Solar occultation

A solar occultation observation can be taken either at sunset or sunrise. The following case study is for a sunset, but sunrises can be treated similarly, only altitudes of observation will increase with time. Well before the instrument’s line of sight to the Sun intersects with the top layers of the atmosphere, the measurement cycle is started and reference spectra are recorded (at a rate of 1 spectrum per second).

Once the top of the atmosphere is reached, solar light is absorbed and the intensity of the recorded signal starts to decrease until the Sun gets so flattened that the spectrometer slit moves out of the diffracted solar disk. The altitude is retrieved from the spacecraft attitude and orbital data provided by ESOC.

One of the main advantages of solar occultations is that it is a self-calibrated technique in terms of transmission: dividing a spectrum obtained during the occultation by a reference solar spectrum recorded outside the atmosphere removes the solar signature and leaves a transmittance containing only information about the composition of the Venus atmosphere.

Fig 1: Illustration of a solar occultation and the evolution of the recorded transmittance (credit: BISA)
(.mpg file, 23 MB)(.wmv file, 2MB)(.avi file, high quality download, 662MB !!)

The right hand side of Fig1. illustrates the evolution of the transmittance during a typical solar occultation. At the beginning, the recorded spectrum is identical to the reference one obtained outside the atmosphere, and the transmittance (ratio of one spectrum w.r.t. the refeerence one) is 1.0. 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.

Analysing SOIR specra thus allows us to determine which molecule are present in the Venus atmosphere and in which quantity.

Fig 2 illustrates the signal intensity measured during a typical occultation. As one can see, the direct solar intensity observed before the occultation is not constant. This is due to a drift of the satellite during the measurement, resulting in a slight apparent motion of the slit over the solar disk.

Fig 2: Evolution in time of the signal intensity (sunset September 3 rd 2006). The plotted intensity is an average value over all 320 pixels (wavelenghts) for each of the recorded order. The 8 curves correspond to the 8 spectra recorded in one second (‘bin 1’ and ‘bin 2’ part of the detector for each of the 4 selected orders). First direct solar radiation is measured when the light path does not cross the Venusian atmosphere. During the occultation, the observed signal decreases. Finally the light path is intercepted by the cloud cover lying at 60-65 km and no light reaches the detector anymore. The dashed line shows the linear regression representing the solar reference spectrum. At time t, the observed signal is I(t) corresponding to an effective reference spectrum I₀(t). (credit: BISA)

 

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• What is a solar occultation - What can we learn from such observations?
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• Retrieval of atmospheric infomation
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  • HCl
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