Magnetic field induced by convective flow in Europa’s subsurface ocean

Abstract

Movements of Europa in Jupiter’s magnetic field generate an induced magnetic field in the moon’s interior. Its measurements by the Galileo space probe led to the discovery of Europa’s subsurface ocean. However, interactions of the ocean flow with Jupiter’s background magnetic field also generate the motionally induced electrical currents in the ocean and the corresponding ocean-induced magnetic field (OIMF), which has not yet been studied in detail. A single study estimated the OIMF $\le$  $20$  nT using a simplified scaling relation. In this paper, we revisit this estimate using a physically consistent modeling setup. Based on the numerical simulations of ocean convection, we show that two modes can exist in Europa’s ocean. Mode I is dominated by a prograde zonal flow at the equator with negligible radial and meridional flows. Mode II is characterized by Hadley-like meridional circulation cells in both hemispheres and a retrograde zonal flow at the equator. The scaling analysis based on our dataset strongly indicates that Mode II is appropriate for Europa’s ocean with velocities around $0.3$  m/s. We then calculate Europa’s OIMF using a time-domain EM induction solver, which properly accounts for self-induction and diffusion of the magnetic field in the silicate and ice layers, and implicitly covers the full temporal spectrum. Our calculations suggest that even under the most favorable circumstances ($150$  km thick ocean with a conductivity of $18$  S/m located under a 1 km thick ice layer) the magnitude of Europa’s OIMF forced by the flow in Mode II is approximately $1$  nT, at the lower bound of the sensitivity of the Europa Clipper measurements and more than one order of magnitude smaller than previously predicted. The discrepancy is primarily caused by a more sluggish ocean flow and a correct treatment of EM induction. Moreover, Europa’s OIMF is affected by the electrical conductivity and thickness of ice and ocean, which we demonstrate in a parametric study.