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The magnetospheric cusp is a funnel-shaped region where shocked solar wind plasma is able to enter the high latitude magnetosphere via the process of magnetic reconnection. The first in-situ analysis of the cusp region at Saturn is presented using data from the Cassini spacecraft, primarily from particle instruments and magnetic field observations. The analysis is presented in three sections. Firstly, two high-latitude spacecraft crossings are confirmed to be cusp observations by: (i) comparing the observed plasma with that of the magnetosheath and the adjacent magnetosphere, (ii) investigating the direction of the observed ions and (iii) analysing the composition of the plasma. The ion observations are shown to be a result of `bursty' reconnection occurring at the dayside magnetopause. The field-aligned distances to the reconnection site are calculated from the observed energy-pitch angle dispersions. Secondly, all the observations of the cusp are presented for the high latitude spacecraft orbits between 2007 and 2013. A comparison of the observations is made as well as classification into groups due to varying characteristics. The locations of the reconnection site are calculated and compared to the literature. The events are also compared to solar wind propagation models to investigate any correlations. Finally, the magnetic field observations of the cusps are analysed, focusing on the diamagnetic depressions. This characteristic is not observed in all of the cusps, and the possible explanations for this are discussed. The data are subtracted from a magnetic field model, and the calculated magnetic pressure deficits are compared to the particle pressures. A high plasma pressure layer in the magnetosphere adjacent to the cusp is discovered to also depress the magnetic field. A summary of the consequences of this work is discussed specifically regarding the ongoing debate of the role of reconnection at Saturn and its influence on magnetospheric dynamics.

This thesis is concerned with the study of magnetic field perturbations associated with large scale electrical current systems that flow along magnetic field lines. We will consider three such current systems within Saturn’s magnetosphere. The first is associated with the subcorotation of plasma in Saturn’s outer magnetosphere. The remaining two systems are directly associated with the northern and southern near-planetary period oscillations (PPOs) observed in Saturn’s magnetic field, as well in other magnetospheric phenomena such as the Saturn kilometric radio emission and related auroral emissions. Presented within this thesis are three detailed studies of the above current systems in Saturn’s northern and southern nightside hemispheres during the 2008 interval of high-latitude orbits of the Cassini spacecraft. The first of these studies is a statistical analysis of the southern hemisphere field-aligned currents, which reveals for the first time their form, magnitude and position is modulated by the phase of the southern PPO system. By exploiting the symmetry properties for the two main field-aligned current systems, they are approximately separated. Due to the smoothing effects of the statistical study the second study focuses on the structure of the southern hemisphere field-aligned current sheets, in terms of their strength and position. The positional modulation is quantified, and the phasing indicates the origin of the southern PPO system is flows in the polar atmosphere. Finally, the third study concerns the northern hemisphere field-aligned current signatures, this reveals that these are modulated in form and magnitude by not only by the northern PPO system, but also the southern PPO system. This provides the first direct evidence of the proposed interhemispheric current flow. Together, these studies provide new insights into the behaviour of the field-aligned currents and can place constrains on the theoretical discussions of the origin and driving of the Saturn’s aurora and periodicities.

The H3+ infrared aurorae of Saturn were analysed using the Cassini Visual and Infrared Mapping Spectrometer (VIMS) to produce two studies of the emission above the limb. The first, following Stallard et al. (2012a), was a case study of the peak altitude of the southern auroral emission above the limb. The results showed that the measured peak emission altitude is dominated by the alignment of the auroral curtain with the limb, and that accounting for this alignment issue results in a peak intensity altitude of 1215 ± 119 km above the 1 bar level. In the second study, a new projection technique mapped the H3+ limb emissions with respect to latitude, altitude and local time. This technique enabled a statistical analysis of the average latitude-altitude structure of the auroral intensity, temperature and density using 511 observations from the years 2005-2010. For the northern auroral oval, the peak emission altitude was found to be 1333 ± 152 km above the 1 bar level, the average temperature for the peak emission layer was 549 −12+34 K and the nadir column density was 1.6−0.9+3.6 × 1015 m-2. Likewise for the southern auroral oval, the peak emission altitude was 1225 ± 193 km, the average temperature was 585 −29+6 K and the nadir column density was 6.2−0.3+0.4 × 1015 m-2. The peak emission latitude was found to be 74º ± 1º for both hemispheres, though the southern auroral emission was found to have a wider latitudinal distribution than the north. The north-south asymmetry in the magnetic field strength was used to explain the hemispheric differences in these properties. Pressure scale height analysis of H3+ revealed that in both hemispheres, the H3+ altitudinal distribution is controlled by its production and loss mechanisms and not gravity.

This thesis is concerned with the structure and dynamics of the polar cap boundary between open and closed field lines in Saturn’s high-latitude magnetosphere, employing data from the Cassini spacecraft to explore methods of identifying such a boundary. The first systematic investigation of the polar cap boundary is presented through a Cassini multi-instrument assessment of observations in various in situ datasets for all the high-latitude boundary region crossings between 2006 and 2009. The average level of coincidence of the polar cap boundary identified in the various in situ data sets is found to be 0.34° ± 0.05° colatitude. The average location of the boundary in the southern (northern) hemisphere is found to be at 15.6° (13.3°) colatitude. The average colatitude of the polar cap boundary during the 2013 high-latitude Cassini orbits was found to be 15.8° (15.1°) in the southern (northern) hemisphere. The associated dayside PCB in the northern hemisphere (NH) is found to be 10.45°. The position of the southern hemisphere (SH) PCB is suggested to be somewhat ordered by the southern planetary-period oscillation (PPO) phase; however, it cannot account for the boundary’s full latitudinal variability. No clear evidence is found of any ordering of the northern PCB location with the northern PPO phase. Solar wind influence on the PCB is investigated; however, a consistent variation is not seen. Finally, FFT wave analysis of Cassini magnetometer data is shown to provide a new method of observing Saturn’s PCB between open and closed magnetic field lines.

Shock waves exist throughout the universe and are fundamental to understanding the nature of collisionless plasmas. The complex coupling between charged particles and electromagnetic fields in plasmas give rise to a whole host of mechanisms for dissipation and heating across shock waves, particularly at high Mach numbers. While ongoing studies have investigated these process extensively both theoretically and via simulations, their observations remain few and far between. This thesis presents a study of very high Mach number shocks in a parameter space that has been poorly explored and identifies reformation using in situ magnetic field observations from the Cassini spacecraft at Saturn's bow shock. This gives an insight into quasi-perpendicular shocks across two orders of magnitude in Alfvén Mach number (MA) and spanning Earth-like to Astrophysical-like regimes. The work here shows evidence for cyclic reformation controlled by specular ion reflection occurring at the predicted timescale of ~0.3 τc, where τc is the ion gyroperiod. The relationship between these reformation signatures, magnetic overshoot and variability are also presented. The final part of this thesis characterises the region downstream of Saturn's bow shock, the magnetosheath. The results show a comprehensive overview of the configuration of the magnetic field in a non-axisymmetric magnetosheath. This non-axisymmetry is revealed to have an impact in the rotation of the magnetic field and is significant enough to influence the magnetic shear at the magnetopause.

The work in this thesis utilises data from the Cassini spacecraft in the analysis of the Saturnian system. Data from the electron spectrometer (ELS), the ion spectrometer (IMS) and the magnetometer (MAG) gathered during the first two years of the mission have been used in two main areas of research. The first is the investigation of the magnetopause, which forms the boundary between the region of space dominated by the planetary magnetic field and currents, and the interplanetary magnetic field. The second concerns the high energy electrons fluxes in the inner magneto sphere, and the effects caused when the icy moons carve out cavities in these populations. We find evidence strongly suggestive of reconnection events as observed at Earth at two separate encounters with the magnetopause. The energisation of the plasma, together with the open field configuration provides us with an estimate for the reconnection voltage at Saturn. It is found to, at times, be comparable to the corotation electric field, which is assumed to dominate the dynamics of the system. The large database of 200 crossings of the magnetopause is also used to derive general characteristics of the electron behaviour at this boundary. We find that boundary layer plasma is often present inside the magnetopause of Saturn, although the evidence suggests that internal plasma may play a role in formation of this layer. The ELS is able to utilise the penetration of high energy electrons directly into the detector to study the 'microsignatures' of the icy moons in the inner magnetosphere. The absorption of the high energy electrons leaves a cavity in the plasma. The subsequent re-filling over time by the surrounding plasma has allowed us to derive diffusion rates at Tethys, Enceladus and Mimas. In general, outward diffusion is faster than inward diffusion, although this varies at Enceladus.

As our abilities to utilise high performance computing to theoretically probe many astrophysical systems increases, a genuine need to relate to real systems becomes ever more important. Here, Saturn s rings can be used as a nearby laboratory to investigate in real time many astrophysical processes. One such system is the narrow F ring and its interaction with its inner shepherd moon Prometheus. Through numerical modelling and direct observations of the in-situ spacecraft Cassini we find new and exciting dynamics. These might help explain some of the asymmetries witnessed in the distribution of embedded moonlets and azimuthal ring brightness known to exist within the F ring. Spatially we find asymmetry in the Prometheus induced channel edges with regards to density, velocity and acceleration variations of ring particles. Channel edges that show fans (embedded moonlets) are also the locations of highly localised increases in densities, velocity and acceleration changes where opposing edges are considerably less localised in their distribution. As a result of the highly localised nature of the velocity and acceleration changes chaotic fluctuations in density were witnessed. However, this could seek to work in favour of creating coherent objects at this channel edge as density increases were significantly large. Thus, density here had a greater chance of being enhanced beyond the local Roche density. Accompanied with these dynamics was the discovery of a non-zero component to vorticity in the perturbed area of the F ring post encounter. By removal of the background Keplerian flow we find that encounters typically created a large scale rotation of ~10,000 km^2. Within this area a much more rich distribution of local rotations is also seen located in and around the channel edges. Although the real F ring and our models are non-hydrodynamical in nature the existence of a curl in the velocity vector field in the perturbed region could offer some interesting implications for those systems that are gas rich.