Region Of Saturn Research Articles

Variability in the H +3 emission of Saturn: Consequences for ionisation rates and temperature

We present an analysis of observations of the auroral/polar regions of Saturn, carried out in 1999, 2004 and 2005, making use of the facility spectrometer, CGS4, on the United Kingdom Infrared Telescope (UKIRT), Mauna Kea, Hawaii. We obtain temperatures of 380 ( ± 70 ) K in 1999 and 420 ( ± 70 ) K in 2004. (The 2005 data has insufficient spectral resolution for a temperature determination to be made.) Our most probable interpretation is that the temperature of Saturn's auroral/polar H + 3 layer should be taken as 400 ( ± 50 ) K. This is lower than the value obtained by Miller et al. [Miller, S., and 10 colleagues, 2000. Philos. Trans. R. Soc. 358, 2485–2502], which is shown to be in error. Our analysis reveals clearly that the line emission due to H + 3 varies considerably, showing nearly an order of magnitude increase when one compares the data obtained in 1999 with those of 2004. Our conclusion is that this variability is (mainly) due to the changing H + 3 column density. By analogy with modelling results obtained for Jupiter, we estimate that the particle (keV electron) precipitation experienced by Saturn must be ∼20 times greater in 2004 than in 1999, to produce this additional ionisation. The H + 3 emission increases, but this is insufficient to offset most of the heating due to the extra particle precipitation, indicating that this ion does not act as a “thermostat” on Saturn, in the same way that it does on Jupiter.

Ion winds in Saturn's southern auroral/polar region

We present profiles of the line-of-sight (l.o.s.) ionospheric wind velocities in the southern auroral/polar region of Saturn. Our velocities are derived from the measurement of Doppler shifting of the H 3 + ν 2 Q(1,0 −) line at 3.953 microns. The data for this study were obtained using the facility high-resolution spectrometer CSHELL on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, during the night of February 6, 2003 (UT). The l.o.s. velocity profiles finally derived are consistent with an extended region of the upper atmosphere sub-corotating with the planet: the ion velocities in the inertial reference are only 1/3 of those expected for full planetary corotation. We discuss the results in the light of recent proposals for the kronian magnetosphere, and suggest that, in this region, Saturn's ion winds may be under solar wind control.

Radial diffusion and losses of energetic protons in the 5 to 12 RS region of Saturn's magnetosphere

Phase space density profiles for protons with first invariants µ = 600 and 3000 MeV/G (integral invariants K = 0.30 and 0.003 G½ RS) previously derived from measurements by the University of Iowa detector G on Pioneer 11 during the 1979 Saturn encounter are analyzed using solutions of the time‐averaged radial diffusion equation in a dipolar planetary magnetic field. A series of loss models ranging from satellite absorption only to satellite plus ring E absorption and added distributed losses is assumed and the corresponding form of the time‐averaged radial diffusion coefficient D(L) (taken to be of the form D(L) = DoLn, where n is an integer) is determined in each case by a minimum‐variance fit to the data‐derived phase space density profiles. The inferred forms for D(L) are characterized by a low‐order L‐dependence (∼L³–L6) and a relatively high amplitude (Do ≃ 10−9–10−8 RS² s−1) in approximate agreement with earlier studies. Loss models that include only satellite absorption result in pronounced Dione macrosignatures in the model phase space density profiles that are not present in the data profiles, indicating a need for additional distributed losses. Absorption by ring E would be sufficient to explain the absence of a Dione macrosignature only if both the presently accepted maximum normal optical depth and mean ring particle size are increased by at least 1 order of magnitude. Charge exchange losses, estimated from observational upper limits on the concentrations of relevant neutral species, also appear to be only marginally important. Pitch angle scattering precipitation losses occurring within the orbit of Rhea at a rate of at least 0.01 times that of the strong pitch angle diffusion limit could in principle provide the necessary distributed losses.

Energetic neutral particles from Jupiter and Saturn

The Voyager 1 spacecraft has detected energetic neutral particles escaping from the magnetospheres of Jupiter and Saturn. These energetic neutrals are created in charge exchange reactions between radiation belt ions and ambient atoms or molecules in the magnetosphere. If the Io torus is assumed to be the dominant Jovian source region for energetic neutrals, the Voyager observations can be used to infer upper limits to the average ion intensities there below about 200 keV. No readily interpretable in‐situ measurements are available in the Io torus at these energies. The middle and outer Jovian magnetospheres may also be a significant source of energetic neutrals. At Saturn, the observed neutral particle count rates are too high to be explained by charge exchange between fast protons and H atoms of the Titan torus. Most of the energetic neutrals may be produced by charge exchanges between heavy ions and a neutral cloud containing H2O in Saturn's inner magnetosphere. If so, the Voyager measurements of energetic neutral fluxes would be the first detected emissions from this region of Saturn's magnetosphere.

Bending waves and the structure of Saturn's rings

The surface mass density profiles at four locations within Saturn's rings are calculated using Voyager spacecraft images of spiral bending waves. Bending waves are vertical corrugations in Saturn's rings which are excited at vertical resonances of a moon, e.g., Mimas, whose orbit is inclined with respect to the mean plane of the rings. Bending waves propagate toward Saturn by virtue of the rings' self-gravity; their wavelength depends on the local surface mass density of the rings. Observations of bending waves can thus be used to determine the surface density in regions of Saturn's rings near vertical resonances. The average surface density of the outer B ring near Mimas' 4:2 inner vertical resonance is 54 ± 10 g cm −2. Surface density in this region probably varies by ∼ 30% over radial length scales of tens of kilometers; and irregular radial structure is present on similar length scales in this region. Surface densities ranging from 24 g cm −2 to 45 g cm −2 are found in the A ring. Small scale variations in surface density are not seen in the A ring, consistent with its more uniform optical appearance.

Plasmas in Saturn's magnetosphere

The solar wind plasma analyzer on board Pioneer 11 provides first observations of low‐energy positive ions in the magnetosphere of Saturn. Measurable intensities of ions within the energy per unit charge (E/Q) range 100 eV to 8 keV are present over the planetocentric radial distance range ∼4–16 Rs in the dayside magnetosphere. The plasmas are found to be rigidly corotating with the planet out to distances of at least 10 Rs. At radial distances beyond 10 Rs, the bulk flows appear to be in the corotation direction but with lesser speeds than those expected from rigid corotation. At radial distances beyond the orbit of Rhea at 8.8 Rs, the dominant ions are most likely protons, and the corresponding typical densities and temperatures are 0.5 cm−3 and 106 °K, respectively, with substantial fluctuations. Identification of the mass per unit charge (M/Q) of the dominant ion species is possible in certain regions of Saturn’s magnetosphere via the angular distributions of positive ions. A large torus of oxygen ions is located inside the orbit of Rhea, and the densities are >10 cm−3 over the radial distance range ∼4–7.5 Rs. Density maxima appear at the orbits of Dione and Tethys where oxygen ion densities are ∼50 cm−3. The dominant oxygen charge states are O2+ and O3+ in the radial distance ranges ∼4–7 Rs and 7–8 Rs, respectively. The observations are suggestive of a decrease of ion energies to values less than the instrument energy threshold of E/Q=100 eV at the apparent inward edge of the torus at 4 Rs. Ion temperatures increase rapidly from ∼2 × 105 °K at 4 Rs to 5 × 106 °K at 7.3 Rs. It is concluded that the most likely source of these plasmas is the photodissociation of water frost on the surface of the ring material with subsequent ionization of the products and radially outward diffusion. The sources associated with the satellites Dione and Tethys are probably of lesser strength. The presence of this plasma torus is expected to have a large influence on the dynamics of Saturn's magnetosphere, since the pressure ratio β of these plasmas approaches unity at radial distances as close to the planet as 6.5 Rs. On the basis of these observational evidences it is anticipated that quasi‐periodic outward flows of plasma, accompanied by a reconfiguration of the magnetosphere beyond ∼6.5 Rs, will occur in the local night sector in order to relieve the plasma pressure from accretion of plasma from the rings.

Five-color photometry of Saturn and its rings

Analysis of 206 high-quality plates from three recent apparitions taken in five colors has yielded several photometric parameters for Saturn and its A and B rings. Phase curves and geometric albedos are derived for two regions of Saturn and for each ring. The phase coefficients of the rings are found to be independent of the ring-plane inclination angle. A comparison of the phase curves shows that the particles of ring A exhibit a larger phase coefficient than do those of ring B. When examined with a multiple-scattering model using Henyey-Greenstein phase functions, the observations of the ring tilt effect indicate that the particles of ring A may also have lower single-scattering and geometric albedos. The color dependence of the geometric albedo of the particles in ring B is shown to be very similar to that of Europa (J II). We find for ring A an optical thickness of 0.50 (0.45 ≤ τ A ≤ 0.57) and for the Cassini division, 0.018 ± 0.004.

The spectral characteristics and probable structure of the cloud layer of Saturn

Spectrophotometric (photographic and photoelectric) measurements of the intensity of CH4 absorption bands at 6190 and 7250 Å over different regions of Saturn's disk show an increase in intensity toward the poles and a decrease toward the equatorial limb. In the bright equatorial belt of Saturn the methane absorption is about 25–28% less than in the south temperate belt (latitude about −20°).Absolute photoelectric spectrophotometry of Saturn's disk gives a value for the single-scattering albedo of the aerosol particles at λ 6200–6500 Å at the center of the disk. Calculations of the curves of growth for the absorption lines formed in the thin gas and in the cloud layer were made and the comparison with observations of Jupiter and Saturn lead to the mean value of the volume scattering coefficient of the aerosol layer σa < 5 × 10−6 cm−1. In the equatorial region of Saturn σa is larger than in the temperate region by a factor 1.3 to 1.8.The models of Saturn's atmosphere that fit well the observational data preclude the condensation of methane. An aerosol layer of ammonia is more probable in the atmosphere of Saturn. Calculations of the distribution of the ammonia aerosol volume density (Qa) give Qa ≃ 10−9 to 10−7 gm/cm3 for relative abundances of ammonia A = 10−6 to 10−4. Observational estimates of Qa derived from the values of σa give Qa < 10−9 gm/cm3. Apparently Saturn's atmosphere departs from conditions of ordinary convection.Some very interesting variations in the spectral reflectivity of the different regions of Saturn are observed, especially in the ultraviolet. These data, as well as a systematic study of the methane absorptions in the near infrared strong bands are needed in future studies of Saturn's atmosphere.