Plasma Observations Near Saturn: Initial Results from Voyager 1

Abstract

Extensive measurements of low-energy plasma electrons and positive ions were made during the Voyager 1 encounter with Saturn and its satellites. The magnetospheric plasma contains light and heavy ions, probably hydrogen and nitrogen or oxygen; at radial distances between 15 and 7 Saturn-radii (Rs) on the inbound trajectory, the plasma appears to corotate with a velocity within 20 percent of that expected for rigid corotation. The general morphology of Saturn's magnetosphere is well represented by a plasma sheet that extends from at least 5 to 17 Rs, is symmetrical with respect to Saturn's equatorial plane and rotation axis, and appears to be well ordered by the magnetic shell parameter L (which represents the equatorial distance of a magnetic field line measured in units of Rs). Within this general configuration, two distinct structures can be identified: a central plasma sheet observed from L = 5 to L = 8 in which the density decreases rapidly away from the equatorial plane, and a more extended structure from L = 7 to beyond 18 Rs in which the density profile is nearly flat for a distance +/- 1.8 Rs off the plane and falls rapidly thereafter. The encounter with Titan took place inside the magnetosphere. The data show a clear signature characteristic of the interaction between a subsonic corotating magnetospheric plasma and the atmospheric or ionospheric exosphere of Titan. Titan appears to be a significant source of ions for the outer magnetosphere. The locations of bow shock crossings observed inbound and outbound indicate that the shape of the Saturnian magnetosphere is similar to that of Earth and that the position of the stagnation point scales approximately as the inverse one-sixth power of the ram pressure.

Heavy ion formation in Titan's ionosphere: Magnetospheric introduction of free oxygen and a source of Titan's aerosols?

“…The Voyager 1 Titan flyby on November 12,1950 showed a very complex encounter with Saturn's magnetosphere with an induced magnetotail 'Ness et at., 1981', and a bite-out in the magnetospheric electron population for E>500eV (Bridge et al, 1981;Hartle et al, 1982). It was also shown that the upstream plasma was composed of light and heavy ions (Hartle et al, 1982; Sittler complex neutral and ion chemistry was predicted to occur in its upper atmosphere and ionosphere (Yung et al, 1984;Yung, 1987;Toublanc et al, 1995;Cravens et al, 1997).…”

Thermal plasma and neutral gas in Saturn's magnetosphere

“…Since the low-energy plasma is tied to a magnetic field line and the field lines rotate with Saturn, every plasma component must move with the same velocity. The Voyager 1 and 2 plasma science experiments (PLS) have provided most of our current knowledge on the thermal plasma at Saturn [Bridge et al, 1981[Bridge et al, , 1982Lazarus and McNutt, 1983;Sittler et al, 1983;Richardson, 1986]. This instrument has four Faraday cup detectors which measure currents with energies from 10 to 5950 eV [see Bridge et al, 1977].…”

Pickup ion distributions from three-dimensional neutral exospheres

“…Titan has no detectable intrinsic magnetic field, but it has an ionopause on the ram side. These results were found during Voyager 1's flyby of Titan on November 12, 1980 [Bridge et al, 1981;Ness et al, 1981;Gurnett et al, 1981;Hartle et al, 1982;Neubauer et al, 1984;Sittler et al, 2005] [Keller et al, 1998;Yung, 1987;Yung et al, 1984;Toublanc et al, 1995] predicted the presence of significant quantities of CH 4 and H 2 . CH 4 was subsequently measured in situ by the Ion Neutral Mass Spectrometer (INMS) on Cassini by Waite et al [2005], who reported a density of ∼3 × 10 6 cm −3 on orbit TA at an exobase altitude of 1429 km (or r c = 4004 km from center) with a temperature of 145 K. A Chamberlain exosphere is chosen for Titan.…”