Investigating temporally variable magnetospheric dynamics at Saturn
Creators: name: Agiwal, Omakshi
Publisher: name: Imperial College London
Year: 2021

description: In this thesis, we investigate the diverse range of processes that control the dynamics of Saturn's magnetosphere, using magnetic field observations from the Cassini mission. The magnetic environment around Saturn is highly dynamic, driven externally by the solar wind; and internally by various drivers, such as a mysterious phenomenon unique to Saturn known as planetary period oscillations which modulate Saturn’s magnetosphere every ~10.6 hours (the expected planetary rotation period), the planetary atmosphere (from both, high and low latitudes), and the rapid-rotation of the equatorial plasma supplied by one of Saturn’s moons Enceladus. The duality of internal and external modulation makes Saturn’s magnetosphere a unique structure in our solar system, as compared to the predominantly externally and internally driven magnetospheres of planets such as Earth and Jupiter, respectively. The diversity of dynamics in Saturn's magnetosphere, coupled with its near perfect alignment of the spin/dipole axes, provides an ideal system in which we can investigate and compare these drivers of temporal variability. In our first study, we investigate the characteristics of planetary period oscillations (PPOs) and the effect of the solar wind in Saturn’s nightside equatorial magnetosphere, using magnetic field measurements from the Cassini End of Mission and empirical models of these systems presented by Cowley et al., (2017) and Arridge et al., (2008). We empirically constrain key characteristics of these systems and the magnetospheric structure over the ~10 month End of Mission interval. Both drivers are found to be relatively in steady state on ~20 hour timescales, however this assumption breaks down on greater than week-long timescales. Intervals of greater solar wind forcing are found to be consistent with thicker current sheets in the magnetosphere, and vice versa. Extending our analysis to other intervals from the Cassini mission reveals seasonal variability in the strength of the PPO systems between northern and southern solstice. We additionally find that the PPOs may be capable of driving an energetic circulation process known as magnetic reconnection within Saturn's magnetosphere. Mass and energy circulation within the Saturnian system is thought to be dominated by the rapid rotation of the Enceladus plasma, with the solar wind only playing a minor role. In our second study, we investigate the likelihood and relative contribution of PPO driven magnetic reconnection towards global circulation using the Cowley et al., (2017) model. We predict that PPO driven reconnection is on-average likely to occur once every 6 PPO cycles (1 PPO cycle ~10.6 hours). While active, this phenomenon may drive the magnetosphere with comparable strength to the Enceladus plasma; however, due to its relatively infrequent occurrence, the contributions of this phenomenon towards magnetospheric circulation are more comparable to solar wind driving of Saturn’s magnetosphere on year-long timescales. Despite this, our analysis shows that this process may be responsible for removing up to ~50% of the mass added to Saturn’s magnetosphere by Enceladus on year-long timescales. Further investigations are needed to better constrain these mass-loss rates. In our final study, we look closer to the planet with the final set of Cassini orbits, known as the Grand Finale. During this time the spacecraft traversed the previously unexplored region between Saturn and its equatorial rings. The azimuthal magnetic field observations reveal the presence of temporally variable, low-latitude field-aligned currents which are thought to be driven by velocity shears in the thermospheric neutral zonal winds at magnetically conjugate latitudes. By process of elimination, we consider atmospheric waves as the most plausible driver of temporal variability in the zonal winds in the low-latitude thermosphere, and empirically constrain the region in which they perturb the atmosphere to be between ±25° latitude. We determine the maximum temporal variability in the peak neutral zonal winds over the Grand Finale interval to be ~350 m/s assuming steady-state ionospheric Pedersen conductances. We show that the ionospheric currents measured must be in steady-state on ~10 minute timescales, and axisymmetric over ~2 hours of local time in the near-equatorial ionosphere. The studies presented in this thesis show clearly how magnetic field measurements can be used in conjunction with empirical models to constrain key characteristics of perturbation systems affecting planetary magnetospheres, as well as characteristics of the magnetospheric structure itself. Our work clearly shows that there is still much more information to be gleaned from the Cassini dataset despite the mission having ended. Although, we find that the lack of longitudinal coverage during the mission leaves certain gaps in our knowledge about the spatial asymmetries in Saturn’s magnetosphere, and the systems that drive it, that will have to be bridged by global simulations of Saturn's magnetosphere in the foreseeable future.