Motional induction in Ganymede's ocean

TL;DR

This study models Ganymede's subsurface ocean magnetic signals caused by oceanic flows, showing they can be detected by spacecraft, especially in deep ocean scenarios with high magnetic Reynolds numbers.

Contribution

It introduces a coupled modeling approach combining thermal convection, dynamo, and induction models to analyze Ganymede's ocean magnetic signatures.

Findings

Ocean flows generate detectable toroidal magnetic fields.

Surface magnetic signals can reach up to 9 nT in certain scenarios.

Ocean-induced signals dominate at higher spherical harmonic degrees.

Abstract

We investigate the magnetic signature of oceanic circulation in Ganymede's subsurface ocean using kinematic induction modeling. Our approach couples zonal jet flows from rotating thermal convection simulations with magnetic field models incorporating Ganymede's internal dynamo and external contributions from Jupiter. We solve the induction equation in spherical geometry for deep-ocean (493 km) and shallow-ocean (287 km) scenarios with varying magnetic Reynolds numbers. Ocean flows generate a predominantly toroidal magnetic field through the omega-effect, with a weaker poloidal component pervading beyond the conductive ocean layer. For some, but not all, induction configurations, analysis of the time-averaged Lowes-Mauersberger spectra reveals that ocean-induced signals dominate at spherical harmonic degrees $\ell \geq 4$. Deep ocean scenarios with magnetic Reynolds numbers above unity produce surface magnetic signals up to 9 nT. Our results demonstrate that Ganymede's intrinsic magnetic field creates favorable conditions for detecting subsurface ocean dynamics, thus emphasizing the need for low-altitude orbits for the Juice probe.