Observed Magnetic Signatures of the Galilean Moons of Jupiter

S. P. Joy, M. G. Kivelson, K.K. Khurana, R. J. Walker, C. T. Russell, J. Warnecke, L. Bennett (UCLA-IGPP), C. Polanskey (JPL)

The Galileo spacecraft has now completed its initial survey of the Galilean moons of Jupiter. The magnetometer recorded a magnetic perturbation on each of the passes. At Io, the magnetic field observations are consistent with an intrinsic field of maximum surface magnitude 2600 nT with the dipole axis nearly spin aligned. Complexities in the local plasma environment make this interpretation ambiguous and additional data are required to resolve many remaining issues. The field perturbations at Europa have been modeled as a centered dipole in order to estimate the upper limit of a possible internal source. The fit gives a maximum surface strength of 240 nT. The axis of the dipole is oblique to the Europa spin axis and tilted 65 degrees away from the background field. Neither a core dynamo source nor a paramagenetic response to the background field adequately explains this orientation. If a substantial quadrupole moment is identified in data from future Europa passes, then the observations would be consistent with a source generated in a spherical shell near the surface as has been proposed for Uranus (Connerney, 1987) and Neptune (Connerney, 1991). The effects of currents flowing within the plasma may also contribute to the magnetic signature near Europa. Observations at Ganymede confirm the existence of an internal field source of surface magnitude 1500 nT (pole), nearly spin axis aligned, and consistent with a core dynamo source. The initial observations of the magnetic field near Callisto place the upper limit on any intrinsic field at 30 nT. During the remainder of the Galileo mission, including the Europa phase of 1998 - 1999, there will multiple encounters with Europa and a return to Io. The additional data are expected to show whether Io has an intrinsic field and constrain better the field of Europa.