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The work in this thesis describes two experiments which use transfer reactions to perform spectroscopic studies of nuclei in order to improve reaction rates in astrophysical environments. The first experiment is an indirect study of the 34S(p,γ)35Cl reaction rate at energies relevant to classical novae temperatures. By reducing uncertainties in this reaction it may be possible to use 32S/34S isotopic ratio as a diagnostic tool to determine pre-solar grain paternity. A study of the 34S(3He,d)35Cl transfer reaction was performed to identify energy levels in the astrophysically relevant energy region and assign spin and parity to these new states. A new reaction rate has been calculated from this spectroscopic information and is the first experimental measurement of the 34S(p,γ)35Cl reaction rate. Using this new rate it was concluded that it is now possible to determine the paternity of pre-solar grains using the 32S/34S isotopic ratio. The second experiment measured two proton transfer reactions, (3He,d) and (α,t), with the aim of making spin assignments of states above the neutron threshold in 27Al. Combined with information from complementary experiments this information would be used to calculate new 26Al(n,p/α) reaction rates. Direct comparison of the two transfer reactions should allow for low and high spin states to be identified, however due to lower than expected cross sections useful information could not be extracted from the (α,t) reaction. The experimental resolution was insufficient to resolve individual states with the (3He,d) reaction, however due to the selectivity of the reaction it appears that many of the previously known states show low spin behaviour and are likely not relevant to the reaction rate at astrophysical temperatures. In addition, the non-observation of 23 states known to exist in 27Al may indicate they are high spin and further measurements of these states should be performed in order to calculate new 26Al(n,p/α) reaction rates.

The objective of this thesis is to present a viable extension of general relativity free from cosmological singularities. A viable cosmology, in this sense, is one that is free from ghosts, tachyons or exotic matter, while staying true to the theoretical foundations of General Relativity such as general covariance, as well as observed phenomenon such as the accelerated expansion of the universe and inflationary behaviour at later times. To this end, an infinite derivative extension of relativity is introduced, with the gravitational action derived and the non-linear field equations calculated, before being linearised around both Minkowski space and de Sitter space. The theory is then constrained so as to avoid ghosts and tachyons by appealing to the modified propagator, which is also derived. Finally, the Raychaudhuri Equation is employed in order to describe the ghost-free, defocusing behaviour around both Minkowski and de Sitter spacetimes, in the linearised regime.

The outer electron radiation belt is highly dynamic in space and time. Understanding the mechanisms that drive these variations is of high interest to the scientific community because of the negative effects that the radiation belt can have on satellite instrumentation. Evidence in support of a wide range of processes has been uncovered, yet a complete understanding of the relative contribution of each processes, and how these contributions vary over time, is yet to be fully determined. The first body of research (Chapter 4) follows the evolution of the electron radiation belt at geosynchronous orbit through three high-speed solar wind stream induced dropouts. Electron flux, magnetic field, and phase space density results from GOES-13 indicate that outwards adiabatic transport plays a key role in causing radiation belt flux dropouts at GEO. This leads to enhanced magnetopause losses and subsequent outwards radial diffusion. Other loss processes may also play a role. In the second body of research (Chapter 5), the partial moments (electron number density, temperature and energy density) from GOES-13 are compared to different solar wind parameters, both instantaneous and time delayed, in order to develop a coarse predictive capability. Using these partial moments allows for changes in the number of electrons and the temperature of the electrons to be distinguished, which is not possible with the particle flux output from individual instrument channels. It is found that using solely the solar wind velocity as a driver results in predicted values that accurately follow the general trend of the observed moments. Given that electron number density and temperature are the fundamental physical parameters of a plasma, the result is a testable model that addresses elementary plasma properties. Hence, for a Maxwellian plasma, it is possible to infer the flux at any energy, not just energy channels tied to a particular instrument. In the final research study (Chapter 6), the validity of using the cold plasma dispersion relation to infer the magnetic field wave power from the measured wave electric field is tested using Van Allen Probes EMFISIS observations in the chorus wave band (0.1-0.9 fce). Results from this study indicate that for observed wave intensities > 10-3 nT2, using the cold plasma dispersion relation results in an underestimate of the wave intensity by a factor of 2 or greater, 56% of the time over the full chorus wave band, 60% of the time for lower band chorus, and 59% of the time for upper band chorus. Hence during active periods, empirical wave models that are reliant on the cold plasma dispersion relation will underestimate chorus wave intensities to a significant degree, thus causing questionable calculation of wave-particle resonance effects on MeV electrons.

Radiative transfer methods provide a path to uncover the intrinsic properties of astronomical objects from observations. The determination of the shape, size and brightness of many objects is complicated by the interaction of photons with the material in the intervening medium. In this thesis I have explored the use of 3D Monte Carlo radiative transfer codes to investigate a variety of astronomical objects. A model has been created to calculate the transfer function for a simple model of the X-ray irradiated accretion disk around a massive black hole. I have reconciled the observationally derived accretion disk transfer function with a simple geometric model for the structure of the accretion disk in the active galactic nuclei Zw229-15. The results suggest that a change in the amount of flaring in the disk at ≈ 600AU, possibly due to the emergence of a disk wind, can explain the observations. By coupling a Monte Carlo photoionization code to a series of static snapshots of a numerical simulation of a star forming cloud I have been able to estimate the impact of photoionizing feedback on the stellar masses. It is estimated that the stellar mass formed over the course of the simulation is reduced by up to 38% by the action of photoionization feedback. I also illustrate the possible problems associated with stochastic sampling of the stellar initial mass function in clusters, on the number of ionizing photons produced. I have utilized multi-wavelength data for three low surface brightness disk galaxies to show that their dusty interstellar medium has a scale height equal to that of the stellar disk. This is in contrast to the structure seen in higher mass disk galaxies and may play a role in their low star formation rates.

In this thesis we develop a mathematical model to describe the internal structure of an -accretion disc. The method is to consider the standard thin disc as a zero order approximation to a disc with vertical structure. The order of the approximation is controlled by the parameter 1/M2, where M is the Mach number of the azimuthal flow is a fiducial point. The theory is developed analytically as far as possible, using numerical solutions for the final system of ordinary differential equations only. The model expands upon the work of other authors by assuming a disc surface defined by the condition of pressure balance between the disc and its environment. Vertically transported angular momentum is extracted by coupling to these surroundings. In the absence of an external couple, the vertical transport of angular momentum is ignored, as in the standard thin disc. The internal structure and stability of the disc is investigated in both the gas and radiation pressure dominated regions, and the effects of including advection and the vertical transport of angular momentum is discussed. An application of the disc model is presented whereby external heating from X-rays associated with a radio jet are shown to induce mass loss from the disc surface. Such a configuration may undergo symmetry breaking to an asymmetric state in which one jet dominates. This is therefore a possible model for intrinsically one-sided radio sources.

This thesis presents a statistical study of the plasma sheet location and thickness of the plasma sheet boundary layer, including the effect of magnetic and solar wind conditions. Next are presented two case studies of in-situ particle data from the Polar spacecraft as it entered and passed through the near-Earth plasma sheet during substorm activity. These data sets are compared with in-situ magnetic field measurements, from the Magnetic Fields Experiment (MFE), and remote observations of the aurora from the Ultra-Violet Imager (UVI). The latter, in conjunction with ground magnetograms, allow us to place the in-situ plasma flows in the overall context of the substorm phases. In the first case study the spacecraft entered the plasma sheet boundary layer (PSBL) and observed field-aligned ion beams, which were connected with field-aligned signature in MFE. When Polar entered the PSBL its footprint mapped to the poleward edge of an auroral double oval. Then, a decrease in the number and energy of the ions occurred, which coincided with the spacecraft footprint entering the central low-luminosity region of the UV double oval. After the spacecraft entered the CPS an expansion phase onset occurred, and Polar observed large ion count rate increases and depolarisation of the magnetic field. When Polar entered the PSBL in the second case study, which again coincided with entry into the auroral oval, there was evidence of pseudobreakups. Then the PSBL retreated over the spacecraft, which was mirrored in the ionosphere by equatorward motion of the auroral oval. Polar entered to the PSBL and rapidly entered the CPS when a substorm expansion phase onset occurred. Due to the large amount of substorm activity during these case studies, emphasis is placed on the use of both remote observations of the aurora and ground-based data to place the in-situ measurements in context.

This thesis presents a study of heavily obscured AGN using data from the Ginga, ASCA and ROSAT X-ray observatories. Chapter 1 provides a brief introduction into the classification of AGN, the Seyfert Unification Theory, the X-ray properties of Seyfert galaxies and possible emission mechanisms that occur in these objects. This is followed by a short discussion of the performance characteristics of the instruments on-board the observatories mentioned above (chapter 2). Chapter 3 focuses on the X-ray properties of the well-known Seyfert 2 galaxy Mrk 3. The utilization of non-simultaneous Ginga, ASCA and ROSAT observations allows the broad-band (0.1- 30.0 keV) X-ray spectrum of this object to be investigated, including the slope and level of the hard continuum, the presence of Compton reflection, iron Kq, emission and the relatively complex soft X-ray spectrum which comprises an emission feature near 0.9 keV. Following on from this, non-simultaneous Ginga and ASCA observations are employed to analyse the mid-hard X-ray spectrum of a sample of five heavily obscured (with column density Njy > 1023 cm-2) Seyfert 2 galaxies (chapter 4). The main result of these investigations is evidence for large variations in the line-of-sight gas column density in three out of the five objects. The X-ray spectrum of the Seyfert 1.5 galaxy NGC 4151 is also considered in this chapter. Multiple ASCA observations of this source show that the X-ray continuum, in the 1-7 keV band, varies with time in a confusing manner that can only be explained as an effect of the complex absorption that is present in the spectrum. A sharp emission feature present in the X-ray spectrum of NGC 4151 at 0.9 keV is investigated in chapter 5. Using models produced by the photoionization code xstar, this feature is explained as recombination of free electrons directly into the ground state of fully ionized oxygen. In addition, evidence for a feature at 0.9 keV is found for Mrk 348, NGC 4507 and NGC 7582. Chapter 6 discusses the potential of future X-ray satellites such as Chandra, ASTRO-E and in more detail XMM to further the investigations carried out in this thesis.

In order to characterize the features of the Ionospheric Alfvén Resonator (IAR) at high latitudes in both northern and southern hemispheres, both data interpretation and numerical computation are presented. Four IAR events observed by the pulsation magnetometer at Sodankylä in a single month were statistically analysed. It was found that the IAR eigenfrequency separations fluctuate with time. The fluctuation was dominated by plasma density perturbation in the ionosphere. Also, a single IAR event observed by five pulsation magnetometers simultaneously was analysed. The analysis showed the eigenfrequencies of the single IAR detected at different locations are different. Additionally, the eigenfrequency shifts were found to differ at different locations. It indicates that the horizontal scale of a single IAR event could be up to thousands of kilometres. The horizontal structure of the IAR in a large scale is non-uniform. Also, the study has revealed that the visibility of the Spectrum Resonance Structure (SRS) strongly depends on the fluctuation rate of the eigenfrequency separations. Moreover, the first study of IARs in Antarctica was carried out. The IAR occurrence and the relation with the solar activities were investigated statistically. In addition, a numerical model was introduced in this thesis. Based on this model the boundary condition dependence of the IAR was investigated. According to the study the detected eigenfrequencies, the spatial structures of the field and the ratio between the intensities of the total current and the source current strongly depend on the ratio between the wave conductivity and the height integrated Pedersen conductivity in the E region. Also, the numerical study in this thesis has revealed that the eigenfrequency shifts respond to the different features of plasma density perturbations in different ways. The possibility of estimating the plasma density perturbation continuously from the IAR eigenfrequencies observed on the ground is illustrated.

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