Lawrence H. Wasserman

On the reduction of occultation light curves

Abstract In the first two sections of this paper, the two basic methods of reducing occultation light curves—curve fitting and inversion—are reviewed and compared. It is shown that the curve fitting methods have severe problems of nonuniqueness. In addition, in the case of occultation curves dominated by spikes, it is not clear that such solutions are meaningful. The inversion method does not suffer from these drawbacks. Methods of deriving temperature profiles from refractivity profiles are dealt with in the third section. It is shown that, although the temperature profiles are sensitive to small errors in the refractivity profile, accurate temperatures can be obtained, particularly at the deeper levels of the atmosphere. The final section contains a brief discussion of the ambiguities that arise when the occultation curve straddles the turbopause.

Effects of surface roughness on the photometric properties of mars

Abstract Starting with a macroscopically flat surface which at each point scatters according to the Minnaert law with exponent k, we investigate the photometric effects of increasing the large scale roughness of the surface. It is found that the photometric properties of macroscopically rough surfaces can still be described by the Minnaert law, but with an exponent k ∗ , in general not equal to k. In fact, examples are given where k ∗ differs considerably from k. Therefore observed values of the Minnaert exponent cannot be used to infer the small scale surface properties of Mars (or of any planet) unless proper allowance is made for the photometric effects of large scale roughness. The azimuthal dependence of k ∗ provides a sensitive test for the importance of these effects.

The occultation of beta Scorpii by Jupiter. II - The hydrogen-helium abundance in the Jovian atmosphere

The helium abundance in the Jovian atmosphere has been determined by a new method from our observations of the occultation of beta Scorpii by Jupiter. The occultation light curves, consisting of simultaneous records at three wavelengths with a time resolution of 0.01 sec, are dominated by sharp, well-defined intensity peaks or spikes. Because of the wavelength-dependence of the refractivity of the Jovian atmosphere, the arrival time of a given spike at the Earth is also wavelength dependent. A new method for determining the composition of a hydrogen-helium atmosphere from such data is presented, and the principal sources of error are discussed and evaluated.

On the upper atmosphere of Neptune

We have reanalyzed the Mount Stromlo observations of the occultation of BD 17 4388 by Neptune and find that the upper atmosphere is not isothermal as suggested by Freeman and Lynga. For a pure hydrogen atmosphere, our results give a temperature of 135 K at a number density of 1000 trillion per cu cm. Above this level, the possibility of a small overall positive temperature gradient is suggested by the data. The temperature structure is complicated in detail, with local 5 to 10 K fluctuations some 10 km in extent associated with the numerous spikes in the light curve. Concentrations of helium greater than 50 per cent can probably be ruled out.

Jovian Atmosphere: Structure and Composition between the Turbopause and the Mesopause.

The occultation of the star Beta Scorpii by Jupiter was observed at high time resolution in three wavelength channels. The results imply a temperature of 220�K at an altitude in the Jovian atmosphere corresponding to 1014 molecules per cubic centimeter, and temperature fluctuations of 2� to 10�K over vertical scales of 2 to 10 kilometers. They suggest that the vertical eddy diffusion coefficient near the turbopause has a lower limit of 7 x 105K square centimeters per second, and that the turbopause lies above the altitude where the density is 5 x 1013 molecules per cubic centimeter. Below the turbopause, the ratio of hydrogen to helium is consistent with cosmic abundances.

The upper atmosphere of Neptune - An analysis of occultation observations

Abstract An analysis of available observations of the April 7, 1968 occultation of BD −17° 4388 by Neptune yields upper atmosphere temperatures of ∼140°K near the 5 × 1014cm−3 level. The temperature structure of the atmosphere at these levels is complicated and nonisothermal. Diurnal temperature variations are certainly less than 10°K and may be absent. The average temperature decreases by less than 15°K between 0° and 55° latitude.

The occultation of Beta Scorpii by Jupiter. III - Simultaneous high time-resolution records at three wavelengths

Abstract Simultaneous high time-resolution records at three wavelengths of the May 13, 1971 Jupiter occultation of Beta Scorpii are presented. These observations are unique and contain important information about the structure and composition of the Jovian atmosphere. An equally favorable Jupiter occultation should not occur again for several centuries.

The Regulus occultation light curve and the real atmosphere of Venus

Abstract An inversion of the light curve observed during the July 7, 1959, occultation of Regulus by Venus leads to the conclusion that the light curve cannot be reconciled with models of the Venus atmosphere based on spacecraft observations.

The occulation of β Scorpii by Jupiter. IV. Diurnal temperature variations and the methane mixing ratio in the Jovian upper atmosphere

Abstract The nightime cooling of the Jovian atmosphere near the occulation level of 10 14 cm −3 is calculated using the models of Strobel (1973) and Strobel and Smith (1973) . The amount of cooling is found to depend on χ, the methane mixing ratio; μ the mean molecular weight; and the sunrise temperature. Using the range of sunrise (emersion) temperatures observed by Veverka et al . (1974) , the overnight cooling is calculated to be 1.5–5.5°K, if reasonable assumptions are made for χ and μ. The argument may be reversed to show that the agreement in measured sunrise and sunset temperatures obtained by other observers of the β Sco occulation implies that χ cannot be significantly greater than the generally accepted value of 7 ×10 −4 .

Galilean satellites: Observations of mutual occultations and eclipses in 1973

Abstract During the fall of 1973 we observed seven Galilean satellite mutual events: two occultations and two eclipses of Europa and three eclipses of Io. The observations were carried out simultaneously at three wavelengths (0.35, 0.50, and 0.91 μm) with a time resolution of 0.1 sec. Our principal aim was to obtain color information about albedo distributions and limb darkening on the satellites, but it is now clear from our analysis that before such information can be fully extracted from the observed light curves, values of the various geometric parameters (such as satellite radii and impact parameters) must be established accurately using all of the mutual-event data obtained during 1973–1974. At this stage, our observations do not yield any conclusive information about Ios limb darkening. For Europa, however, our best data indicate that this satellite is limb-darkened at both 0.50 and 0.91 μm. A variety of computer-generated model light curves are presented to clarify the question of how sensitive observed light curves are to the brightness distribution on the surface of the occulted or eclipsed satellite.