Constraining the Temporal Variability of Neutral Winds in Saturn's Low‐Latitude Ionosphere Using Magnetic Field Measurements

The Cassini spacecraft completed 22 orbits around Saturn known as the “Grand Finale” over a 5 months interval, during which time the spacecraft traversed the previously unexplored region between Saturn and its equatorial rings near periapsis. The 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 neutral zonal winds at magnetically conjugate thermospheric latitudes. We consider atmospheric waves as a plausible driver of temporal variability in the low‐latitude thermosphere, and empirically constrain the region in which they perturb the zonal flows to be between ±25° latitude. By investigating an extensive range of hypothetical wind profiles, we present and analyze a timeseries of the modeled velocity shears in thermospheric zonal flows, with direct comparisons to empirically inferred angular velocity shears from the Bϕ observations. 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 further show that the ionospheric currents measured must be in steady‐state on ∼10 min timescales, and axisymmetric over ∼2 h of local time in the near‐equatorial ionosphere. Our study illustrates the potential to use of magnetospheric datasets to constrain atmospheric variability in the thermosphere region

Investigating temporally variable magnetospheric dynamics at Saturn

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.Open Acces

Current Events at Saturn: Ring–Planet Electromagnetic Coupling

This study presents a synthesized analysis of in situ and ground-based observations to investigate electromagnetic coupling between Saturn and its rings. During the Cassini Grand Finale, the magnetometer detected gradients in the azimuthal magnetic field B _ϕ connected to Saturn’s B-ring on 17 out of 21 orbits. The B _ϕ gradients indicate that field-aligned currents are flowing into Saturn’s B-ring at ∼1.55–1.67 R _S in the ring plane, preferentially in the southern hemisphere. On average, these currents are magnetically conjugate with ground-based observations of nonsolar enhancements in ${{\rm{H}}}_{3}^{+}$ emissions from Saturn’s ionosphere and detected contemporaneously with ring-sourced, planetward electron beams and field-aligned charged dust grain inflow from the C- and B-rings into Saturn’s atmosphere. Collectively, these observations align with Voyager-era predictions of a phenomenon known as “ring rain,” where charged ring material generated inward of a nominal “critical radius” is drawn into Saturn’s upper atmosphere along the magnetic field. However, we show that the B-ring currents are not likely to be a direct signature of infalling field-aligned ring grains. Instead, we propose that the ring rain generation mechanism naturally results in a sharp gradient in the ionospheric Pedersen conductance at the ∼1.57–1.67 R _S boundary, which, combined with a v × B electric field in the ring ionosphere, could drive the observed B-ring currents. The Pedersen conductance in the high-conductance region of the southern ring ionosphere is constrained to ∼0.07–2 S and is observed to vary within this range on week-long timescales

UK magnetosphere, ionosphere and solar-terrestrial (MIST) awards taskforce: a perspective

“We don’t live in a meritocracy, and to pretend that simple hard work will elevate all to success is an exercise in willful ignorance.” (Reni Eddo-Lodge wrote in her book “Why I’m no longer talking to white people about race” (Published by Bloomsbury, London, p. 79, ISBN: PB: 978-1-4088-7)). This echoes through the academic scientific community, and can be readily seen in the demographics of physics prize winners. Prizes are extremely influential in both projecting how a community is outwardly perceived and actively shaping the community through facilitating career advancement. But how can biases in the awards process be addressed? We do not pretend to have all the answers, nor is there a single solution, but in this perspective article we explore one pragmatic approach to tackling chronic underrepresentation in the space sciences when it comes to nominations for awards and prizes

Constraining the Temporal Variability of Neutral Winds in Saturn's Low-Latitude Ionosphere Using Magnetic Field Measurements

The Cassini spacecraft completed 22 orbits around Saturn known as the “Grand Finale” over a 5 months interval, during which time the spacecraft traversed the previously unexplored region between Saturn and its equatorial rings near periapsis. The 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 neutral zonal winds at magnetically conjugate thermospheric latitudes. We consider atmospheric waves as a plausible driver of temporal variability in the low-latitude thermosphere, and empirically constrain the region in which they perturb the zonal flows to be between ±25° latitude. By investigating an extensive range of hypothetical wind profiles, we present and analyze a timeseries of the modeled velocity shears in thermospheric zonal flows, with direct comparisons to empirically inferred angular velocity shears from the Bϕ observations. 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 further show that the ionospheric currents measured must be in steady-state on ∼10 min timescales, and axisymmetric over ∼2 h of local time in the near-equatorial ionosphere. Our study illustrates the potential to use of magnetospheric datasets to constrain atmospheric variability in the thermosphere region

Science return of probing magnetospheric systems of ice giants

The magnetospheric systems of ice giants, as the ideal and the unique template of a typical class of exoplanets, have not been sufficiently studied in the past decade. The complexity of these asymmetric and extremely dynamic magnetospheres provides us a great chance to systematically investigate the general mechanism of driving the magnetospheres of such common exoplanets in the Universe, and the key factors of influencing the global and local magnetospheric structures of this type of planets. In this paper, we discuss the science return of probing magnetospheric systems of ice giants for the future missions, throughout different magnetospheric regions, across from the interaction with upstream solar wind to the downstream region of the magnetotail. We emphasize the importance of detecting the magnetospheric systems of ice giants in the next decades, which enables us to deeply understand the space enviroNMent and habitability of not only the ice giants themselves but also the analogous exoplanets which are widely distributed in the Universe

Ice Giants — The Return of the Rings

International audiencePlanetary rings are a multifaceted player in the system. Recent advances about Saturn’s rings argue that ring science at the ice giants, with magnetospheric and atmospheric sciences, are essential in advancing our knowledge about solar system evolution, the evolution of the moons and Ocean Worlds, and phenomena observed in the ice giant systems

Ice Giants – The Return of the RingsA Science White Paper for the Planetary Science and Astrobiology Decadal Survey 2023-2032

A Science White Paper for the Planetary Science and Astrobiology Decadal Survey 2023-2032Here we highlight recent advances in our knowledge about Saturn's ring system and bring forward the outstanding science issues that could be addressed by studying the ring systems of the ice giants. We focus on interactions between planetary rings and other elements in the system, including the moons, host planet, and its magnetosphere, and conclude that ring science investigations, in accordance with magnetospheric and atmospheric science disciplines, are essential in advancing our knowledge of solar system evolution, the origin and evolution of the moons and Ocean Worlds, as well as contemporary phenomena observed in the ice giant systems. We request that the study of ice giant ring systems to be considered a top priority for all future ice giant explorations