Fourier-spectrometer TIRVIM is a part of ACS spectral complex aboard Mars-Express orbiter spacecraft. TIRVIM spectral range is 2–16 micron. It can operate as a spectrometer – with the Sun as a standard radiation source (“occultation” mode) or as a spectro-radiometer (“nadir” mode). In occultation mode the spectral resolution is 0.2 cm-1, in nadir mode – 1.3 cm-1. The main scientific objective of the occultation mode is to search for atmosphere minor constituents, of the nadir mode – to monitor the Mars atmosphere vertical thermal profile (by 15-micron CO2 band). The occultation mode is self-calibrated. For absolute calibration in the nadir mode TIRVIM has a rotating inlet flat mirror (single-axis foreoptic) able to point the FOV (2º) to nadir, space, built-in black-body or to another direction in the plane. TIRVIM mass is 12 kg, the power consumption is 15 W.
This work is devoted to the study of solar light that experiences single and multiple scattering in the atmosphere at the twilight time. The work is based on polarimetric observations of the twilight sky that were conducted in 1997 and 2000 for different color bands. Basing on the results of observations, the contribution of single-scattered light depending on the wavelength for the light stage of twilight is obtained. The correlation of color and polarization properties of the twilight sky is investigated with an account of multiple scattering.