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This thesis is a theoretical study of the magnetosphere-ionosphere coupling current system in Jupiter's middle magnetosphere associated with the breakdown of corotation of iogenic plasma and the jovian main auroral oval. The study initially investigates the effects of the ionospheric Pedersen conductivity and the iogenic plasma mass outflow rate. Wide ranges of values of these parameters are inputted to the model and the results are compared for dipole and current sheet field models. It is shown that previous results, obtained using 'reasonable' spot values, are generally valid over wide ranges of the parameters. The study then investigates the effect of precipitation-induced enhancements of the Pedersen conductivity. Previous models have assumed constant conductivity, whereas it is expected to be significantly elevated by strong field-aligned currents. A model of the dependence of the Pedersen conductivity on the field-aligned current is developed and incorporated into the model. The findings help reconcile the theoretical results with observation. Specifically, the plasma is maintained closer to rigid corotation out to much further distances than theory previously predicted, the equatorial radial current exhibits a sharp rise in the inner region of the middle magnetosphere and plateaus off thereafter, in line with Galileo magnetic field data, and the field-aligned current is concentrated in a peak in the inner region, which is to where the main oval is usually mapped. Finally, the study investigates the effect of self-consistently including magnetosphere-ionosphere decoupling due to field-aligned voltages. The results show that for typical jovian conditions the effect is very small, such that the modification to the system parameters is generally two orders of magnitude below the values of the parameters themselves. The conclusion is that the assumption made in previous work, that the effect of the field-aligned voltages is small, is generally valid.

The studies contained within this thesis focus on the large-scale azimuthal and radial current systems of Jupiter's middle magnetosphere, i.e. currents with radial ranges of 20-50 RJ. In the first study using magnetometer data from Pioneer-10 and -11, Voyager-1 and -2, and Ulysses, it is discovered that the azimuthal current in the middle magnetosphere is not axi-symmetric as had been assumed for the last twenty-five years, but that it is stronger on the nightside than on the dayside at a given radial distance. A simple empirical model is formulated, which reasonably describes the data in the domain of interest both in radial distance and local time, and allows direct calculation of the current divergence associated with the asymmetry. In a similar way, in the following chapter the radial currents have been computed for the dawn sector of the jovian magnetosphere along various fly-by trajectories. Combination of these radial current estimations with the azimuthal current model allows the total divergence of the equatorial current to be calculated. These current densities mapped to the ionosphere are surprisingly large at ~1A m-2. In order to carry the current, the magnetosphere electrons must be strongly accelerated along the field lines into the ionosphere by voltages of the order of 100 kV. The resulting energy flux is enough to produce deep, bright (Mega Rayleigh) aurora and thus provides the first natural explanation of the main jovian auroral oval. In the final study, newly-available data from the Galileo orbiter mission are combined with the fly-by data in order to compare them to the model derived in the first study. The model is then re-derived for the entire data set, which significantly improves the associated fractional errors.

To begin understanding how the architecture of hot Jupiter planetary systems can be so radically different from that of our own solar system, requires the dynamical evolution of planets to be known. By measuring the sky-projected obliquity � of a system it is possible to determine the dominant process in the dynamical evolution. If a transiting exoplanet that crosses the disc of its host star passes over a starspot, then the amount of received intensity from the star will change. By modelling the position of the anomaly in the lightcurve it is possible to precisely determine the position of the starspot on the stellar disc. If the position of the starspot can be found at two distinct times using two closely spaced transits, then it is possible to measure. Before now there was no definitive model capable of accurately modelling both a planetary transit and a starspot. This research focuses on the development of prism which is capable of accurately modelling a transit containing a starspot anomaly. Due to the nature of the parameter space a new optimisation algorithm was developed, gemc, which is a hybrid between a genetic algorithm and MCMC.

Dynamics of baroclinic jets on extrasolar planets is studied using three-dimensional general circulation models (GCMs) which solve the traditional hydrostatic primitive equations. The focus is on: i ) baroclinic ow and instability on hot-Jupiters; ii ) detailed GCM intercomparison in a commonly used extrasolar planet setup; and, iii ) equatorial superrotation on Earth-like planets. Stability, non-linear evolution and equilibration of high-speed ageostrophic jets are studied under adiabatic condition relevant to hot-Jupiters. It is found that zonal jets can be baroclinically unstable, despite the planetary size of the Rossby deformation scale, and that high resolution is necessary to capture the process. Non-linear jet evolution is then used as a test case to assess model convergence in ve GCMs used in current hot-Jupiter simulations. The GCMs are also tested under a diabatic condition (thermal relaxation on a short timescale) similar to that used in many hot-Jupiter studies. In the latter case, in particular, the models show signi cant inter- and intra-model variability, limiting their quantitative prediction capability. Some models severely violate global angular momentum conservation. The generation of equatorial superrotation in Earth-like atmospheres, subject to \Held & Suarez-like" zonally-symmetric thermal forcing is also studied. It is shown that transition to superrotation occurs when the meridional gradient of the equilibrium surface entropy is weak in this setup. Two factors contribute to the onset of superrotation | suppression of breaking Rossby waves (generated by midlatitude baroclinic instability) that decelerate the equatorial ow, and, generation of inertial and barotropic instabilities in the equatorial region that provide the stirring to accelerate the equatorial ow. In summary, forcing condition and physical setup used in current hot-Jupiter simulations severely stretch model performance and predictive capability. Superrotation in Earth-like conditions may be common, but its strength decreases with resolution. Broadly, numerical convergence must be assessed in GCM experiments for each problem or setup considered.

To date more than 1500 exoplanets have been discovered. A large number of these are hot Jupiters, Jupiter-sized planets orbiting < 0.1 au from their parent stars, due to limitations in observational techniques making them easier to detect than smaller planets in wider orbits. This is also, for the same reasons, the class of exoplanets with the most observational constraints. Due to the very large interaction between these planets and their parent stars they are believed to be tidally locked, causing a large temperature contrast between the permanently hot day side and colder night side. There are still many open questions about these planets. Many are observed to have inflated radii, i.e. the observed radius is larger for a given mass than evolutionary models predict. A mechanism that can transport some of the stellar heating into the interior of the planet may be able to explain this. The presence of hazes or clouds has been inferred on some planets, but their composition and distribution remain unknown. According to chemical equilibrium models TiO and VO should be present on the day side of the hottest of these planets, but these molecules have not yet been detected. Cold traps, where these molecules condense out on the night side, have been suggested to explain this. The efficiency of the heat redistribution from the day side to the night side has been found to vary significantly between different planets; the mechanism behind this is still unknown. To begin to answer many of these questions we need models capturing the three-dimensional nature of the atmospheres of these planets. General circulation models (GCMs) do this by solving the equations of fluid dynamics for the atmosphere coupled to a radiative transfer scheme. GCMs have previously been applied to several exoplanets, but many solve simplified fluid equations (shallow water or primitive equations) or highly parametrised radiation schemes (temperature-forcing, gray or band-averaged opacities). We here present an adaptation of the Met Office Unified Model (UM), a GCM used for weather predictions and climate studies for the Earth, to hot Jupiters. The UM solves the full 3D Euler equations for the fluid, and the radiation scheme uses the two-stream approximation and correlated-k method, which are state of the art for both Earth and exoplanet GCMs. This makes it ideally suited for the study of hot Jupiters. An important part of this work is devoted to the adaptation of the radiation scheme of the UM to hot Jupiters. This includes calculation of opacities for the main absorbers in these atmospheres from state-of-the-art high temperature line lists, the calculation of k-coefficients from these opacities, and making sure all aspects of the scheme perform satisfactorily at high temperatures and pressures. We have tested approximations made in previous works such as the two-stream approximation, use of band-averaged opacities and different treatments of gaseous overlap. Uncertainties in current models, such as the lack of high temperature line broadening parameters for these atmospheres, are discussed. We couple the adapted radiation scheme to the UM dynamical core, which has been tested independently. Our first application is devoted to one of the most well-observed hot Jupiters, HD 209458b. Differences between previous modelling works and our model are discussed, and we compare results from the full coupled model with results obtained using a temperature-forcing scheme. We have also developed a tool to calculate synthetic phase curves, and emission and transmission spectra from the output of our 3D model. This enables us to directly compare our model results to observations and test the effect of various parameters and model choices on observable quantities.

On December 30th 2000, the Cassini-Huygens spacecraft reached the perijove milestone in its continuing journey to the Saturnian system. During an extended six-month encounter, the Composite Infrared Spectrometer (CIRS) returned spectra of the Jovian atmosphere, rings and satellites from 10 to 1400 cm^-1 (1000 to 7 µm) at a programmable spectral resolution of 0.5 to 15 cm^-1. The improved spectral resolution of CIRS over previous infrared instrument-missions to Jupiter, the extended spectral range and higher signal-to-noise performance provide significant advantages over previous data-sets. Both optimal-estimation retrieval and radiance-differencing are used to investigate the global variation of upper-tropospheric temperature, ammonia, phosphine and cloud opacity between ± 60˚ latitude. The analysis methods are shown to successfully reproduce Jovian conditions with results consistent with previous investigations. The composition results in particular are well characterised and suggest an important role played by mixing and transport within the upper-troposphere. Interpretation and validation of the retrieved results is conducted via the construction of a simple dynamic model incorporating transport, diffusion and (photo)chemistry.