The XMM-Newton observatory provides unrivalled capabilities for detecting low surface brightness emission features from extended and diffuse sources, by virtue of its large fieldof- view (15 arcminutes in radius) and high effective area (2150 cm2 at 1 keV). XMM-Newton observes X-ray emission along its entire line-of-sight, whether that be the intended, generally distant astronomical target, or from much closer, for example within the Solar System. The main motivation of this thesis was to characterise one source of locally produced, diffuse Xray emission; that of solar wind charge exchange (SWCX) interactions between solar wind ions and neutral atoms in the Earth’s exosphere. Whilst SWCX is a source of background for astrophysicists concerned with studies of Galactic and extragalactic emission, it provides a diagnostic of the charge-state distribution of the solar wind and mass transport around the Earth’s magnetosheath. This thesis describes an archival study of XMM-Newton observations to identify those affected by temporallyvariable SWCX emission. 3.4% of 3012 XMM-Newton observations studied unambiguously contain a variable exospheric SWCX signal; they are preferentially detected around the subsolar point of the Earth’s magnetosheath. This thesis contains a detailed investigation into the temporal and spectral characteristics of the SWCX-affected observations. It also contains a study of one particular observation, whose emission likely resulted from Coronal Mass Ejection plasma moving through the vicinity of the Earth. A model of exospheric SWCX is presented to provide some predictive power, using the orbital and target-pointing parameters of XMM-Newton during a particular observation. The model is in reasonable agreement with the observed fluxes for approximately 60% of cases. Finally, an idea for a future wide-field X-ray imager with an accompanying plasma monitor and magnetometer is presented. This would observe plasma dynamics in the Earth’s magnetosheath via the mechanism of SWCX emission occurring in this region.

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The Epoch of Reionization is one of the most active areas of research in astronomy because of its importance in the history of the Universe. Numerous studies are currently probing the redshifts at which we believe this reionization to have occurred (z =6 - 15) in search of the first galaxies, which / produced. the ionizing radiation responsible for the transition from a neutral intergalactic medium. These sources are believed to have strong emission in the Lyman alpha line. In this thesis, work carried out on a survey making use of gravitational lensing to search for Lyman alpha emitting galaxies between z =7 and 11 is presented. Gravitational lensing is a very powerful tool in the search for extremely high redshift sources since it allows observers to detect sources too faint to be detected by conventional unlensed means. Archival data from the ISAAC facility on the Very Large Telescope has been rigorously reduced and analysed using the principle of matched filters. Unfortunately no extremely high redshift candidates have been detected but three low redshift (z =1.68, 1.41 and 1.16) galaxy candidates have been observed. Mapping the magnification afforded by gravitational lensing has allowed the estimation of possible luminosity functions, which have been compared against the work of contemporary Lyman alpha surveys. This thesis has a detailed description of all the data obtained, the reduction technique used, the matched filter detection algorithm and analysis into the possible luminosity function with special interest paid to the advantages afford by gravitational lensing.