Limits on an intrinsic dipole moment in Europa

Abstract

Galileo magnetometer data acquired on four passes by Europa have been used to investigate whether a fixed permanent dipole moment is present in the interior of the moon in addition to the induced dipole moment previously identified. The data from several low‐altitude passes have been fitted to models of increasing complexity. The simplest models of the magnetic field fit the measurements to internal multipoles fixed to the satellite, first through lowest‐order dipole and subsequently through second‐order quadrupole. In these models the magnetic field of Jupiter supplemented by a uniform magnetic field that is allowed to change from pass to pass is used to describe the external field as a first‐order approximation. Superimposing an induced magnetic dipole moment that is driven by the time variation of the measured external magnetic field of Jupiter improves the fits and reduces the rms error that characterizes the difference between the model and the data. In more ambitious models the plasma currents in the vicinity of Europa are approximated as wire currents forming an Alfvén wing. The inductive response is found to be ∼97% of the theoretical maximum inductive response for a highly conducting sphere. The largest surface equatorial field caused by a fixed internal dipole moment is less than 25 nT. This should be compared with the magnitude of the induced field, which can be of order 100 nT. We thereby confirm the presence of an inductive response and conclude that the dipole coefficients of the constant intrinsic field contribute at most in a very minor way to the magnetic field.