Ganymede is the only Solar System moon known to generate a permanent magnetic field. Jovian plasma motions around Ganymede create an upstream magnetopause, where energy flows are thought to be driven by magnetic reconnection. Simulations indicate Ganymedean reconnection events may be transient, but the nature of magnetopause reconnection at Ganymede remains poorly understood, requiring an assessment of reconnection onset theory. We present an analytical model of steady-state conditions at Ganymede’s magnetopause, from which the first Ganymedean reconnection onset assessment is conducted. We find that reconnection may occur wherever Ganymede’s closed magnetic field encounters Jupiter’s ambient magnetic field, regardless of variations in magnetopause conditions. Unrestricted reconnection onset highlights possibilities for multiple X-lines or widespread transient reconnection at Ganymede. The reconnection rate is controlled by the ambient Jovian field orientation and hence driven by Jupiter’s rotation. Future progress on this topic is highly relevant for the JUpiter ICy moon Explorer (JUICE) mission.

A sustained dipolar magnetic field between the current sheet outer edge and the magnetopause, known as a cushion region, has yet to be observed at Saturn. Whilst some signatures of reconnection occurring in the dayside magnetodisc have been identified, the presence of this large-scale structure has not been seen. Using the complete Cassini magnetometer data, the first evidence of a cushion region forming at Saturn is shown. Only five potential examples of a sustained cushion are found, revealing this phenomenon to be rare. This feature more commonly occurs at dusk compared to dawn, where it is found at Jupiter. It is suggested that due to greater heating and expansion of the field through the afternoon sector the disc is more unstable in this region. We show that magnetodisc breakdown is more likely to occur within the magnetosphere of Jupiter compared to Saturn.

Ganymede is the only Solar System moon that generates a permanent magnetic field. Dynamics inside Ganymede’s magnetosphere is likely driven by energy-transfer interactions on its upstream magnetopause. Previously in Kaweeyanun et al. (2020), we created a steady-state analytical model of Ganymede’s magnetopause and predicted global-scale magnetic reconnection to occur frequently throughout the surface. Using the same model, this paper provides the first assessment of Kelvin-Helmholtz (K-H) instability growth on the magnetopause in isolation from reconnection effects. The linear K-H instability growth rate is calculated at Ganymede’s equatorial magnetopause flank points under the magnetohydrodynamic with finite Larmor radius effect (MHD-FLR) theory, which accounts for inter-flank growth rate asymmetry due to large gyroradii of Jovian plasma ions. The calculation gives growth rates between γ ≈ 0.01-48 /s with notable enhancement at the equatorial flank point closer to Jupiter. Then, the ideal MHD K-H instability onset condition is evaluated across the entire Ganymedean magnetopause. We find the conditions along both magnetopause flanks to be K-H favorable at all latitudes with growth rates similar to those at respective equatorial flank points. Using Mercury’s magnetopause case as a comparison, we determined that nonlinear K-H vortices are viable at Ganymede based on the calculated growth rates, but the vortex growth will likely be suppressed once global reconnection is taken into account.

Jupiter’s giant magnetosphere is a complex system seldom in a configuration approximating steady state, and a clear picture of its governing dynamics remains elusive. Crucial to understanding how the magnetosphere behaves on a large scale are disturbances to the system on length-scales comparable to the cavity, which are communicated by magnetohydrodynamic waves in the ultra-low-frequency band (≤ 1 mHz). In this study we used magnetometer data from multiple spacecraft to perform the first global heritage survey of these waves in the magnetosphere. To map the equatorial region, we relied on the large local-time coverage provided by the Galileo spacecraft. Flyby encounters performed by Voyager 1 and 2, Pioneer 10 and 11, and Ulysses provided local-time coverage of the dawn sector. We found several hundred events where significant wave power was present, with periods spanning ~5-60 min. The majority of events consisted of multiple superposed discrete periods. Periods at ~15, ~30 and ~40 min dominated the event-averaged spectrum, consistent with the spectra of quasi-periodic pulsations often reported in the literature. Most events were clustered in the outer magnetosphere close to the magnetopause at noon and dusk, suggesting that an external driving mechanism may dominate. The most energetic events occurred close to the planet, though more sporadically, indicating an accumulation of wave energy in the inner magnetosphere or infrequent impulsive drivers in the region. Our findings suggest that dynamics of the system at large scales is modulated by this diverse population of waves, which permeate the magnetosphere through several cavities and waveguides.

Magnetic reconnection at the magnetopause (MP) energises ambient plasma via the< release of magnetic energy and produces an “open” magnetosphere allowing solar wind particles to directly enter the system. At Saturn, the nature of MP reconnection remains unclear. The current study examines electron bulk heating at MP crossings, in order to probe the relationship between observed and predicted reconnection heating proposed by Phan et al. (2013) under open and closed MP, and how this may pertain to the position of the crossings in the Δβ-magnetic shear parameter space. The electron heating for 70 MP crossings made by the Cassini spacecraft from April 2005 to July 2007 was found using 1d and 3d moment methods. Minimum variance analysis was used on the magnetic field data to help indicate whether the MP is open or closed. We found better agreement between observed and predicted heating for events suggestive of locally ‘open’ MP. For events suggestive of locally ‘closed’ MP, we observed a cluster of points consistent with no electron heating, but also numerous cases with significant heating. Examining the events in the Δβ-magnetic shear parameter space, we find 83% of events without evidence of energisation were situated in the ‘reconnection suppressed’ regime, whilst between 43% to 68% of events with energisation lie in the ‘reconnection possible’ regime depending on the threshold used. The discrepancies could be explained by a combination of spatial and temporal variability which makes it possible to observe heated electrons with different conditions from the putative reconnection site.

Permanently magnetic Ganymede carves a distinct magnetosphere inside Jupiter’s larger magnetic domain, confined by ambient Jovian plasma and magnetic field along the upstream magnetopause. As Ganymede traverses the Jovian plasma sheet, ambient magnetoplasma conditions vary at half-synodic period (5.27 hr), leading to same-period oscillation in the Chapman-Ferraro (C-F) magnetic field produced by the magnetopause. We assess the C-F magnetic field as a unique excitation source for Ganymede’s subsurface ocean. The magnetopause field is shown to diffuse into but not through the ocean, causing magnetic induction provided the liquid layer thickness is >~30 km. Then using an analytical model, we calculate maximum variation of the C-F field and estimate ~1-10 nT inductive response from the ocean subjected to the magnetopause’s movement, a range detectable by the JUpiter ICy moon Explorer (JUICE) spacecraft. Hence, Ganymede’s magnetopause may become a viable tool for future induction-based study of the moon’s subsurface ocean.

The Juno spacecraft’s polar orbits have enabled direct sampling of Jupiter’s low-altitude auroral field lines. While various datasets have identified unique features over Jupiter’s main aurora, they are yet to be analyzed altogether to determine how they can be reconciled and fit into the bigger picture of Jupiter’s auroral generation mechanisms. Jupiter’s main aurora has been classified into distinct “zones”, based on repeatable signatures found in energetic electron and proton spectra. We combine fields, particles, and plasma wave datasets to analyze Zone-I and Zone-II, which are suggested to carry the upward and downward field-aligned currents, respectively. We find Zone-I to have well-defined boundaries across all datasets. H+ and/or H3+ cyclotron waves are commonly observed in Zone-I in the presence of energetic upward H+ beams and downward energetic electron beams. Zone-II, on the other hand, does not have a clear poleward boundary with the polar cap, and its signatures are more sporadic. Large-amplitude solitary waves, which are reminiscent of those ubiquitous in Earth’s downward current region, are a key feature of Zone-II. Alfvénic fluctuations are most prominent in the diffuse aurora and are repeatedly found to diminish in Zone-I and Zone-II, likely due to dissipation, at higher altitudes, to energize auroral electrons. Finally, we identify sharp and well-defined electron density depletions, by up to two orders of magnitude, in Zone-I, and discuss their important implications for the development of parallel potentials, Alfvénic dissipation, and radio wave generation.