New high-resolution (0.02 nm) solar spectrum in the 0.7-1.7 μm domain obtained from a reanalysis of TGO observations
Résumé
Measurement of the infrared solar spectrum from the Earth's surface is difficult or even impossible outside observation windows allowed by the atmosphere. Existing solutions then call on modeling the Sun, correcting ground atmospheric transmission effects on measurements, or directly performing measurements from space. This opportunity was seized with the infrared observations of the interplanetary probe Trace Gas Orbiter (TGO) in orbit around Mars launched to study its atmosphere thanks to spectrometers installed onboard on the Atmospheric Chemistry Suite (ACS) platform. The ACS-NIR (Near InfraRed) spectrometer probed the Martian atmosphere in the 0.7-1.7 μm spectral range thanks to its solar occultation mode. In this mode, ACS-NIR is pointed at the Sun and its line of sight gradually passes through the Mars atmosphere as the satellite moves through its orbit. The high-resolution solar spectrum is measured directly when the line of sight is above the atmosphere. A 10-month observation plan (October 2020 - August 2021) has been specifically requested and deployed for this objective. The observations consisted in recording all the diffraction orders of the ACS-NIR by continuously varying the frequency of its AOTF (Acousto-Optic Tunable Filters, a component used to separate the orders). We will first present how we have processed the diffraction orders to obtain the best possible solar spectrum on the 0.7-1.7 μm band. We will show how the use of pieces of image of the same order of diffraction makes it possible both to avoid the spectral contamination between successive orders and to improve the detection of the solar lines at the ends of each order where the intensity is weak due to the Blaze function. We will then show the final version of the high resolution solar spectrum that we obtained (R=0.02 nm) which will be compared to the Toon ground spectrum taken as a reference. We will finish by discussing the parts of the spectrum that exhibit solar lines that are absent from the corresponding parts of the reference spectrum. They are located in spectral bands not observable from Earth.