Magnetized or unmagnetized: Ambiguity persists following Galileo's encounters with Io in 1999 and 2000

Magnetometer data from Galileo's close encounters with Io on October 11, 1999, and February 22, 2000, do not establish clearly either the existence or absence of an internal magnetic moment because they were acquired in regions where plasma currents contribute large magnetic perturbations. Data from an additional encounter on November 26, 1999, with closest approach beneath Io's south polar regions, were lost. The recent passes add to our understanding of the interaction of the torus with Io and its flux tube and tighten the limits on possible internal sources of magnetic fields. Simple field-draping arguments account for some aspects of the observed rotations. Interpretations in terms of both a magnetized and an unmagnetized Io are considered. Data from the February 2000 pass (closest approach altitude 201 km, latitude 18°) rule out a strongly magnetized Io (surface equatorial field larger than the background field) but do not rule out a weakly magnetized Io (surface equatorial field of the order of Ganymede's but smaller than the background field at Io). Models suggest that if Io is magnetized, its magnetic moment is not strictly antialigned with the rotation axis. The inferred tilt is consistent with contributions from an inductive field analogous to that observed at Europa and Callisto. If an induced field is present, the currents would flow in the outer mantle or aesthenosphere. Wave perturbations differ on flux tubes that do or do not link directly to Io and its ionosphere suggesting that the latter flux tubes are virtually stagnant in Io's frame and that a unipolar inductor appropriately models the currents linking Io to Jupiter's ionosphere.