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.

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.

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.