Stellar occultations

One of the advantages of the solar occultation method is that the concentrations are derived from the measurement of a ratio of 2 fluxes, and therefore are not substantially affected by instrument calibration errors or solar spectral features. Because of the observing geometry involved, this technique provides good vertical resolution. The major limitation results from the limited number of observations due to the sunrise or sunset contraints. Better coverage can be obtained by considering, in addition, lunar and stellar occultations. 

Figure 4-19b. Vertical profiles of chemical compounds retrieved from satellite observations based on occultation methods. Upper Panel Nighttime ozone number density (cm-3) measured on 24 March 2002 from the tropopause to the mesopause levels (15°N, 115°E) by the GOMOS instrument on board the ENVISAT spacecraft (stellar occultation). Courtesy of J.L. Bertaux and A. Hauchecorne, Service d Aeronomie du CNRS, France. Lower Panel Water vapor mixing ratio (ppmv) between the surface and 50 km altitude (33°N, 125°W) measured by SAGE II on 11 January 1987 (solar occultation). Courtesy of M. Geller, State University of New York.

A 1987 stellar occultation by Charon did not reveal an atmosphere, but it did provide a fairly good estimate of the diameter of this satellite. Stellar occultations of small objects are rare. Fortunately, in 1988 Pluto occulted a star and the light curve was observed from a number of ground-based telescopes and from the air-bome Kuiper observatory (Millis et al., 1993). Clear evidence of a tenuous atmosphere as well as fairly good estimates of the diameter were obtained. Another fortunate event was the alignment of Charon s orbital plane as seen from Earth. In 1988, that plane could be observed edge on and, consequently, Charon passed directly in front of Pluto and disappeared completely behind it. This orientation helped in the determination of the radii and in the separation of the spectra of both objects (Binzel Hubbard, 1997). 

Binzel, R. P. Hubbard, W. B. (1997). Mumal events and stellar occultations. In Pluto and Charon, 85-102, ed. S. A. Stem D. J. Tholen. Tucson University of Arizona Press. 

Stellar occultations provide an opportunity to probe Titan s atmospheric structure and composition. On December 14,2004 two stellar occultations were observed (the stars Spica and Shaula were occulted). The data for Titan s northern and southern hemisphere were compared. At the time when the occultations occurred it was late winter in the northern hemisphere. Water was found, however, in the Spica data below 575 kilometers, as predicted by earlier models. The restriction of the water ice clouds to the northern hemisphere, where it was late-winter, suggests a seasonal dependence on the distribution of Titan s water ice clouds (Larsen et al., 2006 [194]). 

The already mentioned NICMOS instrument on the Hubble Space telescope was used to measure a sample of KBOs (84 objects were selected) photometrically and first results were presented by Noll et al 2005 [250]. Stellar occultations provide another effective means to study these objects (see Roques and Doressoundiram, 2005 [281]). These observations were made with the WHT in La Palma and with the Very Large Telescope of ESO. The influence of a stellar fly-by encounter on the KBOs was investigated by Kobayashi, Ida, and Tanaka, 2005 [187]. 

Let us consider the transit of a planet. The stellar flux will be reduced by the amount of the ratio of the area of the planet to the star. If the planet has an atmosphere, then some flux from the star will pass through the optically thin part of the planet s atmosphere. In the case of stellar occultations by giant planets in the solar system, the limb of the giant planets is either defined by the cloud tops or by the 1 bar level. 

Roques, R, Doressoundiram, A. The Kuiper belt explorated by stellar occultations. In Bulletin of the American Astronomical Society, vol. 37, p. 746 (2005)... 

Note that all of these contributions are dependent on the wavelength X. The observed flux from a star can change either by intrinsic variations due to stellar activity or by transiting planets that occult a small part of the star. Spectral lines from a star show a center to limb variation. This is explained by the theory of line formation. Different lines are formed at different depths in the stellar atmosphere. As a crude rule we may state that the deeper the line, the higher it is formed in the atmosphere. If a planet passes in front of a star it obscures only a specific part of the star s visible surface. The variation of line profiles across the disc is several tens of a percent. Therefore,... 

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