Can Ganymede’s magnetopause interactions help us probe its subsurface ocean?

The permanent magnetic field of Jupiter’s moon Ganymede is thought to arise from an Earth-like dynamo in the moon’s outer core, alongside a secondary time varying magnetic field induced by convection in the moon’s subsurface ocean. Magnetic fields of Jupiter and Ganymede meet along a current boundary known as the upstream magnetopause, whose location depends on delicate pressure balance and presence of plasma-magnetic interactions including magnetic reconnection. As Ganymede traverses the Jovian plasma sheet, magnetopause conditions vary at half-Jovian synodic period (~5.4 hr), leading to equal-period oscillations of Chapman-Ferraro (C-F) currents and subsequently Ganymede’s magnetospheric field. In this work, we (1) demonstrate that magnetic perturbations from C-F currents will cause induction in Ganymede’s subsurface ocean, and (2) constrain the extent of inducing perturbations based on the (yet unknown) range of Ganymede’s magnetopause motions. Our analysis indicates maximum ocean inductive responses of magnitude order ~1-10 nT. Although improved magnetopause tracking is required to further constrain the response value, the magnitude order lies comfortably within resolution range of the magnetometer aboard the JUpiter ICy moon Explorer (JUICE). Hence, magnetopause interactions may become a viable tool for future induction-based study of Ganymede’s subsurface ocean.