Rabah Ikhlef | Université Nice Sophia Antipolis (original) (raw)

Papers by Rabah Ikhlef

L'objectif est de comprendre d'effet de la turbulence optique sur la mesure au sol du ray... more L'objectif est de comprendre d'effet de la turbulence optique sur la mesure au sol du rayon solaire. La mesure du rayon solaire avec précision est importante pour les modèles de structure et d'évolution solaire et stellaire. En plus l'activité solaire a une influence certaine sur le climat terrestre. Le travail de thèse a porté sur la calibration et l'exploitation de données issues des télescopes SODISM2, dédié à la mesure du rayon solaire par imagerie pleine, et MISOLFA un moniteur de turbulence qui permet d'estimer les paramètres spatio-temporels de celle-ci. Les premières mesures de MISOLFA montrent sa capacité pour l'extraction des profils de la turbulence et des paramètres intégrés à partir des fluctuations des angles d'arrivées observées sur le bord solaire. Les paramètres spatiaux issus des fluctuations d'intensité dans la voie pupille montrent un bon accord avec les paramètres issus de la voie image. Les premières mesures du temps caractér...

Monthly Notices of the Royal Astronomical Society, 2016

Ground-based solar observations are strongly affected by optical turbulence. The concept of a new... more Ground-based solar observations are strongly affected by optical turbulence. The concept of a new instrument which allows one to measure both spatial and temporal parameters of atmospheric turbulence has been proposed in the late 1990s. The instrument MISOLFA (Moniteur d'Images Solaire Franco-Algérien) is based on this concept and has been developed over the past 10 years in the framework of a ground-based solar astrometry programme and in parallel to the development of several night time turbulence monitors at Calern Observatory, south of France. In this paper, we first describe its instrumental concept, the technical choices that were made to meet the specifications and discuss the difficulties encountered. Using numerical simulations, we present and test the methods that can be used in order to estimate the turbulence parameters from both MISOLFA image and pupil planes. The effect of finite outer scale on Fried parameter estimation from a simple estimate of the angle-of-arrival variance is clearly shown. Finally, we present the first results obtained with the instrument fully operating in its two observing planes. We obtained a mean value of angle-of-arrival coherence time of 5.3 ms, and good agreement is found between spatial parameters obtained with image and pupil planes. First estimates of the atmospheric structure constant C 2 n (h) and outer scale L 0 (h) profiles are also presented which illustrates the profiling capacities of the new instrument.

SPIE Proceedings, 2016

PICARD is a mission devoted to solar variability observations through imagery and radiometric mea... more PICARD is a mission devoted to solar variability observations through imagery and radiometric measurements. The main goal is to provide data for scientific investigation first in the area of solar physics, and second in the assessment of the influence of the solar variability on the Earth climate variability. PICARD contains a double program with in-space and on-ground measurements. The PICARD spacecraft was launched on June 15, 2010, commissioned in-flight in October of the same year and was retired in April 2014. The PICARD ground-based observatory is operational since May 2011. We shall give a short overview of the PICARD instrumentation. New estimates of the absolute values of the total solar irradiance, of the solar spectral irradiance at typical wavelengths, and of the solar oblateness will be given. We will also report about helioseismic studies. Finally, we will present our current results about solar radius variations after six years of solar observation.

Journal of Physics: Conference Series, 2013

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

SPIE Proceedings, 2011

PICARD is a space mission developed mainly to study the geometry of the Sun. The satellite was la... more PICARD is a space mission developed mainly to study the geometry of the Sun. The satellite was launched in June 2010. The PICARD mission has a ground program which is based at the Calern Observatory (Observatoire de la Côte d'Azur). It will allow recording simultaneous solar images from ground. Astrometric observations of the Sun using ground-based telescopes need however an accurate modelling of optical effects induced by atmospheric turbulence. Previous works have revealed a dependence of the Sun radius measurements with the observation conditions (Fried's parameter, atmospheric correlation time(s) ...). The ground instruments consist mainly in SODISM II, replica of the PICARD space instrument and MISOLFA, a generalized daytime seeing monitor. They are complemented by standard sun-photometers and a pyranometer for estimating a global sky quality index. 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's parameter, size of the isoplanatic patch, the spatial coherence outer scale and atmospheric correlation times). This paper presents an overview of the ground based instruments of PICARD and some results obtained from observations performed at Calern observatory in 2011.

PICARD is a mission devoted to solar variability observation, which aims at perpetuating valuable... more 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.

For the last thirty years, ground time series of the solar radius have shown different variations... more 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 ...

L'objectif est de comprendre d'effet de la turbulence optique sur la mesure au sol du ray... more L'objectif est de comprendre d'effet de la turbulence optique sur la mesure au sol du rayon solaire. La mesure du rayon solaire avec precision est importante pour les modeles de structure et d'evolution solaire et stellaire. En plus l'activite solaire a une influence certaine sur le climat terrestre. Le travail de these a porte sur la calibration et l'exploitation de donnees issues des telescopes SODISM2, dedie a la mesure du rayon solaire par imagerie pleine, et MISOLFA un moniteur de turbulence qui permet d'estimer les parametres spatio-temporels de celle-ci. Les premieres mesures de MISOLFA montrent sa capacite pour l'extraction des profils de la turbulence et des parametres integres a partir des fluctuations des angles d'arrivees observees sur le bord solaire. Les parametres spatiaux issus des fluctuations d'intensite dans la voie pupille montrent un bon accord avec les parametres issus de la voie image. Les premieres mesures du temps caracter...

Astronomy & Astrophysics, Aug 1, 2018

Context. In 2015, the International Astronomical Union (IAU) passed Resolution B3, which defined ... more Context. In 2015, the International Astronomical Union (IAU) passed Resolution B3, which defined a set of nominal conversion constants for stellar and planetary astronomy. Resolution B3 defined a new value of the nominal solar radius (R N = 695 700 km) that is different from the canonical value used until now (695 990 km). The nominal solar radius is consistent with helioseismic estimates. Recent results obtained from ground-based instruments, balloon flights, or space-based instruments highlight solar radius values that are significantly different. These results are related to the direct measurements of the photospheric solar radius, which are mainly based on the inflection point position methods. The discrepancy between the seismic radius and the photospheric solar radius can be explained by the difference between the height at disk center and the inflection point of the intensity profile on the solar limb. At 535.7 nm (photosphere), there may be a difference of ∼330 km between the two definitions of the solar radius. Aims. The main objective of this work is to present new results of the solar radius in the near-ultraviolet, the visible, and the nearinfrared from PICARD space-based and ground-based observations. Simulations show the strong influence of atmosphere effects (refraction and turbulence) on ground-based solar radius determinations and highlight the interest of space-based solar radius determinations, particularly during planet transits (Venus or Mercury), in order to obtain more realistic and accurate measurements. Methods. Solar radius observations during the 2012 Venus transit have been made with the SOlar Diameter Imager and Surface Mapper (SODISM) telescope on board the PICARD spacecraft. We used the transit of Venus as an absolute calibration to determine the solar radius accurately at several wavelengths. Our results are based on the determination of the inflection point position of the solar limb-darkening function (the most common solar radius definition). A realistic uncertainty budget is provided for each solar radius obtained with the PICARD space-based telescope during the 2012 Venus transit. The uncertainty budget considers several sources of error (detection of the centers of Venus and Sun in PICARD images, positions of Sun and Venus from ephemeris (planetary theory), PICARD on-board timing, PICARD spacecraft position, and optical distortion correction from PICARD images). Results. We obtain new values of the solar radius from the PICARD mission at several wavelengths and in different solar atmosphere regions. The PICARD spacecraft with its SODISM telescope was used to measure the radius of the Sun during the Venus transit in 2012. At 535.7 nm, the solar radius is equal to 696 134 ± 261 km (combined standard uncertainty based (ξ) on the uncertainty budget). At 607.1 nm, the solar radius is equal to 696 156 ± 145 km (ξ), and the standard deviation of the solar radius mean value is ±22 km. At 782.2 nm, the solar radius is equal to 696 192 ± 247 km (ξ). The PICARD space-based results as well as PICARD ground-based results show that the solar radius wavelength dependence in the visible and the near-infrared is extremely weak. The differences in inflection point position of the solar radius at 607.1 nm, 782.2 nm, and 1025.0 nm from a reference at 535.7 nm are less than 60 km for the different PICARD measurements.

Astronomy & Astrophysics, Sep 1, 2014

Context. For the past thirty years, modern ground-based time-series of the solar radius have show... more Context. For the past thirty years, modern ground-based time-series of the solar radius have shown different apparent variations according to different instruments. The origins of these variations may result from the observer, the instrument, the atmosphere, or the Sun. Solar radius measurements have been made for a very long time and in different ways. Yet we see inconsistencies in the measurements. Numerous studies of solar radius variation appear in the literature, but with conflicting results. These measurement differences are certainly related to instrumental effects or atmospheric effects. Use of different methods (determination of the solar radius), instruments, and effects of Earth's atmosphere could explain the lack of consistency on the past measurements. A survey of the solar radius has been initiated in 1975 by Francis Laclare, at the Calern site of the Observatoire de la Côte d'Azur (OCA). Several efforts are currently made from space missions to obtain accurate solar astrometric measurements, for example, to probe the long-term variations of solar radius, their link with solar irradiance variations, and their influence on the Earth climate. Aims. The Picard program includes a ground-based observatory consisting of different instruments based at the Calern site (OCA, France). This set of instruments has been named "Picard Sol" and consists of a Ritchey-Chrétien telescope providing full-disk images of the Sun in five narrow-wavelength bandpasses (centered on 393.37, 535.7, 607.1, 782.2, and 1025.0 nm), a Sun-photometer that measures the properties of atmospheric aerosol, a pyranometer for estimating a global sky-quality index, a wide-field camera that detects the location of clouds, and a generalized daytime seeing monitor allowing us to measure the spatio-temporal parameters of the local turbulence. Picard Sol is meant to perpetuate valuable historical series of the solar radius and to initiate new time-series, in particular during solar cycle 24. Methods. We defined the solar radius by the inflection-point position of the solar-limb profiles taken at different angular positions of the image. Our results were corrected for the effects of refraction and turbulence by numerical methods. Results. From a dataset of more than 20 000 observations carried out between 2011 and 2013, we find a solar radius of 959.78 ± 0.19 arcsec (696 113 ± 138 km) at 535.7 nm after making all necessary corrections. For the other wavelengths in the solar continuum, we derive very similar results. The solar radius observed with the Solar Diameter Imager and Surface Mapper II during the period 2011-2013 shows variations shorter than 50 milli-arcsec that are out of phase with solar activity.

EAS Publications Series

The Franco-Algerian Monitor of Solar Images (MISOLFA) was developped in order to study the effect... more The Franco-Algerian Monitor of Solar Images (MISOLFA) was developped in order to study the effect of optical turbulence on diameter measurements from ground-based solar observations. Some first results obtained with MISOLFA are presented.

SPIE Proceedings, 2010

PICARD is a space mission developed to observe the Sun at high angular resolution. One of the mai... more PICARD is a space mission developed to observe the Sun at high angular resolution. One of the main space objectives of PICARD is to measure the solar diameter with few milli arc-seconds accuracy. A replica of the space instrument will be installed at Calern Observatory in order to test our ability to make such measurement from ground with enough accuracy. High angular resolution observations with ground-based instrument are however limited by atmospheric turbulence. The seeing monitor MISOLFA is developed to give all observation conditions at the same moments when solar images will be recorded with the twin PICARD instruments. They will be used to link ground and space measurements. An overview of the PICARD mission and the solar ground-based experiments will be first given. Optical properties of MISOLFA will be after presented. The basic principles to measure atmospheric parameters and the methods used to obtain them from solar images will be given. Finally, some recent results obtained at Calern Observatory will be presented and discussed.

PICARD is a space mission launched in June 2010 to study mainly the geometry of the Sun. The PICA... more 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 Côte 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 r 0 , size of the isoplanatic patch, the spatial coherence outer scale L 0 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 r 0 and present its statistics over 2 years at Calern Observatory with a global mean value of 3.5cm.

The Astrophysical Journal, 2015

The potential relationship between solar activity and changes in solar diameter remains the subje... more The potential relationship between solar activity and changes in solar diameter remains the subject of debate and requires both models and measurements with sufficient precision over long periods of time. Using the PICARD instruments, we carried out precise measurements of variations in solar diameter during the rising phase of solar cycle 24. From new correction methods we found changes in PICARD space telescope solar radius amplitudes that were less than ±20 mas (i.e. ±14.5 km) for the years 2010-2011. Moreover, PICARD ground-based telescope solar radius amplitudes are smaller than ±50 mas from 2011 to 2014. Our observations could not find any direct link between solar activity and significant fluctuations in solar radius, considering that the variations, if they exist, are included within this range of values. Further, the contribution of solar radius fluctuations is low with regard to variations in total solar irradiance. Indeed, we find a small variation of the solar radius from space measurements with a typical periodicity of 129.5 days, with ±6.5 mas variation.

L'objectif est de comprendre d'effet de la turbulence optique sur la mesure au sol du ray... more L'objectif est de comprendre d'effet de la turbulence optique sur la mesure au sol du rayon solaire. La mesure du rayon solaire avec précision est importante pour les modèles de structure et d'évolution solaire et stellaire. En plus l'activité solaire a une influence certaine sur le climat terrestre. Le travail de thèse a porté sur la calibration et l'exploitation de données issues des télescopes SODISM2, dédié à la mesure du rayon solaire par imagerie pleine, et MISOLFA un moniteur de turbulence qui permet d'estimer les paramètres spatio-temporels de celle-ci. Les premières mesures de MISOLFA montrent sa capacité pour l'extraction des profils de la turbulence et des paramètres intégrés à partir des fluctuations des angles d'arrivées observées sur le bord solaire. Les paramètres spatiaux issus des fluctuations d'intensité dans la voie pupille montrent un bon accord avec les paramètres issus de la voie image. Les premières mesures du temps caractér...

Monthly Notices of the Royal Astronomical Society, 2016

Ground-based solar observations are strongly affected by optical turbulence. The concept of a new... more Ground-based solar observations are strongly affected by optical turbulence. The concept of a new instrument which allows one to measure both spatial and temporal parameters of atmospheric turbulence has been proposed in the late 1990s. The instrument MISOLFA (Moniteur d'Images Solaire Franco-Algérien) is based on this concept and has been developed over the past 10 years in the framework of a ground-based solar astrometry programme and in parallel to the development of several night time turbulence monitors at Calern Observatory, south of France. In this paper, we first describe its instrumental concept, the technical choices that were made to meet the specifications and discuss the difficulties encountered. Using numerical simulations, we present and test the methods that can be used in order to estimate the turbulence parameters from both MISOLFA image and pupil planes. The effect of finite outer scale on Fried parameter estimation from a simple estimate of the angle-of-arrival variance is clearly shown. Finally, we present the first results obtained with the instrument fully operating in its two observing planes. We obtained a mean value of angle-of-arrival coherence time of 5.3 ms, and good agreement is found between spatial parameters obtained with image and pupil planes. First estimates of the atmospheric structure constant C 2 n (h) and outer scale L 0 (h) profiles are also presented which illustrates the profiling capacities of the new instrument.

SPIE Proceedings, 2016

PICARD is a mission devoted to solar variability observations through imagery and radiometric mea... more PICARD is a mission devoted to solar variability observations through imagery and radiometric measurements. The main goal is to provide data for scientific investigation first in the area of solar physics, and second in the assessment of the influence of the solar variability on the Earth climate variability. PICARD contains a double program with in-space and on-ground measurements. The PICARD spacecraft was launched on June 15, 2010, commissioned in-flight in October of the same year and was retired in April 2014. The PICARD ground-based observatory is operational since May 2011. We shall give a short overview of the PICARD instrumentation. New estimates of the absolute values of the total solar irradiance, of the solar spectral irradiance at typical wavelengths, and of the solar oblateness will be given. We will also report about helioseismic studies. Finally, we will present our current results about solar radius variations after six years of solar observation.

Journal of Physics: Conference Series, 2013

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

SPIE Proceedings, 2011

PICARD is a space mission developed mainly to study the geometry of the Sun. The satellite was la... more PICARD is a space mission developed mainly to study the geometry of the Sun. The satellite was launched in June 2010. The PICARD mission has a ground program which is based at the Calern Observatory (Observatoire de la Côte d'Azur). It will allow recording simultaneous solar images from ground. Astrometric observations of the Sun using ground-based telescopes need however an accurate modelling of optical effects induced by atmospheric turbulence. Previous works have revealed a dependence of the Sun radius measurements with the observation conditions (Fried's parameter, atmospheric correlation time(s) ...). The ground instruments consist mainly in SODISM II, replica of the PICARD space instrument and MISOLFA, a generalized daytime seeing monitor. They are complemented by standard sun-photometers and a pyranometer for estimating a global sky quality index. 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's parameter, size of the isoplanatic patch, the spatial coherence outer scale and atmospheric correlation times). This paper presents an overview of the ground based instruments of PICARD and some results obtained from observations performed at Calern observatory in 2011.

PICARD is a mission devoted to solar variability observation, which aims at perpetuating valuable... more 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.

For the last thirty years, ground time series of the solar radius have shown different variations... more 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 ...

L'objectif est de comprendre d'effet de la turbulence optique sur la mesure au sol du ray... more L'objectif est de comprendre d'effet de la turbulence optique sur la mesure au sol du rayon solaire. La mesure du rayon solaire avec precision est importante pour les modeles de structure et d'evolution solaire et stellaire. En plus l'activite solaire a une influence certaine sur le climat terrestre. Le travail de these a porte sur la calibration et l'exploitation de donnees issues des telescopes SODISM2, dedie a la mesure du rayon solaire par imagerie pleine, et MISOLFA un moniteur de turbulence qui permet d'estimer les parametres spatio-temporels de celle-ci. Les premieres mesures de MISOLFA montrent sa capacite pour l'extraction des profils de la turbulence et des parametres integres a partir des fluctuations des angles d'arrivees observees sur le bord solaire. Les parametres spatiaux issus des fluctuations d'intensite dans la voie pupille montrent un bon accord avec les parametres issus de la voie image. Les premieres mesures du temps caracter...

Astronomy & Astrophysics, Aug 1, 2018

Context. In 2015, the International Astronomical Union (IAU) passed Resolution B3, which defined ... more Context. In 2015, the International Astronomical Union (IAU) passed Resolution B3, which defined a set of nominal conversion constants for stellar and planetary astronomy. Resolution B3 defined a new value of the nominal solar radius (R N = 695 700 km) that is different from the canonical value used until now (695 990 km). The nominal solar radius is consistent with helioseismic estimates. Recent results obtained from ground-based instruments, balloon flights, or space-based instruments highlight solar radius values that are significantly different. These results are related to the direct measurements of the photospheric solar radius, which are mainly based on the inflection point position methods. The discrepancy between the seismic radius and the photospheric solar radius can be explained by the difference between the height at disk center and the inflection point of the intensity profile on the solar limb. At 535.7 nm (photosphere), there may be a difference of ∼330 km between the two definitions of the solar radius. Aims. The main objective of this work is to present new results of the solar radius in the near-ultraviolet, the visible, and the nearinfrared from PICARD space-based and ground-based observations. Simulations show the strong influence of atmosphere effects (refraction and turbulence) on ground-based solar radius determinations and highlight the interest of space-based solar radius determinations, particularly during planet transits (Venus or Mercury), in order to obtain more realistic and accurate measurements. Methods. Solar radius observations during the 2012 Venus transit have been made with the SOlar Diameter Imager and Surface Mapper (SODISM) telescope on board the PICARD spacecraft. We used the transit of Venus as an absolute calibration to determine the solar radius accurately at several wavelengths. Our results are based on the determination of the inflection point position of the solar limb-darkening function (the most common solar radius definition). A realistic uncertainty budget is provided for each solar radius obtained with the PICARD space-based telescope during the 2012 Venus transit. The uncertainty budget considers several sources of error (detection of the centers of Venus and Sun in PICARD images, positions of Sun and Venus from ephemeris (planetary theory), PICARD on-board timing, PICARD spacecraft position, and optical distortion correction from PICARD images). Results. We obtain new values of the solar radius from the PICARD mission at several wavelengths and in different solar atmosphere regions. The PICARD spacecraft with its SODISM telescope was used to measure the radius of the Sun during the Venus transit in 2012. At 535.7 nm, the solar radius is equal to 696 134 ± 261 km (combined standard uncertainty based (ξ) on the uncertainty budget). At 607.1 nm, the solar radius is equal to 696 156 ± 145 km (ξ), and the standard deviation of the solar radius mean value is ±22 km. At 782.2 nm, the solar radius is equal to 696 192 ± 247 km (ξ). The PICARD space-based results as well as PICARD ground-based results show that the solar radius wavelength dependence in the visible and the near-infrared is extremely weak. The differences in inflection point position of the solar radius at 607.1 nm, 782.2 nm, and 1025.0 nm from a reference at 535.7 nm are less than 60 km for the different PICARD measurements.

Astronomy & Astrophysics, Sep 1, 2014

Context. For the past thirty years, modern ground-based time-series of the solar radius have show... more Context. For the past thirty years, modern ground-based time-series of the solar radius have shown different apparent variations according to different instruments. The origins of these variations may result from the observer, the instrument, the atmosphere, or the Sun. Solar radius measurements have been made for a very long time and in different ways. Yet we see inconsistencies in the measurements. Numerous studies of solar radius variation appear in the literature, but with conflicting results. These measurement differences are certainly related to instrumental effects or atmospheric effects. Use of different methods (determination of the solar radius), instruments, and effects of Earth's atmosphere could explain the lack of consistency on the past measurements. A survey of the solar radius has been initiated in 1975 by Francis Laclare, at the Calern site of the Observatoire de la Côte d'Azur (OCA). Several efforts are currently made from space missions to obtain accurate solar astrometric measurements, for example, to probe the long-term variations of solar radius, their link with solar irradiance variations, and their influence on the Earth climate. Aims. The Picard program includes a ground-based observatory consisting of different instruments based at the Calern site (OCA, France). This set of instruments has been named "Picard Sol" and consists of a Ritchey-Chrétien telescope providing full-disk images of the Sun in five narrow-wavelength bandpasses (centered on 393.37, 535.7, 607.1, 782.2, and 1025.0 nm), a Sun-photometer that measures the properties of atmospheric aerosol, a pyranometer for estimating a global sky-quality index, a wide-field camera that detects the location of clouds, and a generalized daytime seeing monitor allowing us to measure the spatio-temporal parameters of the local turbulence. Picard Sol is meant to perpetuate valuable historical series of the solar radius and to initiate new time-series, in particular during solar cycle 24. Methods. We defined the solar radius by the inflection-point position of the solar-limb profiles taken at different angular positions of the image. Our results were corrected for the effects of refraction and turbulence by numerical methods. Results. From a dataset of more than 20 000 observations carried out between 2011 and 2013, we find a solar radius of 959.78 ± 0.19 arcsec (696 113 ± 138 km) at 535.7 nm after making all necessary corrections. For the other wavelengths in the solar continuum, we derive very similar results. The solar radius observed with the Solar Diameter Imager and Surface Mapper II during the period 2011-2013 shows variations shorter than 50 milli-arcsec that are out of phase with solar activity.

EAS Publications Series

The Franco-Algerian Monitor of Solar Images (MISOLFA) was developped in order to study the effect... more The Franco-Algerian Monitor of Solar Images (MISOLFA) was developped in order to study the effect of optical turbulence on diameter measurements from ground-based solar observations. Some first results obtained with MISOLFA are presented.

SPIE Proceedings, 2010

PICARD is a space mission developed to observe the Sun at high angular resolution. One of the mai... more PICARD is a space mission developed to observe the Sun at high angular resolution. One of the main space objectives of PICARD is to measure the solar diameter with few milli arc-seconds accuracy. A replica of the space instrument will be installed at Calern Observatory in order to test our ability to make such measurement from ground with enough accuracy. High angular resolution observations with ground-based instrument are however limited by atmospheric turbulence. The seeing monitor MISOLFA is developed to give all observation conditions at the same moments when solar images will be recorded with the twin PICARD instruments. They will be used to link ground and space measurements. An overview of the PICARD mission and the solar ground-based experiments will be first given. Optical properties of MISOLFA will be after presented. The basic principles to measure atmospheric parameters and the methods used to obtain them from solar images will be given. Finally, some recent results obtained at Calern Observatory will be presented and discussed.

PICARD is a space mission launched in June 2010 to study mainly the geometry of the Sun. The PICA... more 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 Côte 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 r 0 , size of the isoplanatic patch, the spatial coherence outer scale L 0 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 r 0 and present its statistics over 2 years at Calern Observatory with a global mean value of 3.5cm.

The Astrophysical Journal, 2015

The potential relationship between solar activity and changes in solar diameter remains the subje... more The potential relationship between solar activity and changes in solar diameter remains the subject of debate and requires both models and measurements with sufficient precision over long periods of time. Using the PICARD instruments, we carried out precise measurements of variations in solar diameter during the rising phase of solar cycle 24. From new correction methods we found changes in PICARD space telescope solar radius amplitudes that were less than ±20 mas (i.e. ±14.5 km) for the years 2010-2011. Moreover, PICARD ground-based telescope solar radius amplitudes are smaller than ±50 mas from 2011 to 2014. Our observations could not find any direct link between solar activity and significant fluctuations in solar radius, considering that the variations, if they exist, are included within this range of values. Further, the contribution of solar radius fluctuations is low with regard to variations in total solar irradiance. Indeed, we find a small variation of the solar radius from space measurements with a typical periodicity of 129.5 days, with ±6.5 mas variation.