Improved Models of Ganymede's Permanent and Induced Magnetic Fields Based on Galileo and Juno Data

Abstract

Near Ganymede, the magnetic field is a superposition of Jupiter's magnetospheric magnetic field, the field arising from sources within the moon, the field generated by plasma currents driven by the interaction of flowing magnetospheric plasma with the conducting moon, and the field arising from ionospheric currents. Previous fits to Ganymede's internal field have not identified the contributions of plasma and ionospheric currents, although their contributions can obscure the signature of sources internal to the moon. Fortunately, using magnetohydrodynamic simulations whose output agrees well with the measurements acquired on close passes by Galileo and Juno, we can estimate the moon-scale contributions of plasma sources. By subtracting the magnetic signatures of plasma and ionospheric currents from the measured field, we approximate measurements made in a current-free region. We fit the corrected data from different sets of flybys either as a sum of low order spherical harmonics or as a permanent dipole moment plus an induced dipole with approximately the same root-mean-square errors. For the induced dipole model, data from multiple flybys occurring at different phases of Jupiter's rotation are used to represent the time-variation of the external field at Ganymede. Compared with earlier estimates, the magnitude of the permanent dipole moment did not significantly change in either analysis. However, for the permanent plus induced dipole model, the induction efficiency decreases from 0.84 to ∼0.72. The reduced efficiency places new constraints on the thickness of the ice shell above the ocean and the ocean's depth and conductivity.