Plasmas observed with the Galileo spacecraft during its flyby over Io's northern polar region

On 8 August 2001 the tape recorder on board the Galileo spacecraft provided high-resolution measurements of plasmas during a 1-hour interval centered on the closest approach to Io at an altitude of 200 km. During this period the spacecraft trajectory was positioned at a radial distance from Jupiter of 5. 9 R-J (Jupiter radii) and approached Io from upstream in the torus plasma flow, then over the northern polar region of Io, and subsequently into the downstream wake in the torus. The upstream and downstream ion densities as calculated from the moments of the measured three-dimensional velocity distributions were about 1200/cm(3). These plasmas were rigidly corotating with Jupiter at 57 km/s. The upstream ions were characterized by the sum of four Maxwellian distributions, and the downstream distributions exhibited a similar set of thermal distributions with half the density of that found upstream and the remainder as pickup heavy ions freshly injected by the interaction of the torus flow with Io's ionosphere/atmosphere. The boundary of the Hill (Lagrange) sphere for Io's neutral gases which were co-orbiting with this moon was about 5 R-Io (Io radii) from its center as identified by the onset of decreasing number densities and bulk flow speed as the spacecraft approached Io. Closer to Io, and just before its entry and departure from magnetic field lines intercepting its northern polar surface, the diversion of incoming torus plasma by Io's face toward this flow and the subsequent downstream flow around Io were observed. Over Io's polar cap the torus plasma density was reduced by a factor of about 5 relative to that measured upstream and downstream from the Io interaction. The bulk flow speeds of the torus plasma were reduced to several km/s, an effect that is expected for the slow transport of magnetic flux in Io's conducting interior. A unique set of conditions allowed the detection of a cooler ion species with extremely high values of mass/charge in the range of 500 to 1000 amu. These ions are interpreted as clusters of SO2 molecules, or SO2 snowflakes,'' which briefly exist from the fresh injection and subsequent cooling of volcanic gases.