Motional induction in Ganymede's ocean

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.

Figures (10)

• Figure 1: Top: Meridional cross-sections of the geostrophic zonal velocity ($U_\phi$) for the deep- and shallow-ocean scenarios. For each scenario, two scales are provided, depending on the dissipation mechanism thought to operate (see text for details). Bottom: geostrophic velocity versus normalized radius $s/r_o$, assuming that dissipation is governed by Ekman friction. The vertical line at $s/r_o=0.811$ (resp. $0.887$) marks the tangent cylinder for the deep- (resp. shallow-) ocean.
• Figure 2: Numerical ocean-induction setup for Ganymede in a deep-ocean scenario with $R_m^{\textsc{e}}=1.77$. Ganymede's internal structure is depicted, featuring the ice crust, the deep ocean with its simulated zonal flow, the internal ice layer, the silicate mantle, and the metallic core. Yellow tubes visualize the magnetic field lines. The bottom-right panel shows how magnetic field lines are sheared by the zonal (east–west) jet flow in the ocean (blue/red denote westward/eastward jets, respectively), thereby reflecting the secondary magnetic field induced by such dynamics.
• Figure 3: Top: Hammer maps of the radial component of the magnetic field induced by Ganymede's ocean zonal flow, $\delta B_r^i = \hat{\mathbf{r}} \cdot \mathbf{B}_{\mathrm{mi}}$, at the surface of the ocean $r=r_o$. The bottom (resp. top) scale of the colorbars corresponds to $R_m=R_m^{\textsc{q}}$ (resp. $R_m=R_m^{\textsc{e}}$). Bottom: Spherical harmonic decomposition of toroidal and poloidal magnetic energy for the full range of tested magnetic Reynolds numbers. The spectra are time-averaged. Colored envelopes show energy with flow; black envelopes show energy without flow.
• Figure 4: Top: magnetic power spectra at Ganymede's surface $r=r_g$, computed for the total magnetic field $\mathbf{B}$. Bottom: magnetic power spectra at $r=r_g$, due to motional induction in the ocean alone, computed for $\mathbf{B}_\mathrm{mi}$ (see text for details). All spectra are time-averaged. The slope of the 0.2 nT detection threshold is caused by the amplification of measurement errors during downward continuation from spacecraft altitude to Ganymede's surface.
• Figure S1: Schematic spherically symmetric structure of Ganymede considered in this study. The mantle and ice layers are assumed to be electrically insulating.