Observations of plasmas in the Io torus with the Galileo spacecraft

On December 7, 1995, the Galileo spacecraft passed through the Io torus near the magnetic equatorial plane of Jupiter. High– resolution measurements of positive ions and electrons in the energy/charge ( E/Q ) range of 0.9 V to 52 kV were acquired over the jovicentric radial distances of 5.6 to 7.8 Rj . At radial distances beyond Io's orbit at 5.9 Rj the plasma instrument (PLS) observed intense magnetically field‐aligned beams of electrons with average energies of hundreds of eV to several keV. The directional energy fluxes at the spacecraft position ranged from ∼1.5 to 90 erg cm −2 S −1 sr −1 in directions parallel and antiparallel to the magnetic field. The corresponding range of energy fluxes into Jupiter's ionosphere was ∼9 to 560 erg Cm −2 s −1 , sufficient for the production of auroral emissions at far‐ultraviolet wavelengths with brightnesses in the range of 0.06 to 3 mega‐Rayleighs. The largest energy fluxes were observed at ∼7.8 Rj and were comparable to those observed during the close flyby of Io. The torus plasmas were observed to rigidly corotate with Jupiter out to a radial distance of ∼7 Rj and are slower at greater radial distances. Fluctuations of the bulk ion speeds and densities were found to be superposed upon the general pattern of corotation and are suggestive of interchange of magnetic flux tubes in the plasma torus. For an isolated field‐aligned electron beam, the radial component of plasma flow was directed toward Jupiter at a speed of ∼4 km s −1 . The other electron beams also occurred coincidentally with the fluctuations in the radial component of flow. The observations of these low‐energy electron beams are consistent with the absence of magnetically field‐aligned acceleration of electrons into Jupiter's ionosphere. The electron acceleration appears to be due to intense heating of the ionospheric plasmas associated with large currents driven by the interchange motions in the torus plasmas.