Christopher Stephen Arridge

The structure of Jupiter's magnetosphere reflects the combined effects of a strong intrinsic field and an extended hinged current sheet whose morphology varies with solar wind forcing. We present a study of Jupiter's magnetodisc that combines the UCL‐AGA magnetodisc code with Khurana's generalized, hinged current sheet models (1992/2022) to fit Juno MAG data from orbits PJ6–PJ12 and PJ34 using a “rolling‐window” approach. Periods of relative stability (“quiescent” magnetosphere) are identified during each pass, reducing the impact of transient solar‐wind and reconfiguration events. The underlying magnetospheric parameters are then determined and current sheet morphology constrained. Estimated magnetopause distances are also obtained and compared to the results obtained by Joy et al. (2002, https://doi.org/10.1029/2001ja009146 ), showing most probable subsolar standoff distances between 46 and 64 and results consistent with dawn‐dusk asymmetry and/or a cushion region along the dusk flank. This approach reveals substantial per‐perijove magnetospheric and current sheet geometry variability, as well as capturing changes in hot‐plasma content and effective magnetodisc radius within a single pass. Correlations between model parameters link extended magnetospheres to larger magnetic field sweepback onset distances, with weaker negative trends linking magnetospheric compression with the Alfvén wave propagation speed and hot plasma content with the distance at which hinging becomes significant. Finally, both MDISC + Khurana model variants are shown to match, and often outperform, the optimized Connerney (2020) baseline current disc model across most perijoves analyzed.

We present an application of the latest UCL‐AGA magnetodisc model (MDISC) to the study of the magnetic and plasma conditions in the near‐Ganymede space. By doing this, we provide a comparison with measurements from Juno's most recent flyby of the Jovian moon, perijove 34 (PJ34). We find good agreement between the model results and the magnetometer data, pointing toward a hot plasma index value (2.7190.024)10⁷ Pa m T⁻¹ and an effective magnetodisc radius (79.51.1) Jupiter radii for the Jovian magnetosphere, for the duration of the trajectory, suggesting a configuration with middling levels of expansion. We also predict the plasma conditions observed by Juno during the same flight‐path, as well as the typical conditions over the orbit of Ganymede, with the magnetic and hot plasma pressures assuming dominant roles. Finally, these results are compared with functional fits of a compilation of Galileo flyby data obtained in the vicinity of Ganymede's orbit, suggesting Juno experienced somewhat similar conditions, despite a systematic overestimation in magnetic field intensity in the near‐Ganymede space.

Magnetic reconnection and current disruption are two key processes in driving energy conversion and dissipation in planetary magnetospheres. At the Earth, the two processes usually occur at different locations: the current disruption process occurs more frequently in the near-Earth magnetotail ∼10 R _E, while the magnetotail reconnection process is expected to take place in the more distant region where the current sheet is thinner. Occasionally, under very intense solar wind perturbations, reconnection could be located closer to the Earth where current disruption processes usually operate. But it is unclear what the situation is at giant planets, in which the plasma environment is very different from the Earth. In this study, we investigate a middle-Jupiter reconnection event at ∼43 R _J. During the event, the inferred integrated cross-field currents were substantially reduced, which we argue is a signature of current disruption. In this case, we suggest that magnetic reconnection could be colocated with a current disruption process in the Jovian magnetosphere, which is roughly analogous to the situation in the extremely perturbed Earth’s magnetosphere.

Since arriving at Jupiter, Juno has observed instances of field-aligned proton and electron beams, in both the upward and downward current regions. These field-aligned beams are identified by inverted-V structures in plasma data, which indicate the presence of potential structures aligned with the magnetic field. The direction, magnitude and location of these potential structures is important, as it affects the characteristics of any resultant field-aligned current. At high latitudes, Juno has observed potentials of 100’s of kV occurring in both directions. Charged particles that are accelerated into Jupiter’s atmosphere and precipitate can excite aurora; likewise, particles accelerated away from the planet can contribute to the population of the magnetosphere.Using a time-varying 1-D spatial, 2-D velocity space Vlasov code, we examine magnetic field lines which extend from Jupiter into the middle magnetosphere. By applying and varying a potential difference at the ionosphere, we can gain insight into the effect these have on the plasma population, the potential structure, and plasma densities along the field line. Utilising a non-uniform mesh, additional resolution is applied in regions where particle acceleration occurs, allowing the spatial and temporal evolution of the plasma to be examined. Here, we present new results from our model, constrained, and compared with recent Juno observations, and examining both the upward and downward current regions.

The Langmuir Probe (LP) onboard Cassini was one of the three experiments that could measure the cold inner magnetospheric plasma, along with the Radio and Plasma Waves Science (RPWS) and the Cassini Plasma Spectrometer (CAPS). While the century-old LP theory looks quite straight-forward, in reality things are much more complicated.The operation of the LP is quite simple: by applying positive bias voltages, the probe attracts the electrons and repels the ions of the surrounding plasma. From the resulting current-voltage curve characteristics of the ambient electrons can be estimated, i.e. density and temperature. When negative bias voltages are applied to the probe the characteristics of the ambient ions can be estimated, i.e. density, temperature, and mass.Though the LP operation and interpretation are quite simple and straightforward, there are assumptions made and therefore the theoretical models may not always reflect the actual plasma conditions in Saturn’s magnetosphere. For this study we are focused on the effect of the photoelectrons, i.e. electrons that are generated by the incident sunlight on Cassini’s surfaces, which are difficult to be observed and corrected for in a laboratory plasma.We developed a robust algorithm that identifies the transitions of the LP in and out of shadow caused by the Saturn and its rings. The LP data inside and outside the eclipses are compared using the algorithm developed. In this presentation we will discuss the impact of the photoelectron generation from the spacecraft surfaces to the LP current-voltage curves, and understand the variations of the measured plasma density connected with the photoelectrons.

Saturn's magnetosphere is an efficient emitter of Energetic Neutral Atoms (ENAs), given the presence of an extended neutral cloud around the planet that originates from the icy moon, Enceladus. The ENA emission is symptomatic of the global circulation of plasma in Saturn's magnetosphere. Energetic ions are injected from the outer magnetosphere following magnetotail dynamics and reconnection events. These ions then charge exchange with the neutral cloud, which is mostly confined to the spin plane, resulting in ENA production. The global ENA emission is dynamic, displaying sudden brightening on the nightside, and discrete rotating enhancements which circle the planet for many hours as energetic ions drift with the bulk plasma flow. These latter features have been linked with rotating signatures in the ultraviolet auroras, suggesting coupling via some transient system of field-aligned currents that forms following injection events. Indeed these injection events occur so often as to form Saturn’s dawn auroral arc.Our characterization of the ENA emission at Saturn is made possible using imagery from the Ion-Neutral Camera (INCA) that flew onboard Cassini. Observations were made over the entire mission lifetime. We present for the first time a statistical analysis of the complete INCA image set, using equatorial projections of the flux distribution to reveal the time-averaged morphology of Saturn's ENAs. We used a comprehensive data processing and equatorial projection algorithm to calibrate, clean and filter for all high inclination orbit days. In the final average pictures, many of the projected pixels consist of between tens to hundreds of days continuous exposure, all captured with a line-of-sight > 50° elevation (above the projection plane) and within 30 RS distance from the spacecraft.We find clear toroidal ENA distributions in O and H, and all INCA energy bands, with the emission dropping off sharply inside 5 RS radial distance in all cases. Average peak intensities occur at radial distances from ~7 RS (O, 170-230 keV) to ~10 RS (H, 24-55 keV). All toroids are offset towards the dayside by several RS, most clearly in the 24-55 keV H image, with a maximum intensity at ~13-14 RS from the planet centre on the dayside, compared to only ~10 RS on the nightside. The H ENA distribution is also enhanced around midnight local times, as previously observed in an early morphological study of 2007 data by Carbary et al. [2008], a net effect associated with reconnection return flows and transient ENA enhancements in this sector.We also explore possible organisation of the average global ENA intensity by Saturn’s rotating current systems associated with planetary period oscillations (PPOs, e.g., Provan et al. [2018]). We find that the ENA intensity is statistically modulated by periodic changes in expected plasma sheet thickness as controlled by field-aligned current interactions, a pattern evident in both north and south rotating system frames. With a thicker plasma sheet, more energetic ions are available to charge exchange within the background neutral cloud, and the LOS integral measure increases as a result (and vice versa). In this long-term picture, this effect may dominate over other possible PPO modulation effects on the appearance or evolution of transient ENA injection signatures.

The Jovian magnetosphere can undergo significant changes in its size due to varying external (e.g. solar activity) or internal (e.g. hot plasma pressure) conditions. Simulations have shown that the difference in the dayside stand-off distance between a compressed and an expanded state can be around 30 Jovian radii.In this work, we study the compressibility of the Jovian magnetosphere using a large ensemble of axisymmetric models, obtained from the recently updated UCL/AGA magnetodisc code. Each model is defined by its size (via the stand-off distance) and hot plasma content (via the hot plasma index). Using these models as virtual magnetopause crossings we estimate the compressibility index, calculated via changes in the dayside stand-off distance as a function of the external pressure, which characterises the overall response of the magnetosphere.We find that the system size plays an important role in the Jovian case, as we observe a change in the compressibility index as a function of the stand-off distance. This has also been noted in existing studies on Saturn, using a linear relation between changes in the stand-off distance and the external pressure.As a complementary study, we also estimate the compressibility index using magnetopause crossings from recently published catalogues based on JUNO data.