Abstract
Abstract. Stellar light passing through the Earth atmosphere is affected by refractive effects, which should be taken into account in retrievals from stellar occultation measurements. Scintillation caused by air density irregularities is a nuisance for retrievals of atmospheric composition. In this paper, we consider the influence of scintillation on stellar occultation measurements and on the quality of ozone retrievals from these measurements, based on experience of the GOMOS (Global Ozone Monitoring by Occultation of Stars) instrument on board the Envisat satellite. In GOMOS retrievals, the scintillation effect is corrected using scintillation measurements by the fast photometer. We present quantitative estimates of the current scintillation correction quality and of the impact of scintillation on ozone retrievals by GOMOS. The analysis has shown that the present scintillation correction efficiently removes the distortion of transmission spectra caused by scintillations, which are generated by anisotropic irregularities of air density. The impact of errors of dilution and anisotropic scintillation correction on the quality of ozone retrievals is negligible. However, the current scintillation correction is not able to remove the wavelength-dependent distortion of transmission spectra caused by isotropic scintillations, which can be present in off-orbital-plane occultations. This distortion may result in ozone retrieval errors of 0.5–1.5% at altitudes 20–40 km. This contribution constitutes a significant percentage of the total error for bright stars. The advanced inversion methods that can minimize the influence of scintillation correction error are also discussed.
Highlights
The stellar occultation measurements have a set of beneficial features that are important for long-term monitoring, such as a self-calibration measurement principle, global coverage, good vertical resolution, and a wide altitude range of measurements from the troposphere to the thermosphere
The stellar spectra observed through the Earth atmosphere are attenuated by absorption and scattering, but they are modified by refractive effects
At altitudes below 25–30 km, the weak scintillation as- GOMOS scintillation correction efficiently eliminates modsumption is violated due to multi-path propagation and ray ulation caused by anisotropic scintillations: the remaining crossing, resulting in further degradation of the scintil- error is below 1%
Summary
The stellar occultation measurements have a set of beneficial features that are important for long-term monitoring, such as a self-calibration measurement principle, global coverage, good vertical resolution, and a wide altitude range of measurements from the troposphere to the thermosphere. Since stars are point sources of quite low-intensity light, special instruments are needed for recording stellar spectra. The stellar spectra observed through the Earth atmosphere are attenuated by absorption and scattering (this phenomenon is used in reconstruction of chemical composition of the atmosphere), but they are modified by refractive effects. Refraction in the atmosphere transforms parallel incident rays into diverging beams, resulting in dilution of the light intensity registered at the satellite level. This effect is known as refractive attenuation (or refractive dilution). The dependence of atmospheric refractivity on wavelength leads to a differential bending of rays of different color in the atmosphere; this effect is known as chromatic refraction.
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