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Ground-Based Measurements of the Solar Diameter During the Rising Phase of Solar Cycle 24

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Ground-Based Measurements of the Solar Diameter During the Rising Phase of Solar Cycle 24 A&A 569, A60 (2014) Astronomy DOI: 10.1051/0004-6361/201423598 & c ESO 2014 Astrophysics Ground-based measurements of the solar diameter during the rising phase of solar cycle 24 M. Meftah1, T. Corbard2, A. Irbah1,R.Ikhlef2,3,F.Morand2, C. Renaud2, A. Hauchecorne1, P. Assus2, J. Borgnino2, B. Chauvineau2,M.Crepel1, F. Dalaudier1,L.Damé1, D. Djafer4, M. Fodil3, P. Lesueur1,G.Poiet1, M. Rouzé5, A. Sarkissian1,A.Ziad2, and F. Laclare2 1 Université Versailles St-Quentin; Sorbonne Universités, UPMC Univ. Paris 06; CNRS/INSU, LATMOS-IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France e-mail: [email protected] 2 Laboratoire Lagrange, UMR 7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, Bd. de l’Observatoire, 06304 Nice, France e-mail: [email protected] 3 CRAAG, Observatoire d’Alger, BP 63 Bouzareah, Alger, Algérie 4 Unité de Recherche Appliquée en Énergies Renouvelables (URAER), Centre de Développement des Energies Renouvelables (CDER), BP 88, Ghardaïa, Algérie 5 CNES – Centre National d’Études Spatiales, Rue Édouard Belin, 31000 Toulouse, France Received 7 February 2014 / Accepted 5 August 2014 ABSTRACT 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. Key words. astrometry – Sun: fundamental parameters – Sun: activity 1. Introduction a value for the solar radius of 959.63 arcsec that he derived from heliometer measurements made by 29 observers (Wittmann Measurements of the solar radius are of great interest within the 1977) during the period 1873–1886. In solar modeling, this scope of the debate on the role of the Sun in climate change canonical value has been commonly used and was adopted by (Rozelot 2001a,b; Schröder 2001). The solar radius is mainly the International Astronomical Union (IAU). Solar radius mea- related to the knowledge of the solar atmosphere. However, it surements (mostly from ground) are plotted in Fig. 1,show- is very difficult to measure this fundamental parameter of astro- ing inconsistent results that are probably caused by different in- physical interest. Solar radius determination is one of the oldest struments, different spectral domains of measurements, differ- problems in astrophysics. Systematic measurements of the so- ent calculation methods, different definitions (Haberreiter et al. lar radius have been made since Antiquity (Rozelot & Damiani 2008b), and different sites where the conditions of observations 2012). At the end of the nineteenth century, an investigation of are not comparable. Thus, the accurate absolute value of the the value of the solar radius obtained by meridian observations solar radius is not the subject of a consensus. Indeed, the Earth was carried out by Arthur Auwers (Auwers 1891). He published Article published by EDP Sciences A60, page 1 of 15 A&A 569, A60 (2014) 970 961.5 961 965 960.5 960 MDI Canonical solar radius: 959.63 arc−seconds Solar astrolabes 960 Solar eclipses HMI SDS 955 Heliometer SODISM Meridian transit time Mercury transit 959.5 Transit time Venus transit 950 Photoelectric drift scan Others... Solar radius [arc−seconds] Solar radius [arc−seconds] 959 Solar eclipse (Sri Lanka, 2010) SDS (last revision) 945 SoHO−MDI Picard−SODISM 958.5 SDO−HMI 1600 1700 1800 1900 2000 1970 1980 1990 2000 2010 2020 Year Year Fig. 1. Left: solar radius measurements (red symbols) made since the seventeenth century (Rozelot & Damiani 2012). The mean value of all these measurements is close to 960 arcsec. Right: focus on solar radius measurements made since 1970. Solar Disk Sextant (SDS) measurements (Sofia et al. 2013) are represented with black circles. Solar and Heliospheric Observatory – Michelson Doppler Imager (SoHo-MDI) and Solar Dynamics Observatory – Helioseismic and Magnetic Imager (SDO-HMI) records are represented with blue symbols. The Solar Diameter Imager and Surface Mapper (SODISM) measurement obtained during the transit of Venus is represented with a green symbol. Adassuriya et al. (2011) found a solar radius of 959.89 ± 0.18 arcsec (see magenta circle symbol) during solar eclipse in Sri Lanka. atmosphere generates various hindrances that make morphome- • to determine the relation between the turbulence parameters tric and photometric studies subject to discussion concerning and the measured solar radius; the distinction between solar activity and atmospheric effects • to determine whether small-angle scattering by aerosols merged in ground-based measurements. They include refraction, could also impact significantly the metrologic accuracy; turbulence, scattering, extinction, and diurnal alternation. It is • to compare the solar radius measurements obtained with suspected that the past inconsistencies of the temporal depen- SODISM II and ground-based instruments (to identify pos- dence of the solar radius measured from the ground stem pri- sible biases); marily from such contingencies (Ribes et al. 1991; Delache & • to continue solar radius measurements with ground-based Kroll 1994; Badache-Damiani et al. 2007). Efforts have been instruments. made in the past to understand and quantify the effects of at- mospheric disturbances on ground-based observations (Brown 1982; Lakhal et al. 1999). The interpretation of ground-based 2. Historical solar radius measurements at Calern observations, however, remains controversial to date, and recent Observatory measurements obtained outside the atmosphere (balloon flights The solar radius survey was initiated in 1975 with the Solar and space instruments) indicate that the canonical value of the Astrolabe at Calern Observatory (France). Simultaneously with solar radius is under-estimated. Ideally, space instrumentation is visual observations on the same instrument, a program of required for solar radius measurements, but this instrumentation charge-coupled device (CCD) records was conducted, which is a high-level technical challenge given the desired accuracy (a started in 1989. The coherence of visual and CCD measure- few milli-arcsec), and mission duration in a harsh environment ments thus obtained over ten years permitted qualifying the (BenMoussa et al. 2013). From the ground, the instruments are ff whole visual series, which appeared to be free of systematic per- not a ected by degradation due to space environment, and main- sonal effects (Laclare et al. 1999). The DORAYSOL (Définition tenance can be easily provided. If, in addition, the atmospheric ff et Observation du Rayon Solaire) instrument was then de- e ects are properly monitored and taken into account, they rep- signed and also developed at Calern Observatory. The princi- resent our best chance to build the needed long time-series ple of this instrument remains the same as that of the Solar records. That is why an important program of measurements Astrolabe (timing the crossing of a parallel of altitude by the from the ground is associated with the space operations dur- Sun), but a prism at a varying angle enables more daily
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