Solar occultation

Zander, R., C. P. Rinsland, C. B. Farmer, J. Namkung, R. H. Norton, and J. M. Russell III, Concentrations of Carbonyl Sulfide and Hydrogen Cyanide in the Free Upper Troposphere and Lower Stratosphere Deduced from ATMOS/Spacelab 3 Infrared Solar Occultation Spectra, . /. Geophys. Res., 93, 1669-1678(1988). 

During the last 15 years Fourier transform spectrometers have been used successfully to sound the stratosphere and upper troposphere. One of the most important successes has been the Atmospheric Trace MOlecule Spectroscopy (ATMOS) project (e.g. Farmer et al., 1987 Gunson et al., 1996). The ATMOS instrument flew aboard Spacelab 3 and the Atmospheric Laboratory for Applications and Science (ATLAS) Space Shuttle missions (Table 1). ATMOS performed solar occultation measurements and a variety of trace gases in the upper troposphere and stratosphere have been retrieved. 

The first passive remote sensing experiment to measure successfully the abundance of atmospheric aerosols from space was the Stratospheric Aerosol Measurement (SAM II) aboard Nimbus 7 (McCormick et al. 1979). This experiment was a single channel radiometer observing in solar occultation and was the forerunner of SAGE. Stratospheric aerosols have also been measured by their infrared absorptions (e.g. HALOE). 

AEROSOL RETRIEVAL ALGORITHM FOR SATELLITE SOLAR OCCULTATION AND INFRARED EMISSION MEASUREMENTS ... 

Duffell H., Oppenheimer C., and Burton M. (2001) Volcanic gas emission rates measured by solar occultation spectroscopy. Geophys. Res. Lett. 28, 3131—3134. 

Figure 14. Solar occultation spectra measured in this balloon experiment. Solar zenith angle a, 76.6° and b, 92.0°. (Reproduced with permission from Ref. 6. Copyright 1983, American Geophysical Union.)...

In the solar occultation method a space-borne detector (e.g., photomultiplier tube, photolytic array detector, Fourrier transformed interferometer) points towards the Sun, and during brief periods (sunrise, sunset), when the optical path penetrates into the atmosphere (see Figure 4.19a), measures the attenuation of the solar radiation by the absorbing compounds. The intensity at frequency v detected by the spacecraft is... 

Figure 4..19a. Geometry of solar occultation. McCormick et aZ.(1979) Assuming that this absorbant is uniformly distributed in each atmospheric layer (and hence that its concentration varies only with height), the integrated concentration N along the optical path with tangent height i 0 (slant column density) can be derived from the observation of lu and Iv,00 through Eq. (4.61), when...

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.

Pommereau, J-P., and J. Piquard, Ozone and nitrogen dioxide vertical distributions by UV-visible solar occultation from balloons. Geophys Res Lett 21, 1227, 1994b. 

Fischer, H., F. Fergg, D. Rabus, and P. Burkert (1985). Stratospheric H20 and HN03 profile derived from solar occultation measurements. J. Geophys. Res. 90, 3831-3843. 

The principal advantage of the solar occultation approach is the relatively strong signal obtained by using the Sun as a source. Its principal disadvantage is that the measurements can be acquired only at local sunrise or sunset. 

Another technique to measure the water content in a planetary atmosphere is using solar occultation in the IR. Such experiments can be made during sunrise or sunset. Before the instrument s line of sight to the Sun intersects with the top layers of the atmosphere, the measurement cycle is started, and as soon as the top of the atmosphere is reached, solar light is absorbed and the intensity of the recorded signal starts to decrease. When molecules start to absorb the radiation, structures appear in the spectrum. Those structures are characteristic of a specific molecule and their amplitude or depth are in direct relation to the quantity of this species present in the sounded atmosphere. 

Vertical distributions of the molecular density and mixing ratios of H2O and HDO in the Venus mesosphere were measured by Fedorova et al., 2008 [129]. The experiment was carried out on board of Venus express mission (SOIR instrument, 2.32-4.35 pm). The atmosphere was sounded during solar occultation in the range of altitudes from 65 to 130 km. An enrichment of D to hydrogen indicates the escape of water from Venus. Bertaux et al., 2007 [25] report on the detection of a warm layer at 90-120 km. ... 

Fedorova, A., Korablev, O., Vandaele, A.-C., Bertaux, J.-L., Belyaev, D., Mahieux, A., Neefs, E., WUquet, W.V., Drummond, R., Montmessin, F., Villard, E. HDO and H2O vertical distributions and isotopic ratio in the Venus mesosphere by Solar occultation at infrared spectrometer on board Venus express. J. Geophys. Res. (Rlanets) 113, EOOB22 (2008)... 

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