim$5% of the total number of ionizing photons from OB stars escape the neutral disk, highlighting the importance of radiative transfer through the multiphase ISM. The DIM in NGC 1068 demonstrates a bipolar asymmetry common among active galaxies, with high-excitation emission confined to two diametric sectors aligned along the radio jet axis and low-excitation emission uniformly distributed over the disk. We show that this bipolar morphology can be understood using the developing picture of NGC 1068, in which its burried Type 1 Seyfert nucleus is visible only through scattered polarized light. The DIM excitation is therefore anisotropic, with high-excitation gas along the jet axis photoionized by direct nuclear continuum and low-excitation gas away from the axis, illuminated by indirect nuclear emission scattered into it. Photoionization models using power-law continua, indicate that the filling factor of this DIM is restricted to the range $10\sp{-5} \ \sbsp{\sim}{<}\ \phi\ \sbsp{\sim}{<}\ 10\sp{-4}$ in order to reproduce the observed DIM emission characteristics.

The temporal and spatial relationships between hard X-ray and UV emission in solar flares provide stringent restrictions on the physical processes responsible for solar flare emission. In this thesis, we present results from a detailed analysis of temporal profiles and spatially resolved hard X-ray images from RHESSI alongside high cadence observations of the UV continuum from TRACE for two solar flares: A C6.5 flare from 16 July 2002 06:38--06:43 UT and an M8.5 flare from 17 July 2002 07:00--07:05 UT. For both flares we find a significant temporal correlation between the UV and hard X-ray emission over the duration of both events. In the spatial analysis for the 16 July 2002 event, we note a significant spatial separation between the correlated UV sources and their hard X-ray counterparts leading to a complex flare topology invoking loop-loop interactions. We suggest a three-dimensional reconnection scenario in an attempt to address the spatial discrepancy in the hard X-ray and UV correlated emission.

We use the unprecedented spectral and spatial resolution of RHESSI to explore the behavior of electrons and their associated currents in solar flares. Spectral images are used to determine an estimate of the effective surface area for the different independent substructures within each event. The incident electron spectra at those flaring footpoints are derived from the RHESSI photon spectra. We find that, over a wide range of flare X-ray magnitudes, the integrated photon flux above 20 keV asymptotically approaches a limiting value, suggesting a saturation of the photon production in flares. The inferred particle fluxes in the beam, together with this saturation limit, are used to determine the energy loss mechanism dominating the energetic particle transport in solar flares.

This thesis presents two-dimensional hydrodynamic disk simulations with embedded protoplanets, emphasizing the non-linear dynamics in the co-orbital region. In particular, it demonstrates how a protoplanetary disk responds to embedded low mass planets at the inviscid limit. Since the potential vorticity (PV) flow is not conserved, due to the spiral shocks and possibly boundary layer effects emanating from the planet, the PV profile develops inflection points which eventually render the flow unstable. Vortices are produced in association with the potential vorticity minima. Born in the separatrix region, these vortices experience close encounters with the planet, consequently exerting strong torques on the planet. The existence of these vortices, if confirmed, have important implications on planetary migration rates. The formation of vortices is discussed in more detail and a key parameter is found which depends solely on planet mass and sound speed. With this key parameter, one can predict the disk evolution, PV growth rates, and threshold conditions for forming vortices in the co-orbital region. An analytical estimate for the change of PV due to shocks is compared to the actual change in PV in the hydrodynamic simulations. They match well except in the inner region where vortices form. In addition, extensive resolution tests were carried out but uncertainties remain about the physics of this particular region.

The access of incident solar wind plasma to the surface of Mercury is dependent on the magnetic fields in the vicinity of the planet. We present the structure of the Hermean magnetosphere obtained by the Toffoletto-Hill (1993) model of a magnetically interconnected ("open") magnetosphere modified for the strength of the magnetic field of Mercury and scaled to account for the difference in planetary radii between Mercury and the Earth. We calculate open regions for the access of incident particles to the surface as a function of the interplanetary magnetic field (IMF) direction and of the solar wind pressure. Predictions are also made for cases of limited field penetration. These results are compared with existing sodium data. Although quantitatively crude, the model gives a good qualitative approach on how to interpret the sodium emission. We predict emission is stronger in the southern hemisphere for a positive Bx component, and in the north for a negative Bx. The Bz component regulates the size and position of the open field line region, with more negative Bz's corresponding to more equatorial open flux regions.

Results from the analysis of HST FOS, WFPC1, IUE, and CTIO 4m observations of the morphology, physical conditions, and chemical abundances in the unusually dusty H II region N88A in the Small Magellanic Cloud are presented. HST WFPC1 imagery of N88A indicate the nebula is bi-lobed due to a dark dust lane cutting across the center of the nebula. The presence of a high ionization arc, apparently connecting two semi-stellar objects within the nebula, is reported. The derived reddening curve for the nebula is flatter in the UV than the general SMC extinction curve, suggesting the presence of large grains. Carbon and possibly nitrogen abundances are enriched in the nebula. The conclusion is that SMC N88A is a very young H II region forming out of a small dusty C- and N-rich molecular cloud which is being disrupted by star formation in a larger OB association in the area.

Four monochromatic (H$\alpha$, (N II), (O III) and 5500 A continuum) mosaics of the inner region of the Orion Nebula have been made by using data obtained with the Wide Field Planetary Camera 2 of the Hubble Space Telescope. These mosaics have a common area of $7\sp\prime52\sp{\prime\prime} \times 9\sp\prime25\sp{\prime\prime}$ which is the largest area of the Orion Nebula covered by the Hubble Space Telescope images in a single study. Making use of these Wide Field Planetary Camera 2 data, a survey of compact sources and stars down to V $\approx$ 22 has been done. A total of 489 sources that are not Herbig-Haro objects have been found. 145 of them can be classified as protoplanetary disks (proplyds). The proplyds have been classified into eight different forms. The forms which show strong evidence of interaction with a stellar force are concentrated near $\Theta\sp1$C Ori. Those forms that do not show such evidence are more widely spread.

We used Smoothed Particle Hydrodynamics to study the formation and evolution of mass streams that transfer material from a circumbinary disk to the stars of a close binary. These streams form when the disk is hot enough and/or viscous enough (thickness-to-radius ratio c &ge; 0.05, for viscosity parameter alpha > 0.01). We modeled binaries with different mass ratios and eccentricities and calculated the spectral energy distribution of the binary-disk system, for an optically thick disk. We focused on the variation of the excess in the N band (effective wavelength &sim;11mum). The result of the mass streams is a considerable infrared excess that varies over the binary period. This variability is observable and can be used as a diagnostic element in the study of very close binary systems, because its presence is a sign of stream accretion. There is a periodicity in this variation, with a period equal to the binary period. The general pattern the excess follows is that it is larger at the periastron of the orbit. We also examined the role of the temperature profile in the formation of the streams. We found that for a hotter disk temperature profile ( T &sim; 1/R1/2) streams form even for moderate values of thickness-to-radius ratio (c &ap; 0.03). Thus, systems like DQ Tau (c &ap; 0.03--0.045) may have such streams, something that previously was questioned.