Europa and Callisto: Induced or intrinsic fields in a periodically varying plasma environment

Abstract

Abstract. Magnetometer data from four Galileo passes by the Jovian moon Europa and three passes by Callisto are used to interpret the properties of the plasma surrounding these moons and to identify internal sources of magnetic perturbations. Near Europa the measurements are consistent with a plasma rich in pickup ions whose source is freshly ionized neutrals sputtered off of the moon's surface or atmosphere. The plasma effects vary with Europa's height above the center of Jupiter's extended plasma disk. Europa is comet-like when near the center of the current sheet. It is therefore likely that the strength of the currents coupling Europa to Jupiter's ionosphere and the brightness of a Europa footprint will depend on System III longitude. Magnetic perturbations on the scale of Europa's radius can arise from a permanent dipole moment or from an induced dipole moment driven by the time-varying part of Jupiter's magnetospheric field at Europa's orbit. Both models provide satisfactory fits. An induced dipole moment is favored because it requires no adjustable parameters. The inductive response of a conductive sphere also fits perturbations on two passes near Callisto. The implied dipole moment flips direction as is predicted for greatly differing orientations of Jupiter's magnetospheric field near Callisto in the two cases. For both moons the current carrying shells implied by induction must be located near the surface. An ionosphere cannot provide the current path, as its conductivity is too small, but a near surface ocean of-10 km or more in thickness would explain the observations.

Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations

“…These currents would produce a secondary (or induced) magnetic field that adds to the background field of Jupiter and to fields arising from currents flowing in the ambient plasma to give the total observed field. Khurana et al (1998) and Kivelson et al (1999) recently showed that the dominant large-scale features of magnetic field perturbations observed by the Galileo spacecraft during flybys of both Europa and Callisto are consistent with induced magnetic dipoles, as had also been pointed out by Neubauer (1998). At both moons, the observed magnetic field perturbations are approximately those expected for moons responding as perfectly conducting spheres.…”

“…As a result, this moon is exposed to a roughly constant field during most of the time interval between consecutive current sheet crossings (see Fig. 1b of Kivelson et al 1999). This is not the case of Europa, where the importance of the current sheet field relative to Jupiter's dipole field is small.…”

“…Such a response requires a globally distributed highly conducting medium located close to the surface of the satellites. Khurana et al (1998) and Kivelson et al (1999) interpreted this result as support for the presence of salty subsurface oceans. For Europa, the same conclusion was reached independently by K. Kuramoto et al (unpublished manuscript, 1998), extending a previous study by Kuramoto et al (1998).…”

“…1a of 1b of Kivelson et al (1999) that the average of B r and B ϕ over one synodic cycle are both close to 0 (i.e., small compared to their oscillation amplitudes). Together with the near-constancy of B θ , this implies that the inducing field, which is the time-varying component of the external field seen by the moon, is simply given by the equatorial projection of Jupiter's background field.…”

“…Figures 1a and 1b of Kivelson et al (1999) each show one cycle of Jupiter's background field at the location of Europa and Callisto respectively, as a function of the moon's west longitude, or equivalently as a function of time during one synodic period (11.23 h for Europa and 10.18 h for Callisto). The field was computed from the empirical model of Khurana (1997), which includes the contribution of the current sheet and the O 6 model of Connerney (1992) for Jupiter's internal field.…”