Passage through lo's ionospheric plasmas by the Galileo spacecraft

Abstract

On February 22, 2000, the Galileo spacecraft passed by the moon Io at a closest approach distance of 206 km. This altitude was sufficiently low that the plasma analyzer observed the three‐dimensional ion velocity distributions as functions of energy/charge (E/Q) at the top of Io's ionosphere. The ionospheric ion distributions consisted of two populations, a warm distribution with density and temperature of ∼8000 cm−3 and 10,000 K and a cool population with 3000 cm−3 and 2300 K. This cooler temperature is in the range of those observed remotely for some volcanic plumes. The bulk speed of these ions was 2 km s−1 with respect to Io's surface. In order to identify the mass/unit charge (M/Q) of the ionospheric ions, the E/Q spectra of the pickup ions in this region were examined. The most probable M/Q of the primary population of these ions was inferred to be 64. Remote spectroscopic observations of Io's surface and atmosphere suggest that these ions are S2+ and/or SO2+, although SO+ and SO3+ are not eliminated as possibilities. There is some evidence for lesser densities of heavier ions such as S3+ and S4+. The densities, temperatures, and bulk flow velocities of torus ions were measured as the spacecraft approached Io from the upstream direction. A combination of fits to the E/Q spectra observed with Galileo, the determinations of the M/Q of the primary ions with the plasma instrumentation on this spacecraft during other passages, and the previous identification of the primary ions with Voyager 1 plasma and remote observations find the following primary composition for the unperturbed torus near Io: O++ (50 cm−3, 30 eV), O+ (200 cm−3, 30 eV), S++ (400 cm−3, 90 eV), and S+ (100 cm−3, 90 eV). At a radial distance from Io of ∼19,700 km (10.9 Io radii), changes in the torus ion density, temperature, and bulk flow velocity provide evidence that a cloud of neutral gases is co‐orbiting with Io and is providing a substantial interaction with the torus ions.