Analytical assessment of Kelvin-Helmholtz instability growth at Ganymede's upstream magnetopause

Abstract

Ganymede is the only Solar System moon that generates a permanent magnetic field. Dynamics within the Ganymedean magnetosphere is thought to be driven by energy-transfer interactions on its upstream magnetopause. Previously in Kaweeyanun et al. (2020), https://doi.org/10.1029/2019GL086228 we created a steady-state analytical model of Ganymede's magnetopause and predicted global-scale magnetic reconnection to occur frequently throughout the surface. This paper subsequently provides the first assessment of Kelvin-Helmholtz (K-H) instability growth on the magnetopause. Using the same analytical model, we find that linear K-H waves are expected on both Ganymedean magnetopause flanks. Once formed, the waves propagate downstream at roughly half the speed of the external Jovian plasma flow. The Ganymedean K-H instability growth is asymmetric between magnetopause flanks due to the finite Larmor radius effect arising from large gyroradii of Jovian plasma ions. A small but notable enhancement is expected on the sub-Jovian flank according to the physical understanding of bulk plasma and local ion flows alongside comparisons to the well-observed magnetopause of Mercury. Further evaluation shows that nonlinear K-H vortices should be strongly suppressed by concurring global-scale magnetic reconnection at Ganymede. Reconnection is therefore the dominant cross-magnetopause energy-transfer mechanism and driver of global-scale plasma convection within Ganymede's magnetosphere.