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NASA NTRS1980

If you've seen one magnetosphere, you haven't seen them all - Energetic particle observations in the Saturn magnetosphere

The present paper deals with a study of Saturn's magnetosphere, using data of the Pioneer 11 Cosmic ray experiment. At the orbit of Saturn, the nominal energy density of the interplanetary magnetic field and of the solar wind has decreased by 2 orders of magnitude over their value at 1 AU. The Pioneer inbound trajectory near the noon meridian and the outbound trajectory toward the dawn meridian reveal a magnetosphere which is in many aspects similar to that of the earth, but has important differences due to the imprint of Saturn's moons and rings. The magnetotail and polar regions were not observed. However, the presence of solar cosmic rays offers strong evidence for an open magnetotail configuration. The detection of energetic particles at Saturn means that 4 of the 6 planets inside 10 AU possess stable magnetospheres.

Mcdonald, F. B.↗
NASA NTRS1981

Extreme ultraviolet observations from Voyager 1 encounter with Saturn

Hydrogen Lyman-alpha, helium and molecular hydrogen band ultraviolet emissions from Saturn and the Titan atmosphere are considered. The Saturn H2 band excitation mechanism, while qualitatively similar to that of Jupiter, is closely related to the solar flux. The occurrence of auroras at 80 deg latitude suggests an earth-like magnetotail activity. No ion emissions have been detected from the Saturn magnetosphere. Although nitrogen emissions excited by particles have been detected in the Titan dayglow and bright limb scans, no such emission has been detected from its dark atmosphere. Enhancement of the nitrogen emission is observed in the region of interaction between Saturn's corotating plasmasphere and the Titan atmosphere.

Broadfoot, A. L.↗
NASA NTRS1981

Energetic charged particles in Saturn's magnetosphere - Voyager 1 results

Voyager 1 provided the first look at Saturn's magnetotail and magnetosphere during relatively quiet interplanetary conditions. This report discusses the energetic particle populations of the outer magnetosphere of Saturn and absorption features associated with Titan and Rhea, and compares these observations with Pioneer 11 data of a year earlier. The trapped proton fluxes had soft spectra, represented by power laws in kinetic energy with an exponent of 7 in the outer magnetosphere and 9 in the magnetotail. Structure associated with the magnetotail was observed as close as 10 Saturn radii on the outbound trajectory. The proton and electron fluxes in the outer magnetosphere and in the magnetotail were variable and appeared to respond to changes in interplanetary conditions. Protons with energies greater than or approximately equal to 2 MeV had free access to the magnetosphere from interplanetary space and were not stably trapped outside about 7.5 Saturn radii.

Vogt, R. E.↗
NASA NTRS1981

High-spectral-resolution imagery of Saturn

High-spectral-resolution images of Saturn over the wavelengths of the 6196.8-A feature of CH4 were obtained and used to extract line profiles for the observed feature over the disk of Saturn and to determine the spatial variation of the equivalent width at the time of the observations. The result of the observations is consistent only with a single class of models for the vertical structure of the atmosphere of Saturn, namely, the homogeneous scattering models. Reflecting-layer or two-layer models are at a striking variance with the observations. With these data, the requirements for new observations to determine the finer details of the vertical structure of Saturn's atmosphere can be specified.

Smith, W. H.↗
NASA NTRS1981

Saturn's ring current and inner magnetosphere

The Voyager 1 magnetic field observations at Saturn are shown in a graph. The departure of the oberved magnetic field from the field of a dipole is considered. The observed field magnitude is appreciable less than that of the model dipole at small radial distances and greater than the model dipole in the more distant magnetosphere. These characteristics can be understood by introducing a model current system similar to a system originally applied to observations of the Jovian magnetic disk. Saturn's ring current has important implications for charged-particle motion in Saturn's magnetosphere, particularly the absorption of trapped radiation by its many satellites and rings. The absorption signature observed by the Voyager 1 cosmic ray experiment near the orbital position of Rhea illustrates well the effects of Saturn's ring current on charged particle trajectories.

Connerney, J. E. P.↗
NASA NTRS1981

Measurement of stratospheric aerosol on Saturn using an eclipse of Titan

Measurements of the aerosol content in the Saturn atmosphere made during an eclipse of Titan by Saturn are presented. Observations of the ingress of Titan into the shadow of Saturn on December 20.08, 1979 were made by the 74-in telescope at Kottamia Station in Egypt at an effective wavelength of 6728 A with FWHM of 1000 A. The deviation of the observed light curve after a satellite darkening of 3 mag from that predicted for a clear Saturn atmosphere is interpreted in terms of an aerosol layer decreasing in extinction coefficient from a base altitude 5 km below the planetary tropopause to an altitude of up to 75 km above the tropopause. For an aerosal layer extending to 75 km above the tropopause calculations reveal possible optical depths ranging from 0.020 to 0.050. The range of the optical depth is comparable to that of the Jupiter atmosphere, and suggests that similar processes may be responsible for the stratospheric haze on both planets.

Smith, D. W.↗
NASA NTRS1981

Voyager at Saturn

The spacecraft design and operations and mission results of the Voyager Project are reviewed with particular emphasis on the results of the Saturn encounters. The mission was undertaken by two spacecraft: Voyager 2, launched first on August 20, 1977 on a lower energy trajectory to Jupiter, Saturn, Uranus and Neptune; and Voyager 1, launched on September 5, 1977 on a higher energy trajectory to closer flybys of Jupiter and Saturn. The Voyager spacecraft make use of earlier Mariner and Viking Orbiter design concepts, with additional capabilities for solar independent power, long-range communications and autonomy as well as equipment for the 11 science investigations. The Voyager operations system consists of a ground data system, missions operations system, and support by the Mission Control and Computer Center and Deep Space Network. Among the results of the Jupiter encounter are data concerning the magnetospheric structure and atmospheric dynamics of the planet and the surface features of the Galilean satellites. The encounters with Saturn produced scientific data concerning wind speeds, auroral emissions, and radio emissions from the planet, the fine structure of the previously known and newly discovered rings, the surface features of the satellites and the atmospheric composition and conditions of Titan. Voyager 1 is presently making field and particle measurements on its flight into interstellar space, while Voyager 2 is headed for an encounter with Uranus in January 1986 and Neptune in August 1989.

Heacock, R. L.↗
NASA NTRS1982

Evidence for solar wind control of Saturn radio emission

Using data collected by the Voyager 1 and 2 spacecraft in 1980 and 1981, strong evidence is presented for a direct correlation between variations in the solar wind at Saturn and the level of activity of Saturn's nonthermal radio emission. Correlation coefficients of 57 to 58% are reached at lag times of 0 to 1 days between the arrival at Saturn of high pressure solar wind streams and the onset of increased radio emission. The radio emission exhibits a long-term periodicity of 25 days, identical to the periodicity seen in the solar wind at this time and consistent with the solar rotation period. The energy coupling efficiency between the solar wind with the Saturn radio emission is estimated and compared with that for Earth.

Desch, M. D.↗
NASA NTRS1982

Magnetic field studies by Voyager 2 - Preliminary results at Saturn

Results of Voyager 2 studies of the magnetosphere and planetary magnetic field of Saturn are presented. Magnetometer studies have confirmed the results obtained by Voyager 1, indicating the magnetic field to be that of a centered dipole of moment 0.21 gauss Saturn radii-cubed, tilted approximately 0.8 deg from the rotation axis and a maximum measured field intensity of 1187 nT at latitude 17.3 deg N just before periapsis. Voyager 2 observed multiple bow shock and magnetopause crossings during its inbound and outbound trajectories, which were complementary to those of Voyager 1, including magnetopause crossing at 18.5 Saturn radii on the inbound trajectory, and at 48.4-50.9 Saturn radii outbound indicative of magnetospheric expansion due to changing solar wind conditions. Throughout the outbound passage, the magnetospheric field was observed to be relatively steady and smooth, with no evidence for any azimuthal asymmetry or magnetic anomaly. Results thus are incapable of accounting for the observed periodic modulation of the Saturnian kilometric radio emissions.

Ness, N. F.↗
NASA NTRS1982

Plasma observations near Saturn - Initial results from Voyager 2

Results of plasma measurements made by Voyager 2 in the vicinity of Saturn are discussed and compared with those made by Pioneer 11 and Voyager 1 in a more limited range of latitudes. The initial bow shock crossing on the inbound trajectory closely agreed with the shock position inferred from the external ram pressure in the solar wind, although boundaries on the outbound pass were much further out than expected. Magnetospheric plasma observations reveal the presence of (1) shocked solar wind plasma in the magnetosheath between 30 and 22 Saturn radii; (2) a variable density region between 17 Saturn radii and the magnetopause; (3) an extended thick plasma sheet between 17 and 7 Saturn radii; and (4) an inner plasma torus probably originating from local sources. The ratio of heavy to light ions was observed to vary with distance to the equatorial plane in the dayside magnetosphere, with the heavy ions, probably O(+), more closely confined to the equatorial plane. The plasma data also account for the observed inner boundary of the neutral hydrogen torus discovered by Voyager 1.

Bridge, H. S.↗
NASA NTRS1982

Energetic charged particles in Saturn's magnetosphere - Voyager 2 results

Results from the cosmic-ray system on Voyager 2 in Saturn's magnetosphere are presented. During the inbound pass through the outer magnetosphere, the not less than 0.43-million-electron-volt proton flux was more intense, and both the proton and electron fluxes were more variable, than previously observed. These changes are attributed to the influence on the magnetosphere of variations in the solar wind conditions. Outbound, beyond 18 Saturn radii, impulsive bursts of 0.14to greater than 1.0-million-electron-volt electrons were observed. In the inner magnetosphere, the charged particle absorption signatures of Mimas, Enceladus, and Tethys are used to constrain the possible tilt and offset of Saturn's internal magnetic dipole. At approximately 3 Saturn radii, a transient decrease was observed in the electron flux which was not due to Mimas. Characteristics of this decrease suggest the existence of additional material, perhaps another satellite, in the orbit of Mimas.

Vogt, R. E.↗
NASA NTRS1983

A Pioneer-Voyager study of the solar wind interaction with Saturn

Voyager 1/2 observations are used to confirm and quantify the unusual bluntness of the dayside Saturn magnetopause suggested by the earlier Pioneer 11 measurements. Once corrected for variations in solar wind pressure, the Saturn observations are found to indicate a magnetospheric radius in the terminator plane equal to 1.8 times the nose distance. This ratio is 30% greater than the terrestrial value and is corroborated by the very blunt shape of the Saturn bow shock and large width of the forward magnetosheath. Possible causes and ramifications of these results for the solar wind interaction with Saturn are discussed.

Slavin, J. A.↗
NASA NTRS1983

Saturn's electrostatic discharges - Properties and theoretical considerations

The properties of Saturn's electrostatic discharges (SED) as observed by the Voyager Planetary Radio Astronomy experiment during the two Voyager encounters with Saturn are summarized. Several models for the formation of SED are discussed in light of these observations. The most likely source regions appear to be either the equatorial zone of the planet or the dense part of the B ring near 1.80 Saturn radii. The strengths and weaknesses of each of these possibilities are examined. Neither possibility accounts fully for the observed SED properties in a simple way. A search for an anomaly near 1.80 Saturn radii in the data of other experiments aboard Voyager has been carried out, and at least one and possibly more such experiments do indeed obtain anomalous data at this point in the ring system. There thus appears to be unexplained phenomena at this point, independent of the PRA data, and it is a short step to postulate that a single object may be the cause of all such phenomena.

Evans, D. R.↗
NASA NTRS1983

Saturn meteorology - A diagnostic assessment of thin-layer configurations for the zonal flow

Voyager imaging, infrared, and radio observations for Saturn have been recently interpreted by Smith et al. (1982) as an indication that the jet streams observed at the cloud tops extend to depths greater than the 10,000-bar level. This analysis assumes a maximum latitudinal temperature contrast of a few percent, a mean atmospheric rotation rate at depth given by Saturn's ratio period, and no variation with latitude of the bottom pressure level for the zonal flow system. These assumptions are not, however, firmly constrained by observation. The diagnostic analysis of plausible alternative configurations for Saturn's atmospheric structure demonstrates that a thin weather layer system (confined at mid to high latitudes to levels above 200 bar) cannot be excluded by any of the available observations. A quantitative estimate of the effects of moisture condensation (including the differentiation of mean molecular weight) suggests that these might provide the buoyancy contrasts necessary to support a thin-layer flow provided that Saturn's outer envelope is enriched approximately 10 times in water abundance relative to a solar composition atmosphere and strongly differentiated with latitude at the condensation level.

Allison, M.↗
NASA NTRS1983

Electrostatic discharges in Saturn's B-ring

The Voyager observations of electrical discharges in Saturn's rings strongly support earlier speculations on the role played by electrostatics, magnetic fields, and lightning phenomena in the primitive solar system. They also suggest conditions then by direct analogy rather than by extrapolating backwards through time from conditions now. The observed discharges show a pronounced 10h periodicity, which suggests a source in Keplerian orbit at 1.80 + or - 0.01 Saturn radii. In that region, the B ring is thicker than optical depth 1.8 for about 5,000 km. At 1.805 + or - 0.001 Saturn radii, however, the ring is virtually transparent for a gap of width 200 m. It is concluded that a small satellite orbits Saturn at that radius and clears the gap. The gap edges must prevent diffusive filling of the gap by fine material which is especially abundant at this position in the rings and would otherwise destroy the gap in minutes. The discharges represent the satellite's interaction with the outer edge of the gap. Spoke formation may involve the interaction of ring material in the vicinity of the gap.

Warwick, J. W.↗
NASA NTRS1983

The magnetosphere of Saturn

Characterizations of the Saturn magnetospheric activities that have been made possible through data gathered by means of the Pioneer 11 and Voyagers 1 and 2 flybys are reviewed. The spacecraft data confirmed the presence of a Saturn magnetosphere, which features inward diffusion and energization of trapped energetic particles as well as cosmic ray albedo neutron decay as a source of inner belt protons. A dense plasma was identified in the region covering the outer magnetosphere to the magnetopause, which extends outward 17.3-23.6 Saturn radii on the sunward side and from 30.3-70 radii on the far side, according to the satellite measurements. Characteristics of the particle populations and behaviors of the outer magnetosphere, the slot region, the inner magnetosphere, and the ring region are reviewed, as are those of the magnetic field, the solar wind-magnetospheric interaction, and the Titan-magnetospheric interaction are described. Further investigation of the longitudinal asymmetry of decimetric radio emissions from Saturn is recommended.

Schardt, A. W.↗
NASA NTRS1983

Origin and evolution of the Saturn system

A review is provided of current concepts concerning the formation of the Saturn system and the subsequent history of the planet, its satellites, and rings. Emphasis is placed upon numerical models of Saturn's evolution and interior models of its satellites. Alternative theories are presented and assessed for the origins of the Saturn system, the rings of Saturn, and the atmosphere of Titan.

Pollack, J. B.↗
NASA NTRS1983

Saturn's small satellites - Voyager imaging results

Voyagers 1 and 2 provided images of sufficient resolution for morphologic and photometric studies of Saturn's small satellites. These objects, all very difficult to observe from earth, orbit Saturn at distances of 2.3 to 6.3 Saturn radii (just outside the A ring to the orbit of Dione) and range in mean diameter from 22 to 188 km. All are irregularly shaped (long/short axis ratios range from 1.4 to 2.0) and probably heavily cratered. While impacts have apparently been important in shaping these objects, observed crater densities suggest that the present forms may have survived for about 4 billion years. Geometric albedos vary from 0.4 to at least 0.8. These albedos and the few color data available are similar to those of larger Saturn satellites which are known to have surfaces made predominantly of water ice. The range of observed albedos could be explained by minor variations in the amount of dark, opaque contaminants.

Thomas, P.↗