Doyle Hall

Ultraviolet Spectrometer Observations of Neptune and Triton

Results from the occultation of the sun by Neptune imply a temperature of 750 � 150 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane, acetylene, and ethane at lower levels. The ultraviolet spectrum of the sunlit atmosphere of Neptune resembles the spectra of the Jupiter, Saturn, and Uranus atmospheres in that it is dominated by the emissions of H Lyman α (340 � 20 rayleighs) and molecular hydrogen. The extreme ultraviolet emissions in the range from 800 to 1100 angstroms at the four planets visited by Voyager scale approximately as the inverse square of their heliocentric distances. Weak auroral emissions have been tentatively identified on the night side of Neptune. Airglow and occultation observations of Tritons atmosphere show that it is composed mainly of molecular nitrogen, with a trace of methane near the surface. The temperature of Tritons upper atmosphere is 95 � 5 kelvins, and the surface pressure is roughly 14 microbars.

A comparison of the Voyager 1 ultraviolet spectrometer and plasma science measurements of the Io plasma torus

We have developed a sophisticated package (Colorado Io Torus Emissions Package, or CITEP) to simulate emissions from the Io plasma torus, and have used it to examine the consistency of Voyager in situ and remote observations. CITEP merges ion composition derived from the ultraviolet spectrometer line ratios with measurements of electron densities, electron temperatures, and ion temperatures obtained by the plasma science instrument. The program then predicts the brightness and morphology of torus EUV emissions. We find that the measured brightness is approximately 2 times larger than the model predicts. When scaled up by this factor, the morphology of the model radial emission profile is consistent with the data, but somewhat less sharply peaked at the ribbon location. We examine several possible causes for the brightness discrepancy such as calibration errors, measurement accuracy, inaccuracies of the model, and torus variability, but find no definitive cause.

HOPKINS ULTRAVIOLET TELESCOPE DETERMINATION OF THE IO TORUS ELECTRON TEMPERATURE

Sulfur ion emissions from the Io plasma torus observed by the Hopkins Ultraviolet Telescope (HUT) in 1990 December have been analyzed to determine the effective temperature of the exciting electrons. Spectra were obtained with a long slit that extended from 3.1 to 8.7 Jupiter radii R(sub J) on both dawn and dusk torus ansae. The average temperature of electrons exciting S(2+) emissions from the dawn ansa is (4800 +/- 2400) K lower than on the dusk ansa, a dawn-dusk asymmetry comparable in both sign and magnitude to that measured by the Voyager Ultraviolet Spectrograph (UVS) experiment. Emissions from S(2+) ions are generated in a source region with electron temperatures in the range 32,000-56,000 K; S(3+) ion emissions are excited by electrons that average 20,000-40,000 K hotter. This distinct difference suggests that the S(3+) emission source region is spatially separate from the S(2+) source region. Estimated relative aperture filling factors suggest that the S(3+) emissions originate from a region more extended out of the centrifugal plane than the S(2+) emissions.

Time-Resolved I-Band Photometry of Calibration Spheres and NaK Droplets

Wehave developed a program to obtain photometry of Earth-orbiting satellites using the U.S. Air Force advanced electrooptical system 3.6m telescope’s Visible Imager instrument, which acquires time-series charge-coupled device images at typical rates of 0.1 to 2.1 Hz. Observations of 12 spherical satellites provide measurements of I-band albedos (i.e., reflectances) and evaluation of surface nonuniformities. Best-fit specular diffuse albedo models enable brightness predictions and a means to grade the quality of the spheres as photometric calibration reference objects. CalSphere-4A appears to be the highest quality calibrator of the 12, with the best-fitmodel reproducing 90% of the measurements to within 0:03 stellar magnitudes. This four-decade-old sphere reflects in a predominantly diffuse fashion and has a total albedo of 67% that is somewhat lower than expected for fresh white paint, effects likely due to space weathering. The aluminum Lincoln Calibration Sphere-1 shows predominantly specular reflection (albedos 59%) but with 0:3 magnitude deviations between data and model that recur every 30–90 s, likely due to one or more surface irregularities. Three other predominantly specular aluminum spheres display less frequent but larger amplitude deviations. Observations indicate that sodium/potassium droplets leaked from Soviet radar ocean reconnaissance satellite reactors are highly reflective specular spheres that can also serve as optical calibrators.

Characterization of the near-Earth Asteroid 2002 NY40

Abstract : In August 2002, the near-Earth asteroid 2002 NY40, made its closest approach to the Earth. This provided an opportunity to study a near-Earth asteroid with a variety of instruments. Several of the telescopes at the Maui Space Surveillance System were trained at the asteroid and collected adaptive optics images, photometry and spectroscopy. Analysis of the imagery reveals the asteroid is triangular shaped with significant self-shadowing. The photometry reveals a 20-hour period and the spectroscopy shows that the asteroid is a Q-type.