PICARD is a mission devoted to solar variability observation, which aims at perpetuating valuable historical time-series of the solar radius. PICARD contains a double program with in-space and on-ground measurements using Ritchey-Chrétien telescopes. The PICARD spacecraft was launched on June 15, 2010, commissioned in-flight in October of the same year, and was retired in April 2014. PICARD ground-based observatory is functional since May 2011 in the Plateau de Calern (France), and is still operational today. We shall give an overview of the PICARD instrumentation and the performances of the existing ground-based telescope. We will also present our current results about solar radius variations after eight years of solar observations.
PICARD is a space mission launched in June 2010 to study mainly the geometry of the Sun. The PICARD mission has a ground program consisting mostly in four instruments based at the Calern Observatory (Observatoire de la Cˆote d’Azur). They allow recording simultaneous solar images and various atmospheric data from ground. The ground instruments consist in the qualification model of the PICARD space instrument (SODISM II: Solar Diameter Imager and Surface Mapper), standard sun-photometers, a pyranometer for estimating a global sky quality index, and MISOLFA a generalized daytime seeing monitor. Indeed, astrometric observations of the Sun using ground-based telescopes need an accurate modeling of optical effects induced by atmospheric turbulence. MISOLFA is founded on the observation of Angle-of-Arrival (AA) fluctuations and allows us to analyze atmospheric turbulence optical effects on measurements performed by SODISM II. It gives estimations of the coherence parameters characterizing wave-fronts degraded by the atmospheric turbulence (Fried parameter r0, size of the isoplanatic patch, the spatial coherence outer scale L0 and atmospheric correlation times). We present in this paper simulations showing how the Fried parameter infered from MISOLFA records can be used to interpret radius measurements extracted from SODISM II images. We show an example of daily and monthly evolution of r0 and present its statistics over 2 years at Calern Observatory with a global mean value of 3.5cm.
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.