Neutral Gas and Thermal Plasma in Saturn's Magnetosphere: Implications for Measurements by the Cassini Ion Neutral Mass Spectrometer (INMS)


Sputtering by charged particle and meteoroid bombardment of material from the rings and the surfaces of Saturn's moons is believed to create an extensive neutral cloud of water molecules and water dissociation products in the inner magnetosphere. Subsequent ionization of this neutral gas by electron impact ionization, photoionization by ultraviolet radiation, and by charge transfer with magnetospheric ions acts as a source of thermal plasma (both protons and heavier ions) in Saturn's inner magnetosphere.

Titan acts a source of neutral gas and plasma for the outer magnetosphere of Saturn. Both neutral hydrogen and nitrogen torii are thought to surround Saturn near the orbit of Titan, and plumes of heavy ions from Titan are thought to exist in the outer magnetosphere.

The ion neutral mass spectrometer (INMS) onboard the Cassini Orbiter will make measurements of neutral and ions densities and composition in at least the densest portions of these magnetospheric neutral and plasma torii/clouds. This section of the poster will provide an overview of the these neutral and plasma torii, and the following three sections will review in more detail the clouds/torii associated with the rings, the icy satellites, and Titan.

The atomic hydrogen distribution in Saturn's magnetosphere is illustrated here as an isosurfaces showing the 4000 cm^-3 number density level (red), based on the model of Ip (1996). Also shown are the 60 cm^-3 (magenta) and 20 cm^-3 (yellow) levels for neutral water molecules associated with the icy satellites (Johnson et al., 1989) and the rings (Pospieszaka and Johnson, 1991) respectively. The planned trajectory of the Cassini spacecraft through this region during its first year at Saturn is also shown. Detailed density values along the orange segment of the trajectory are shown below.

Time series plots of the number density along the orange segment of the planned Cassini trajectory shown above. The hydrogen number density associated with the Titan torus is shown in the top panel. The middle panel shows the number density of the neutral water ions associated with the icy satellites (blue) and the rings (red). The water ion density associated with the icy satellites (blue) and the rings (red) is shown in the bottom panel.

Titan's Gas and Plasma Torus

Unlike the icy satellites Titan possesses a substantial atmosphere and hence, an ionosphere. The interaction of Titan's ionosphere with Saturn's outer magnetosphere results in Titan being both a source and a sink of magnetospheric neutrals and ions. The primary processes of Titan's plasma interaction are summarized in the figure below.

A schematic illustration of Titan's interaction with Saturn's magnetospheric plasma. The key processes that lead to neutrals and ions being removed from Titan and added to Saturn's magnetosphere are shown. The solid blue line represents the draping of Saturn's magnetic field around Titan's ionosphere.

Energetic ions may impact Titan's atmosphere sputtering off neutral atoms or molecules. Neutrals may also be ejected from Titan as a result of photochemical reactions. These reactions may energize some of the neutrals beyond the escape speed of Titan. The ejected neutrals will begin to orbit Saturn producing a neutral torus. The main components of this neutral torus are hydrogen and nitrogen.

Analysis of the Voyager ultraviolet spectrometer data by Shemansky and Hall (1992) found that the hydrogen torus was not axially symmetric, but had a strong local time dependence. This local time dependence was demonstrated by Ip (1996) to be the result of orbital perturbations of the hydrogen atoms by solar radiation pressure acting over the ionization lifetime of these neutrals. The solar radiation pressure changes the initial circular orbit of the neutral hydrogen atom into an elliptical orbit with the apogee of the orbit located in the dawn direction. This results in a density cavity on the dawn side of Saturn and spreads out the hydrogen torus radially inward on the dusk side of Saturn.

The global distribution of atomic hydrogen from the model of Ip (1996). The distribution of the hydrogen emitted from Titan is strongly influenced by the solar radiation pressure. The x-axis point in the direction of the sun. The negative y-axis points in the direction of the dawn side of Saturn. A density cavity is clearly visible on this side. In contrast the hydrogen density on the dusk side is spread from Titan's orbit radially inward twords Saturn.

Cross sections of atomic hydrogen density (cm^-3) in the noon(left)-midnight(right) plane (yz-plane). From the model by Ip (1996)

Titan's nitrogen torus forms in a similar fashion as its hydrogen torus. The density distribution of the nitrogen torus depends strongly on the total emmision rate, the initial velocity distribution of the out flowing neutrals and the ionization rate of the neutral nitrogen. Calculations by Ip (1992) suggest that the peak number density near the orbit of Titan can peak between 10-40 cm^-3 depending on the initial conditions. This value drops to around 0.1 cm^-3 in the Tethys-Dione region.

Cross sections of atomic nitrogen density (cm^-3). From the model by Ip (1992)

The atoms and molecules in Titan's neutral torus can be ionized and picked-up by Saturn's magnetic field. This is just one process that can produce a plasma torus around Saturn. . Another process contributing to Titan's plasma torus is Titan itself. Neutrals in Titan's atmosphere can be ionized by impact with Saturn's magnetospheric electrons or by solar extreme ultraviolet photons. Ions created at lower altitudes may outflow down the wake, while ions created at higher altitudes are picked-up by the external plasma and magnetic field. These processes may lead to a Titan plasma plume which wraps around Saturn. As Titan orbits Saturn it may encounter plasma from this plume.

Projection of the magnetic field vectors and contours of the log of the number density from a three-dimensional MHD model of Titan's interaction with Saturn's magnetospheric plasma by Ledvina and Cravens. Ionspheric tail flow is evident as is the draping of the magnetic field. The circle represents Titan. Saturn's magnetospheric flow is from the left. The density shown below an altitude of about 700 km is an artifact from the contouring algorithm.

The Icy Satellites

A torus of neutral water molecules coexists with the icy satellites Enceladus, Tethys, Dione and Rhea, and the Saturn's rings. Bombardment of the satellites by ions present in Saturn's magnetosphere sputters off neutrals. Most of these sputtered neutrals have sufficient energy to escape the gravitational fields of the small satellites. However, they do not have sufficient energy to escape the gravitational field of Saturn. These neutral co-orbit with the satellites forming a gravitationally bound toroidal atmosphere around Saturn. On average these neutrals make several orbits about Saturn before being ionized (via photons, electron impact or charge exchange) and picked-up by Saturn's magnetic field.

The results of the model of Johnson et al. (1989) show that the neutral water densities are highly peaked around the orbital location of the satellites. The water density is mainly confined to the orbital plane. Given this configuration, the satellites should interact with their respective neutral torii, reabsorbing some of the neutrals. Johnson et al. (1989) estimate that less than 10% of the sputtered neutrals will be reabsorbed by the satellite.

Contributions to the water density in the inner torus due to the satellites Enceladus, Tethys, Dione and Rhea.

Cross section of the water density in the inner torus due to the satellites Enceladus, Tethys, Dione and Rhea.

Ionization of the icy satellite neutral torus leads to the formation of an inner plasma torus. Like the neutral water densities, the water ion densities peak around the orbital locations of the satellites. Johnson et al. (1989) concluded that ions from the icy satellites could not account for the ion densities observed in the inner regions of the magnetosphere. They infer that an additional source of neutrals was needed in the region to account for the difference, possibly ring erosion.

Contributions to the water ion density in the inner torus due to the satellites Enceladus, Tethys, Dione and Rhea (right).

The Rings

For radial distances from Saturn less than about 4 Rs, micrometeorite erosion of the main rings may be a significant source of water molecules and water ions. The neutral densities produced by this process can be significant. Neutral density results from the model of Pospieszalska and Johnson show a peak density of about 4 x 10^4 cm^-3 located around a radial distance of 1.4 Rs. At low radial distances the cloud is compressed by Saturn's gravitational pull. The cloud expands as the gravitational field falls off at larger radial distances. This cloud may result in an injection of water molecules into Saturn's atmosphere and is though to be a source of some of the transient features observed in the rings.

Contours of water density through a cross section of the inner torus (r < 3.5 Rs) associated with Saturn's rings and including contributions from the satellites Enceladus, Tethys, Dione and Rhea.

Ionization of the water within the ring cloud forms a heavy ion torus within the orbit of Enceladus (about 4 Rs). The plasma in this torus has a relatively low temperature. It will interact with the water neutral in the cloud and neutral hydrogen escaping from Saturn's atmosphere (see Richardson et al. 1986; Pospieszalska and Johnson, 1991). In these respects the plasma acts similar to ionospheric plasma. However, the structure and distribution of this plasma is typical of toroidal magnetospheric plasma (Pospieszalska and Johnson, 1991).

The plasma density results from the model of Pospieszalska and Johnson show a peak density of 3.5 x 10^4 cm^-3 at roughly the same radial distance as the peak neutral density (1.4 Rs). The magnetospheric fields dominate are more important to the ion dynamics then the gravitational force. This results in much less constriction of the plasma torus near Saturn. In addition the plasma expands less at larger radial distances then the neutral cloud does. There is also less spread out in the radial direction than the neutrals.

Contours of water ion density through a cross section of the inner torus (r < 3.5 Rs) associated with Saturn's rings and including contributions from the satellites Enceladus, Tethys, Dione and Rhea.

Beyond a radial distance of 2.3 Rs both the neutral and plasma densities begin to fall off. At this location the speed of the plasma corotating with the magnetosphere is larger than the escape speed of a neutral. Once an ion at these distances is neutralized it escapes the Saturnian system (baring further ionization). This reduces the neutral density in this region and hence the ion density sense there are less neutrals to ionize. This process is efficient at removing debris from the region near Saturn and may be the determining factor in the location of the outer edge of the main ring Pospieszalska and Johnson, 1991.


Neutral gas and thermal plasma is thought to exist in the magnetosphere of Saturn and to have substantial densities in some locations. The picture of neutral and plasma torii presented in this poster is base on a rather small set of data and extensive modeling and an fresh infusion of data from the Cassini mission will be needed to either confirm it or modify it. The Cassini INMS will be able to carry out critical, and unique, in situ measurements of the neutral gas and its composition in the magnetosphere, and INMS will also be able to assist in the characterization of the thermal plasma by measuring low energy ions and their composition.


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