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.