As on Earth, Jupiter's auroral emission comes from the upward Birkeland currents. The VIP4 magnetic field model maps the main oval to the middle magnetosphere (r ∼ 30 RJ). However, the interpretation of the observed features of the jovian aurora and its link to the middle magnetosphere remains an open question. This thesis reviews the main observational features and existing models of Jupiter's aurora. After comparing the merits and disadvantages of each model, we outline future possible improvements of the models.

During fast fluid flows in Earth's magnetotail, the plasma distribution function often takes the form of one beam flowing through another, which raises the question of whether Bursty Bulk Flows (BBF's) can reasonably be represented in terms of single fluid magnetohydrodynamics (MHD), either in global MHD codes or in thin-filament theory. An exact kinetic solution is compared with exact fluid solutions for a simplified case of cold, collisionless particles in a pipe, under conditions where there are counter-streaming beams similar to the ones that often occur in Earth's magnetotail. The results from kinetic theory differ from standard fluid theory but are exactly consistent with Chew-Goldberger-Low double-adiabatic fluid theory. Double-adiabatic MHD equations are derived for the motion of a thin filament through a medium. Simulation results are presented for a double-adiabatic filament that starts out with lower gas pressure than nearby flux tubes and also for plasma ejected earthward from a patch of reconnection at X ∼ -25 RE. As in earlier calculations for the isotropic case, in both cases the near-equatorial part of the filament moves rapidly earthward. A compressional shock wave forms in the filament near the equatorial plane and propagates earthward. The near-equatorial region of the filament exhibits characteristics similar to a flow burst, while the behavior far from the equatorial plane resembles that of earthward-streaming plasma-sheet boundary layer. In both cases, the double-adiabatic filament becomes firehose unstable after the shock wave reflects from the earthward boundary of the simulation and propagates back into the tail. The tailward-propagating compressional wave, which brakes the earthward flow in the filament, is thus characterized by strong magnetic fluctuations. Within the context of the Near-Earth-Neutral-Line model of substorms, we suggest that firehose instability might cause the intense magnetic-field fluctuations that are observed in the inner plasma sheet at substorm onset. Additional simulations have been carried out to confirm the robustness of our principal conclusion that fast earthward flows in the Earth's plasma sheet should lead to firehose instability.

This optical, near-infrared observational study compares the properties of clusters with and without Wolf-Rayet (W-R) stars in starburst galaxies. Previous studies looked only at the galactic properties. Because this work identifies the specific locations of W-R stars, comparisons of clusters with and without W-R stars are possible and enable us to understand the impact of massive stars on their environment, to gain knowledge of the cosmic formation and evolution of chemical abundances, to constrain the parameters for modeling these systems, and to apply this knowledge to observations of starburst galaxies at higher redshifts. BHK and Ha imagery as well as spectroscopic observations over lambdalambda3500--7000 were taken of 48 clusters in 20 galaxies. Using standard reduction techniques, we calibrated the data to provide the locations of W-R stars and to derive 35 cluster and interstellar medium properties such as age and elemental abundances. These 35 properties of clusters with and without W-R stars are compared for similarities and differences in stellar populations, nebular morphology, and elemental abundances. Most stellar populations of clusters with and without W-R stars confirm current evolutionary theory of massive stars. Clusters with W-R stars are currently younger, bluer clusters which will later evolve into clusters without W-R stars. A few young non W-R clusters may indicate that the IMF/Upper Stellar Mass Limit is not universal; further work to determine the presence of W-R stars in these young clusters is therefore critical. Clusters with and without W-R stars show significant differences in nebular morphology, differences best explained evolutionarily. The clusters with W-R stars typically have a gas configuration similar to a Stromgren sphere so that the dust is heated, causing red H-K colors. Around the clusters without W-R stars, the gas expands into a superbubble configuration because of longer influence from stellar winds. The dust in these superbubbles obscures the stellar light, causing red B-H colors. Neither N/O nor S/O shows enhancements due to Wolf-Rayet stars. Future work will expand these studies into X-ray, ultraviolet, and infrared wavelengths.

The jovian aurora contains a persistent main oval encircling each magnetic pole, which is associated with the upward field-aligned currents in the corotation enforcement current system. It has been suggested by two recent studies that the brightness of the main oval should become temporarily dimmer ∼ 1 hr after arrival of a shock wave in the solar wind, compressing the magnetosphere abruptly, because the difference between the angular velocity of the plasma in the magnetosphere and the rigid planetary rotational speed becomes smaller. But recent observations at Jupiter and Saturn have reported the opposite: the auroral oval brightens, and moves poleward, after the arrival of a solar wind shock. In this thesis, I will quantitatively include the flywheel effect of the neutral gas in the ionosphere in the coupling current system to explain this discrepancy and show that the corotation enforcement current should reverse and strengthen after a compression, and thereby temporarily cause the main oval to become brighter and move poleward. I will also show the differences between the night side and the day side in steady state and after a compression event by applying two different magnetic field models fitted from observations, and try to qualitatively explain the dawn-dusk asymmetry by introducing a region-1 current system analogous to that at Earth, which arises from the detailed interaction between solar wind and magnetosphere. Generally, I expect the day side sector of the main oval to brighten more than the night side sector, and the dawn sector to brighten more than the dusk sector.

Though it has been more than 30 years since the first discovery of the classic Galactic black hole candidate (GBHC) Cygnus X-1, the X-ray emission mechanisms of GBHCs are still not well understood. It is generally agreed that black holes accrete materials from nearby objects or media to form accretion disks around them. The accretion disks can be heated up to a temperature above 1 keV. Such a hot disk emits X- and Gamma-rays, observations of which are essential for understanding the accreting and heating processes. Our multi-wavelength observations of GRS 1758-258 (a GBHC) in August 1997 revealed several properties that are important for constraining the emission model and the geometry of the accretion disk. (1) Its spectrum does not have a significant soft component. This implies that the cold optically thick disk must be small or be mostly covered by a optically thin hot corona. (2) The spectrum has an exponential cutoff around 200 keV, which means that the hot corona is largely thermal. (3) No significant iron lines are detected. This suggests that the reprocessing of the X-ray by the cold disk is negligible. We systematically analyzed RXTE archival data to study the energy dependency of the variability of X-ray flux from four hard X-ray sources: Cygnus X-1, GX 339-4, GRS 1758-258 and 1E 1740.7-2942. Cygnus X-1 was found to have flatter power density spectrum (PDS) shapes at higher energies, while the other three have energy independent PDS shapes. No current models can fully explain these results. A general trend was found among the four sources that the variability anti-correlates with the X-ray flux. We found that the 0.5--10 Hz quasi-periodic oscillations (QPOs) observed in GRS 1915+105 (a GBHC) has peculiar phase lag behaviors. When the QPO fundamental frequency is low (0.5--2.0 Hz), positive phase lags, which mean that hard photons arrive later than soft photons, were observed in both fundamental and first harmonic frequencies. The phase lags have opposite signs at the two frequencies when the fundamental frequency is high (2.0--4.5 Hz). Such strange behaviors can not be explained by current models. We found the flat radio spectrum observed in most GBHCs can be explained by a hybrid thermal/non-thermal plasma, which can also reproduce the power-law tail that have been observed in many GBHCs beyond the thermal X-ray spectrum.

Gamma-ray bursts (GRBs) have become a truly unique puzzle in Astronomy. Unlike quasars and pulsars which were explained within years of their discovery, the origins of GRBs remain a mystery more then thirty years after their discovery. Even the most basic information about these events, such as a concrete estimate of their distance, wasn't known until only a few years ago. The difficulty in our ability to obtain information about the nature of GRBs reflects their transient nature, which occur in random location in the sky with a rate of roughly one a day. Obtaining multi-wavelength observations of such brief events is a situation that astronomy has rarely been faced with. It has only been recently, with the advent of rapid arcminute x-ray localization, that we have started to decern information regarding the intrinsic properties of these events. In this thesis, I present a compilation of research describing our efforts to investigate the intrinsic properties of GRBs. The thesis is broken down into chapters including an introduction to the field of GRB astronomy, three chapters covering a range of spectral and temporal investigation, two of which have been published in the Astrophysics Journal with plans to submit the third and a conclusion outlining current and future work on the promises of using GRBs for cosmological research.

On April 13, 1996, while Russia's Interball Tail spacecraft was travelling through the dayside magnetosheath, NASA's Polar spacecraft traversed the northern cusp. Transients observed in the data from the Interball Tail/SCA-1 particle instrument are shown to be multiple crossings of the magnetopause. Analysis of data from the SCA-1 instrument and magnetic field data from the MIF instrument shows quasi-periodic motions of the magnetopause. Simultaneous observations of changes in cusp precipitation by the Polar/TIDE instrument and changes in the He2+/H+ density ratio in the cusp by the Polar/TIMAS instrument show variations on the same time scale. We show that the variations in the cusp data observed by Polar are the result of changes in the reconnection rate at the magnetopause and that these changes are associated with the magnetopause oscillations observed by Interball.

This thesis presents new tracings of ions in Mercury's magnetosphere that model the variability of the solar wind, sodium and potassium surface fluxes. The magnetic field is given from a modified version of the analytic Toffoletto-Hill (TH93) open magnetosphere model, which also gives the electric potential along open field lines. Its applicability is extended into the closed field line region by the Ding (1995) potential solver, which computes the realistic electric potential that is self-consistent with the magnetic field. Three cases of the solar wind and the interplanetary magnetic field (IMF) are tested, two at aphelion and one at perihelion. Photoions are launched from the scale height for each species, while solar wind ions are backtraced from the surface and to the magnetopause. Photoion results reveal that the escape rate to the solar wind responds to external conditions only loosely: between aphelion and perihelion, the escape ratio was seen to range from 30 to just 40 percent. Therefore, impacts dominate. The prediction that recycling reduces by a factor of 1.5 at perihelion could help explain why the sodium atmosphere is denser at aphelion. The flux capable of sputtering varies between 10--15 of the total dayside flux for strongly southward IMF Bz = -10 nT. In addition, tracings of Hermean ions show that differential escape losses do not exist for potassium photoions such that may explain the variable Na/K ratio in the Hermean atmosphere. Solar wind ion tracings confirm that precipitation to Mercury's surface may happen along closed field lines not only at perihelion, but even at aphelion for realistic cases of southward IMF. The computed total sputtering flux increased by a factor of 1.7 from aphelion to perihelion. Most of the flux capable of sputtering is deposited on a region that is extended in longitude but limited in latitude. We conclude that ion sputtering caused by the solar wind can explain the high-latitude variability seen in imaging data of Mercury's sodium atmosphere. This work is relevant to the NASA/Messenger and ESA/BepiColombo missions, both for mission planning and analysis of results.

Particle acceleration in astrophysical plasmas has been a longstanding and challenging problem and it has been both intensively and extensively highlighted by the recent observations from Compton Gamma Ray Observatory on various astrophysical objects, including Gamma Ray Bursts (GRBs), Active Galactic Nuclei (AGNs) and Galactic Black Hole Candidates (gBHCs). In this work, I study the stochastic particle acceleration due to the resonant interactions between the turbulent plasma waves and particles. I employ the particle orbital theory approach by treating the effects of various waves as perturbations to particle's zeroth-order Hamiltonian. The particle's momentum and pitch angle diffusion coefficients ($D\sb{pp},\ D\sb{\mu\mu})$ are derived for interactions of proton/Alfven-wave, electron/fast-magnetosonic-wave and electron /whistler-wave, though the formalism can be generalized to other type of waves. Based on the Monte Carlo code I have developed, which solves the coupled time-dependent wave, particle, and photon kinetic equations, these results have been or are being applied to the central region of both AGNs and gBHCs, with applications for gamma-ray production and energetic particle outflows. By calculating the particle trajectories under the influence of gravity and radiation pressure near the center of galactic black hole accretion disk, I show that the recent discoveries of relativistic outflows from several X-ray binaries (e.g., GRS1915+105 and GROJ1655$-$40) can be well explained by radiation acceleration. The calculated final jet velocity is in good agreement with the observations and further constraints can be put on the composition and the power of those jets. The isotropic but inhomogeneous distribution of GRBs over the whole sky apparently requires more exotic explanations. A novel model for GRBs from high velocity neutron stars, which escape into our Galactic halo, has been developed. I show, in detail, the difficulties the halo models are facing and propose several scenarios to overcome them. This model predicts that bright burst distribution should deviate from isotropy and it still awaits confrontation with the observations.

This work explores the connection between cosmic rays and light element production in an active galaxy environment. Cosmic rays generated in an active galactic nucleus (AGN) interact with the local, line-emitting gas and spall the light elements, Li, Be and B. Careful consideration of the propagation of cosmic rays from AGNs to Earth yields a variety of models that are consistent with the observed cosmic ray spectrum. However, by using observed upper limits for BIII $\lambda$2066A line emission from AGNs, we are able to rule out certain cosmic ray flux models. This analysis requires a detailed study of boron ionization balance under typical AGN conditions, a study that is carried out here for the first time. Models with a total cosmic ray luminosity $L\sb{CR}=10\sp{45}$ erg s$\sp{-1}$ and a diffusion coefficient in the line emission region of $D\le 10\sp{28}$ cm$\sp2$ s$\sp{-1}$, and those with $L\sb{CR}=10\sp{45}$ erg s$\sp{-1}$ and $D\le 3\times 10\sp{26}$ cm$\sp2$ s$\sp{-1}$ do not satisfy the spectroscopic constraints. However, models with lower cosmic ray luminosities or larger diffusion coefficients are acceptable. The results of spallation in AGNs are also applied to our Galaxy, under the assumption that it has passed through an active phase. An additional source of light elements during this active phase can reproduce the B and Be abundances observed in the halo, and contribute partially to the light element abundances observed in the disk.