Temperature observations in the upper stratosphere and in the mesosphere using Rayleigh scattering at limb: GOMOS/ENVISAT climatology and MARTIC Cubesat constellation

The intensity of sunlight scattered by the atmospheric limb above 30-35 km is primarily due to molecular Rayleigh scattering and can be assumed to be directly proportional to the atmospheric density. From the measured relative density profile it is possible to retrieve an absolute temperature profile up to the upper mesosphere by assuming local hydrostatic equilibrium, the perfect gas law, and an a priori temperature from a climatological model at the top of the atmosphere. This simple technique is applied to Rayleigh lidar observations for over 40 years.

The GOMOS star occultation spectrometer aboard the ESA ENVISAT satellite included spectral channels to observe daytime scattered sunlight at limb close to the star direction. GOMOS Rayleigh scattering profiles in the spectral range 420-480 nm have been used to retrieve temperature profiles in the altitude range 35-85 km with a 2-km vertical resolution. The global climatology built using GOMOS data from 2002 to 2012 showed an agreement better than 2 K with ECMWF analysis in the upper stratosphere but differences up 10 K withe the MSIS climatology in the mesosphere. The temperature evolution obtained at Equator indicates the occurrence of mesospheric temperature inversion layers with global longitudinal extension, descending in about one month from 80 to 70 km.

The technique to derive temperature profiles from Rayleigh scattering at limb can be applied to any other limb-scatter sounder providing observation in the spectral range 350-500 nm. It can bring very valuable temperature observations to numerical weather prediction models in an altitude region where other sources of data are scarce. It is also a good candidate for a future small satellite constellation due to the simplicity of the principle. The MARTIC (Middle Atmosphere Rayleigh Temperature Instrument Constellation) project aims to develop a constellation of 12 Cubesats, orbiting at 3-4 different solar times to cover the tidal impact on middle atmosphere temperature, using a small space-qualified CMOS monochrome camera pointed to the Earth limb, and two band-pass filters, one centred around 450 nm to observe the Rayleigh scattering profile and one centred around 850 nm to detect the eventual presence of Mie scattering due to aerosols and cloud particles. The prototype of MARTIC is in development at LATMOS.