Plasma Observations Near Saturn: Initial Results from Voyager 2

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Abstract
Results of measurements of plasma electrons and poitive ions made during the Voyager 2 encounter with Saturn have been combined with measurements from Voyager 1 and Pioneer 11 to define more clearly the configuration of plasma in the Saturnian magnetosphere. The general morphology is well represented by four regions: (i) the shocked solar wind plasma in the magnetosheath, observed between about 30 and 22 Saturn radii (RS) near the noon meridian; (ii) a variable density region between ∼ 17 RS and the magnetopause; (iii) an extended thick plasma sheet between ∼ 17 and ∼ 7 RS symmetrical with respect to Saturn's equatorial plane and rotation axis; and (iv) an inner plasma torus that probably originates from local sources and extends inward from L ≈ 7 to less than L ≈ 2.7 (L is the magnetic shell parameter). In general, the heavy ions, probably O+, are more closely confined to the equatorial plane than H+, so that the ratio of heavy to light ions varies along the trajectory according to the distance of the spacecraft from the equatorial plane. The general configuration of the plasma sheet at Saturn found by Voyager 1 is confirmed, with some notable differences and additions. The "extended plasma sheet," observed between L ≈ 7 and L ≈ 15 by Voyager 1 is considerably thicker as observed by Voyager 2. Inward of L ≈ 4, the plasma sheet collapses to a thin region about the equatorial plane. At the ring plane crossing, L ≈ 2.7, the observations are consistent with a density of O+ of ∼ 100 per cubic centimeter, with a temperature of ∼ 10 electron volts. The location of the bow shock and magnetopause crossings were consistent with those previously observed. The entire magnetosphere was larger during the outbound passage of Voyager 2 than had been previously observed; however, a magnetosphere of this size or larger is expected ∼ 3 percent of the time.