Open AccessPassage through lo's ionospheric plasmas by the Galileo spacecraft
Author(s) -
Frank L. A.,
Paterson W. R.
Publication year - 2001
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2000ja002503
Subject(s) - ionosphere , ion , physics , population , spectral line , galileo (satellite navigation) , plasma , atomic physics , geophysics , geology , astronomy , geodesy , demography , quantum mechanics , sociology
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 S 2 + and/or SO 2 + , although SO + and SO 3 + are not eliminated as possibilities. There is some evidence for lesser densities of heavier ions such as S 3 + and S 4 + . 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.