Analytical study of energy-transfer processes on Ganymede’s upstream magnetopause

Abstract:

Jupiter’s largest moon Ganymede maintains a permanent magnetic field generated via internal dynamo action – the only Solar System moon to do so. Dynamics within Ganymede’s magnetosphere are thought to be driven by energy-transfer interactions on its upstream magnetopause, particularly magnetic reconnection and Kelvin-Helmholtz (K-H) instability. Previous observations and simulations have speculated prevalence of reconnection events, but their nature remains poorly understood. Meanwhile, there has been no directly study on K-H instability for Ganymede. Here we perform the first assessments of both magnetic reconnection and K-H instability growth on Ganymede’s magnetopause under fundamental plasma theory, using an analytical model of steady-state conditions near the boundary. Magnetic reconnection can occur wherever the Ganymedean magnetic field is partly antiparallel to the external Jovian magnetic field regardless of the moon’s orbital position. Large reconnection electric field strengths (2.6-5.7 mV/m) suggests significant reconnection rates throughout the magnetopause, indicating that multiple X-lines and widespread flux-transfer events are possible. The average reconnection rate is dependent on Ganymede’s orbital position and hence effectively driven by Jupiter’s rotation. Linear K-H instability growth rates are ~0.01-48 s^-1 along Ganymede’s equatorial magnetopause flanks, with faster growth on the near-Jupiter flank due to the finite Larmor radius effect arising from large Jovian plasma ion gyro-radius. At global scale, linear K-H waves are likely prevalent for all magnetopause flank latitudes with similar growth rates to those at the equator. Based on comparison with results from Mercury’s magnetopause, Ganymede’s K-H instability growth rates may be sufficient to form energy-transferring nonlinear K-H vortices, but their formations are likely suppressed by frequent reconnection events. Understanding of these energy-transfer processes will be relevant for not only future research on Ganymede’s magnetosphere, but also planning of the upcoming Jupiter Icy Moons Explorer mission.