PICARD SOL mission, a ground-based facility for long-term solar radius measurement
2012
For the last thirty years, ground time series of the solar radius have shown different variations according to
different instruments. The origin of these variations may be found in the observer, the instrument, the atmosphere
and the Sun. These time series show inconsistencies and conflicting results, which likely originate from
instrumental effects and/or atmospheric effects. A survey of the solar radius was initiated in 1975 by F. Laclare,
at the Calern site of the Observatoire de la Cˆote d’Azur (OCA). PICARD is an investigation dedicated to the
simultaneous measurements of the absolute total and spectral solar irradiance, the solar radius and solar shape,
and to the Sun’s interior probing by the helioseismology method. The PICARD mission aims to the study of the
origin of the solar variability and to the study of the relations between the Sun and the Earth’s climate by using
modeling. These studies will be based on measurements carried out from orbit and from the ground. PICARD
SOL is the ground segment of the PICARD mission to allow a comparison of the solar radius measured in space
and on ground. PICARD SOL will enable to understand the influence of the atmosphere on the measured solar
radius. The PICARD Sol instrumentation consists of: SODISM II, a replica of SODISM (SOlar Diameter
Imager and Surface Mapper), a high resolution imaging telescope, and MISOLFA (Moniteur d’Images SOLaires
Franco-Alg´erien), a seeing monitor. Additional instrumentation consists in a Sun photometer, which measures
atmospheric aerosol properties, a pyranometer to measure the solar irradiance, a visible camera, and a weather
station. PICARD SOL is operating since March 2011. First results from the PICARD SOL mission are briefly
reported in this paper.
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