Richard L. Baron

Images of Excited H3+ at the Foot of the lo Flux Tube in Jupiter's Atmosphere

The electrodynamic interaction between lo and the Jovian magnetosphere drives currents to and from the planets ionosphere, where H3+ emission is excited. Direct images of this phenomenon were obtained with the ProtoCAM infrared camera at the National Aeronautics and Space Administrations 3-m Infrared Telescope Facility. The emissions are localized to the instantaneous foot of the lo flux tube, ≈8� equatorward of the more intense auroral H3+ emission associated with higher magnetic latitudes. The foot of the lo flux tube leads that of (undisturbed) model magnetic field lines passing through lo by 15� to 20� in longitude and is less visible in the northern hemisphere at longitudes where the surface magnetic field strength is greatest. These data favor the unipolar inductor model of the lo interaction and provide insight into the source location and generation of Jovian decameter radio emission.

Thermal maps of jupiter: spatial organization and time dependence of stratospheric temperatures, 1980 to 1990.

The spatial organization and time dependence of Jupiters stratospheric temperatures have been measured by observing thermal emission from the 7.8-micrometer CH4 band. These temperatures, observed through the greater part of a Jovian year, exhibit the influence of seasonal radiative forcing. Distinct bands of high temperature are located at the poles and mid-latitudes, while the equator alternates between warm and cold with a period of approximately 4 years. Substantial longitudinal variability is often observed within the warm mid-latitude bands, and occasionally elsewhere on the planet. This variability includes small, localized structures, as well as large-scale waves with wavelengths longer than ∼30,000 kilometers. The amplitudes of the waves vary on a time scale of ∼1 month; structures on a smaller scale may have lifetimes of only days. Waves observed in 1985, 1987, and 1988 propagated with group velocities less than �30 meters per second.

The oblateness of Uranus at the 1-μbar level

Stellar occulations by Uranus between 10 March 1977 and 23 March 1983 have been used to derive the radius and oblateness of Uranus at approximately the 1-gmbar level. The atmospheric occulation half-light times and scale heights were used, along with updated ring orbit model parameters, to determine the shape of the planetary limb as projected on the sky. A least-squares fit to the limb profile yielded an equatorial radius of Re = 26071 ± 3 km and an oblateness ϵ = (1 − Rp/Re) of 0.0197 ± 0.0010. The corresponding polar radius was Rp = 25558 ± 24 km. Assuming that the planet is in hydrostatic equilibrium and that J2 and J4 are as given by the ring orbit solution of R.G. French et al. (Icarus 73, 349–378 (1988), the inferred rotation period is 17.7 ± 0.6 hr for the latitude range (−30° < θ < 26°) sampled by the observations. This is consistent with G.F. Lindal et al.s (J. Geophys. Res. 92, 14,987–15,001 (1987)) period of 18.0±0.3 hr at θ = +5°, based on Voyager 2 observations, and with a possible equatorial subrotation predicted by P.L. Read (Quart. J. Roy. Met. Soc. 112, 253–272 (1986)) and supported by the cloud motion studies of B.A. Smith et al. (Science 233, 43–64 (1986)).

Photometric variability of Charon at 2.2 μm

Pluto-Charon images obtained on each of four nights at 2.2, 1.2, and 1.7 microns are presently fitted by a two-source image model in which the position of Charon and the ratio of its signal to that of Pluto are free parameters. At 2.2 microns, Charon is fainter than Pluto by magnitudes which, when combined with Pluto-Charon system photometry, yield apparent magnitudes of 15.01 + or - 0.08 for Charon at 0.06 lightcurve phase and 15.46 + or - 0.05 at lightcurve phase 0.42. In view of these results, Charon is variable in this filter bypass due to geometric albedo changes as a function of longitude.