Abstract
Abstract. In this paper, we consider occultations of celestial bodies through the atmospheric limb from low Earth orbit satellites and we show how the usual change of tangent altitude associated with atmospheric refraction is inseparably connected to a variation of the observed apparent intensity, for extended and pointlike sources. We demonstrate, in the regime of weak refraction angles, that atmospheric optical dilution and image deformation are strictly concomitant. The approach leads to the integration of a simple differential equation related to the observed transmittance in the absence of other absorbing molecules along the optical path. The algorithm does not rely on the absolute knowledge of the radiometer pointing angle that is related to the accurate knowledge of the satellite attitude. We successfully applied the proposed method to the measurements performed by two past occultation experiments: GOMOS for stellar and ORA for solar occultations. The developed algorithm (named ARID) will be applied to the imaging of solar occultations in a forthcoming pico-satellite mission.
Highlights
In the terrestrial and planetary atmospheres, electromagnetic waves generally do not propagate along straight lines due to refractivity gradients caused by the vertical variation of the molecular concentration
We derive several interrelated effects of atmospheric refraction and we show the concomitance of image flattening, shift of the apparent tangent altitude and dilution of the incoming irradiance for low Earth orbit (LEO) satellites
Many studies have been focused on the computation and the measurement of amplitude and phase fluctuations induced by a random medium, reflecting important properties of atmospheric turbulence
Summary
In the terrestrial and planetary atmospheres, electromagnetic waves generally do not propagate along straight lines due to refractivity gradients caused by the vertical variation of the molecular concentration. Stellar, planetary and GPS radio occultations, an orbiting spectroradiometer in a low Earth orbit (LEO) measures the atmospheric transmittance as a function of the tangent altitude of the line of sight. High-precision refraction measurements obtained by solar imaging were recorded by the SOFIE instrument (Gordley et al, 2009) onboard the AIM satellite They proposed an elegant method to relate the apparent Sun image flattening to the refraction angles characterizing the tangent rays emitted from the solar disc edges. We present an original method to retrieve the refraction angle profile from the integration of a simple differential equation, defining the ARID algorithm (Atmospheric Refraction by Inversion of Dilution). The vertical inversion techniques coupled to the use of the hydrostatic hypothesis have been extensively studied elsewhere (Hajj et al, 2002)
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