Karen E. Simmons

Photopolarimetry from voyager 2; preliminary results on saturn, titan, and the rings.

The Voyager 2 photopolarimeter was reprogrammed prior to the August 1981 Saturn encounter to perform orthogonal-polarization, two-color measurements on Saturn, Titan, and the rings. Saturns atmosphere has ultraviolet limb brightening in the mid-latitudes and pronounced polar darkening north of 65�N. Titans opaque atmosphere shows strong positive polarization at all phase angles (2.7� to 154�), and no single-size spherical particle model appears to fit the data. A single radial stellar occultation of the darkened, shadowed rings indicated a ring thickness of less than 200 meters at several locations and clear evidence for density waves caused by satellite resonances. Multiple, very narrow strands of material were found in the Encke division and within the brightest single strand of the F ring.

Photometric observations of jupiter at 2400 angstroms.

The photopolarimeter instrument on Voyager 2 was used to obtain a map of Jupiter at an effective wavelength of 2400 angstroms. Analysis of a typical north-south swath used to make this map shows strong absorption at high latitudes by a molecular or particulate constituent in the Jovian atmosphere. At 65� north latitude, the absorbing constituent extends to altitudes above the 50-millibar pressure level.

Mariner 9 ultraviolet spectrometer experiment - Pressure-altitude measurements on Mars

Abstract Ultraviolet spectrometer measurements of the reflectance at 3050 A are modeled to give pressure-altitudes for Mars assuming a quiescent atmosphere. Ultraviolet light that is Rayleigh-scattered by the Mars molecular atmosphere, with allowance for uniform turbidity, is proportional to surface pressure independent of atmospheric temperature structure. All model constants except the over-all scaling factors are found by requiring ultraviolet spectrometer pressures of 47 locations on the planet to be the same when measured at different geometries. The overall scaling factor is found by intercomparison with Mariner 9 occultation pressures. Comparison with other Mars pressure-altitude measurements show deviations from the assumption of uniform turbidity to occur over the Hesperia plateau for ultraviolet measurements obtained during the 13–26 February 1972 time period.

Features in Saturn's rings

A systematic, uniform search of Voyage 2 photopolarimeter system (PSS) data set for all significant features of Saturns rings is described. On August 25, 1981, the PSS observed the occultation of the star Delta Scorpii by the rings of Saturn, and the timing of the data taking was rapid enough that the spatial resolution in the radial direction in the ring plane was better than 100 m. Tabular information and figures for 216 significant features that were found are presented.

High-phase-angle observations of Neptune at 2650 and 7500 Å: Haze structure and particle properties

Abstract Spatially resolved Voyager 2 PPS photometric and polarimetric observations of Neptune at 2650 and 7500 A are presented and analyzed. A vertically homogeneous model with Rayleigh scattering and absorption fit the 2650 A at 137° phase and below, but not the 159° phase data. The 159° phase data imply stratospheric particles of mean radius 0.20 ± 0.05 μm, with a best-fit imaginary refractive index at 2650 A of 0.03. The optical depth of the stratospheric haze in the 5- to 100-mbar region is 0.19 ± 0.08 at 2650 A. The 7500 A data at latitude - 25° constrain the stratospheric haze mean radius to be less than 0.4 μm, with a best-fit value of 0.25 μm. Stratospheric haze optical depth at 7500 A is 0.05 ± 0.02. A continuum absorber is abundant in the troposphere. If it is confined to the region below the methane haze layer, the optical depth of the methane haze can be no larger than about 0.8 (too much limb brightening is produced for larger optical depths). The methane haze optical depth at 7500 A must be greater than 0.1. Models with this optical depth as large as 3 produced too much limb darkening if the continuum absorber is mixed in both the methane haze and deeper cloud. Tropospheric aerosols have phase functions with shalloow backward lobes, described by a double Henyey-Greenstein parameterization with g2 (the asymmetry parameter for the backward lobe) in the range −0.07 to −0.22. The planetary phase integral (the ratio of the spherical albedo to the geometric albedo) at 7500 A is 1.4 ± 0.1; at 2650 A it is 1.26 ± 0.05.

Voyager photopolarimeter observations of Uranian ring occultations

Abstract The Voyager 2 photopolarimeter (PPS) observed the stars σ Sagitarii and§b Persei as they were occulted by the Uranian ring system on January 26, 1986 (A. L. Lane et al. 1986, Science 233, 65–70). Both occultations passed through the ring system so that orbital radii were sampled at two different longitudes in the ring plane for each occultation. The σ Sagitarii occultation was a grazing occultation and only ϵ, Δ, and 1986U1R occulted the star. β Persei was occulted by each of the nine classical rings twice, and each of these rings was identified on ingress and egress, although the 6 ring in β Per ingress is a marginal detection, and the η ring in β Per ingress was not detected. The orbits of the nine classical rings have been successfully modeled by numerous Earth-based stellar occulations (R. G. French et al. 1988, Icarus 73, 349–378), and Voyager occultation profiles of the rings from the radio science occultation (RSS, D. L. Gresh et al. 1989, Icarus 78, 131–168) and the ultraviolet spectrometer occultations (UVS, J. B. Holberg et al. 1987, Astron. J. 94, 178–188) have been published. The PPS occultations provide high resolution profiles of the rings (∼ 10 m/point for σ Sgr, ∼100 m/point for β Per) which contain information on waves (L. J. Horn et al. 1988, Icarus 76, 485–492), edge sharpness, vertical thickness, and azimuthal variations in radial structure within the rings. The PPS profiles of the e ring show structure similar to that seen in the radio, UVS, and Earth-based occultations, although the magnitude of peaks and valleys in optical depth changes with azimuth. 1986U1R, γ, η, α, and 6 all show varying degrees of azimuthal inhomogeneity. The η ring is not present in the β Per ingress data although it shows up clearly in egress, and the γ ring is ony 0.6 km wide at β Per ingress and is opaque to the starlight while on the egress cut it is 2.6 km wide. We also present the results of a rigorous statistical search of the data for previously undetected rings. Several features were identified in the β Per data that were more statistically significant than some of the known ring profiles (6 and η in β Per ingress), but the low signal-to-noise ratio for this occultation prevents the identification of any new rings.

Galileo ultraviolet spectrometer observations of Jupiter's auroral spectrum from 1600–3200 Å

In 1996 and 1997 the Galileo Ultraviolet Spectrometer (UVS) obtained the first measurements of Jupiters nightside midultraviolet (MUV) polar auroral spectrum from 1620 to 3231 A at 13 A resolution. The reduced polar spectra, after removal of off-axis scattered radiation from the sunlit dayside of Jupiter, contain a spectrum that matches laboratory spectra of the H 2 continuum in the a-b dissociative emission transition. This is the first direct identification of the H 2 a-b transition in astronomy. The a-b emission is excited by electron impact exchange reactions with H 2 that peak in cross section near 15 eV. The emission threshold is at 1216 A, and the continuum peaks in intensity in the 2000-2500 A range. Jupiters observed wavelength-integrated MUV H 2 a-b emissions (1620-3231 A) have a photon flux ∼8 times smaller than simultaneously observed wavelength-integrated far-ultraviolet (FUV) H 2 band emissions (1230-1650 A). Because the FUV H 2 emissions have an emission cross section that peaks at higher energies near 50 eV, this FUV/MUV brightness ratio is diagnostic of the secondary electron energy distribution and is consistent with a warm distribution of electrons.

Evidence for material between Saturn's A and F rings from the Voyager 2 photopolarimeter experiment

Abstract The region in the Saturn system between the F ring and the outer edge of the A ring is an area that appears, in images from the imaging experiment, to be virtually devoid of material except for three small satellites. Near the orbit of 1980S28, Atlas—the innermost satellite—the Voyager Photopolarimeter Stellar Occultation data show a discontinuity in count rate which marks a boundary between the tenuous materials near the outer edge of the A ring and the orbit of Atlas. The data pertaining to this region have been examined with the aid of statistics and models generated from other similar ring structures. It is concluded that the discontinuity is real, implying the existence of tenuous material of normal optical depth of 0.01 to 0.006 in this region.

Solar wind latitude variations and multiple scattering from Galileo interplanetary Lyman-α observations

Abstract The Galileo Ultraviolet Spectrometer Experiment (UVS) obtained a map of the celestial sphere from interplanetary Lyman-α (IP Lα) on 13,14 December 1990 during the Earth1 encounter. The Galileo spacecraft was near the downwind interstellar axis during the encounter and the map view directions filled the downwind hemisphere. The ratio between the observation and a single scattering model is attributed to a direct measurement of the multiple scattering correction required to model IP Lα in the inner solar system. Analysis of this data set, referred to as an antisun map, shows that the solar wind charge exchange rate with interplanetary gas is 25% less over the solar poles than in the ecliptic at solar maximum. A model of the interstellar wind based on the antisun map observation exhibits a number density of atomic hydrogen far from the solar system, inside the heliosphere, of 0.16 ± 0.05 cm −3 .

Voyager photopolarimeter observations of Saturn and Titan

Abstract The Voyager 2 photopolarimeter experiment observed the intensity and polarization of scattered sunlight from the atmospheres of Saturn and Titan in the near-UV at 2640 A and in the near-IR at 7500 A. Measurements of Saturns limb brightening and polarization at several phase angles up to 70° indicate that a significant optical depth of UV absorbers are present in the top 100 mbar of Saturns atmosphere in the Equatorial Zone and north polar region, and possibly at other latitudes as well. UV absorbers are prominent in polar regions, suggesting that charged particle precipitation from the magnetosphere may be important in their formation. The whole-body polarization of Titan is strongly positive in both the UV and near IR. If spherical particles are responsible for the polarization, no single size distribution or refractive index can account for the polarization at both wavelengths. The model atmosphere proposed by Tomasko and Smith [1], characterized by a gradient in particle size with altitude, seems capable of explaining the Voyager observations. If non-spherical particles predominate, the Voyager observations place important constraints on their scattering properties.