Thermal plasma and neutral gas in Saturn's magnetosphere

Abstract

Abstract. Saturn's magnetosphere contains plasma and neutral particles from Saturn's atmosphere, the rings, the inner icy satellites, and Titan. This paper reviews the observations of plasma and neutrals near Saturn. Plasma conditions were observed during the Pioneer 11 and Voyager 1 and 2 flybys of Saturn. Neutral H was observed by the Voyagers; neutral OH has been observed by the Hubble Space Telescope. The attempts which have been made to understand the physical processes behind the data are also discussed. Saturn's magnetosphere is dominated by neutrals, with a neutral to plasma density ratio of about 10. The neutrals are probably produced by bombardment of the moons by micrometeorites and heavy ions, although there is a discrepancy between predicted sputtering rates and the amount of sputtering necessary to produce the observed neutral densities. Transport of plasma out of the magnetosphere must be very fast, of the order of a few days. Models of the processes occurring in Saturn's magnetosphere are used to determine the densities of neutral and plasma species which cannot be directly observed. What happens to this plasma? The largest portion recombines back into neutral atoms and goes flying out of the magnetosphere. A significant fraction is transported to the boundary of the magnetosphere and escapes down the magnetotail into the solar wind. Of the rest, some smashes into the rings and moons, knocking off neutrals which then form more plasma, and some enters Titan's and Saturn's atmospheres, providing an energy source to drive atmospheric processes and auroral displays.Most of our knowledge of Saturn's magnetosphere comes from the brief flyby encounters of Pioneer 11 (1979) and Voyager 1 (1980) and 2 (1981). Recent observation• using the Hubble space telescope (HST) also give valuable clues as to the neutral density structure. This paper describes the observations from Saturn's magnetosphere, the current understanding of the

Atmospheres and Plasma Interactions at Saturn's Largest Inner Icy Satellites

“…The most obvious source of OH is dissociation of sputtered H 2 O molecules from satellites' surfaces and Saturn's rings (e.g., Johnson et al 1983;Pospieszalska & Johnson 1989;Shi et al 1995). The model of Richardson (1998) demonstrated that the required source strength of OH radicals is at least an order of magnitude larger than can be supplied by sputtering. Jurac et al (2001) found that the missing water source is even a more severe problem than concluded by Richardson (1998).…”

Mass loading of Saturn's magnetosphere near Enceladus

“…The magnetic field strength in this region is $320 nT, and the electron density is $60/cm 3 [Richardson, 1998]. The background ions and electrons have temperatures of $50 and 5 eV, respectively [Sittler et al, 1983;Saur and Strobel, 2005].…”

First whistler observed in the magnetosphere of Saturn

“…The equatorial proton to water group density ratio was adjusted to give the b value that gave the best agreement (b = 3) with the results of Richardson [1998], as discussed in the previous section. The magnetic mirror force was included in the iteration procedure, but was initially assumed to be zero, i.e., P k = P ?…”

“…It then follows from the equation for the scale height (given earlier) that b = 4(T H +/T W +) 1/2 . The best currently available measurements of the ratio of the temperature of the protons to the temperature of the water group ions are from Richardson [1998]. temperatures given by Richardson [1998] is for b = 3, which corresponds to a ratio of the proton temperature to the water group ion temperature of T H +/T W + = 0.5.…”

“…As in the previous model we use two ion species, singly charged oxygen ions (i.e., m W + = 16) to represent the water group ions and protons. The plasma density is then calculated using the following equation for each species from Richardson [1995Richardson [ , 1998,…”