Supervisors' Foreword; Preface; Acknowledgements; Contents; 1 Introduction; 1.1 Overview; 1.2 Host Stars; 1.2.1 Solar Magnetic Activity; 1.2.2 Flares; 1.2.3 Sun in Time; 1.2.4 Stellar Variability; 1.3 Planetary Upper Atmospheres; 1.3.1 Solar System Giant Planet Ionospheres: The Case of Saturn; 1.3.2 Ionospheres in Exoplanetary Systems; References; 2 Solar and Stellar X-Ray and UV Radiation; 2.1 Instrumentation for Solar and Stellar XUV Observations; 2.1.1 TIMED/SEE; 2.1.2 SORCE/XPS; 2.1.3 SDO/EVE; 2.2 Evolution of Solar Irradiance Over Different Timescales; 2.2.1 Methods

2.2.2 Solar Cycle Variability2.2.3 Rotational Variability; 2.2.4 Flares; 2.2.5 XUV Flux Bands; 2.3 Stellar XUV Irradiance; 2.3.1 Absorption in the Inter-stellar Medium (ISM); 2.3.2 Coronal Model; 2.3.3 Testing the Synthetic Spectra; 2.3.4 Scaling of the Solar Spectrum; References; 3 Thermosphere; 3.1 Thermospheric Model; 3.1.1 Fluid Equations; 3.1.2 Lower Boundary; 3.1.3 Upper Boundary; 3.1.4 Photochemistry; 3.1.5 Model Runs; 3.2 Planets Orbiting Solar-Type Stars; 3.3 Planets Orbiting Other Low-Mass Stars; 3.3.1 Escape Regimes; 3.3.2 Validation of Synthetic Spectra

3.3.3 Highly Enhanced Stellar X-Ray Irradiance3.4 Using Scaled Solar Spectra to Estimate Stellar XUV Emissions; 3.5 Mass Loss Rates; 3.6 Conclusion; References; 4 Ionosphere; 4.1 Ionospheric Model; 4.1.1 Ionisation Mechanisms and Theory; 4.1.2 Fluid Equations; 4.1.3 Chemical Reactions; 4.1.4 Photo-Absorption and Photo-Ionisation Cross-Sections; 4.1.5 Boundary Conditions; 4.1.6 Model Validation for the Case of Saturn; 4.2 High Resolution H2 Photo-Absorption Cross-Section; 4.3 Ionisation in Planetary Atmospheres; 4.3.1 Planets Orbiting at 1 AU from Solar-Type Stars

4.3.2 Planets Orbiting Other Low-Mass Stars at 1 AU4.3.3 Using Scaled Solar Spectra; 4.3.4 Variation with Orbital Distance; 4.3.5 Flares; 4.4 Conclusion; References; 5Conclusions and FutureWork; References; Appendix A Neutral Densities; B Ion Densities from the Thermospheric Model; Index