Plasmas observed near local noon in Jupiter's magnetosphere with the Galileo spacecraft

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Abstract
Measurements of electron beams and thermal ions were achieved by the Galileo spacecraft during the later phases of its mission for three outbound passages of the local afternoon and noon sectors of Jupiter's magnetosphere. During the first two passages the spacecraft passed through the rigidly corotating plasma torus with outer boundary near 8 RJ, into the transition region from torus to plasma sheet, and then into the plasma sheet proper at distances beyond ∼25 RJ. Telemetry coverage for the third passage began in the transition region. At the outer boundary of the torus the ion densities and temperatures were ∼500 cm−3 and 106 K. In the transition region the ion densities decreased with increasing distance to 0.1 cm−3, and temperatures increased to 5 × 107 K. In the plasma sheet the ion densities were typically 0.1 cm−3 with temperatures of 108 K. The azimuthal component of plasma flow slows to ∼70% of the corotational value in the transition region. For the first two passages, strong radial flows toward Jupiter increase with increasing radial distances. For the third passage near local noon the plasma flows are considerably more stagnant than those at local evening. The variations in the flow suggest that the solar wind influence extends to radial distances in the range of 10 RJ. Electron beams parallel and antiparallel to the magnetic field were observed during the passages on field lines connected to the main auroral ring in the ionosphere. Major constraints on the heating/acceleration mechanism for the main ring are (1) the presence of the electron beams at and near the equator and (2) previous remote observations of emissions in the vicinity of the atmospheric methane layer. These constraints support a heating mechanism in this layer driven by magnetospheric convection without acceleration by field‐aligned electrostatic fields. At radial distances 30–44 RJ near local noon the plasma densities exhibited a System III effect, increasing once per planetary rotation near a longitude 300°. In addition, the finding of thermal ion beams directed parallel to the magnetic field near 20 RJ resolves the problem of radial force balance previously identified with Voyager measurements. That is, the outward centrifugal force for these beams is sufficient to balance the inward radial force of the magnetic field.

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