Observations of thermal plasmas in Jupiter's magnetotail

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Abstract
A survey of thermal plasmas and magnetic fields is presented for the orbit of the Galileo spacecraft around Jupiter that occurred during the period May 4 through June 22, 1997. This orbit traversed the magnetotail out to Jovian radial distances of 100.2 RJ in the magnetotail. Perijove was positioned at 9.3 RJ. Three primary ion populations were detected with the plasma analyzer: cool hydrogen ions with temperatures of 10 eV, hot hydrogen ions with temperatures of ∼10 keV, and a third population of heavy ions such as O++, O+, S++, and S+++ with temperatures in the range of 500 eV. Plasma flows near perijove were in the corotational direction but with speeds ∼60% of those for rigid corotation with the planet out to radial distances of ∼18 RJ. In the radial range of 18–26 RJ there were significant radial components for the bulk flows, and the flow components in the corotational direction reached values expected for rigid corotation when the current sheet was crossed. The transient character of the plasma parameters suggests that strong ion plasma acceleration is occurring in this region. The temperatures of the heavy ions increased from 5 × 106 K at 9.3 RJ to ∼108 K at 26 RJ. At distances 50 RJ it varied as R−1.19. The thermal plasma pressure in the current sheet is a factor of ∼10 less than the magnetic pressure at 9.3 RJ at positions above or below the sheet but becomes equal to this magnetic pressure at radial distances >30 RJ. The corresponding values of the ratio of the plasma to magnetic pressure, β, are in the range of 10–100 in the current sheet. The number densities and temperatures of these plasmas are 0.05–0.1 /cm3 and 0.5–1 × 108 K, respectively. In the magnetotail the bulk flows of the thermal plasmas exhibit substantial components in the corotational and radially outward directions, but the bulk speeds of 50–200 km s−1 are significantly less than those for rigid corotation. For this single orbit the average bulk flows were ∼50 km s−1 in the premidnight sector and 200 km s−1 in the early morning sector at radial distances >∼50 RJ. At apojove of 100 RJ an antisunward flow of ∼200 km s−1 is found that is supportive of the magnetospheric “wind” reported for Voyager measurements of energetic charged particles. The 10‐hour periodicity of the pairs of current sheet crossings at the position of the Galileo spacecraft includes a variety of dynamical signatures, which are suggested to be due to the changes in direction and pressures in the solar wind and due to the transient acceleration of plasmas in the current sheet.

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