Solar diameter measurement, new French facility, cycle 24

The size of the sun is of critical importance to solar studies yet this is poorly known, let alone if and how the size varies over time. Paper published this week in Astronomy & Astrophysics.

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. …

Fig.2. Evolution of the solar radius variations over time for ground instruments (Solar Astrolabe, DORAYSOL and SODISMII monthly mean at 782.2 nm), balloon experiment (SDS), and space instrument (MDI) vs. daily sunspot number time-series. For each series, the mean has been
taken as reference value.

 

Ground-based measurements of the solar diameter during the rising phase of solar cycle 24
M. Meftah, T. Corbard, A. Irbah, R. Ikhlef, F. Morand, C. Renaud, A. Hauchecorne, P. Assus, J. Borgnino, B. Chauvineau, M. Crepel, F. Dalaudier, L. Damé, D. Djafer, M. Fodil, P. Lesueur, G. Poiet, M. Rouzé, A. Sarkissian, A.Ziad, and F. Laclare

Paper access is available with registration.

 

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 20000 observations carried out between 2011 and 2013, we find a solar radius of 959.78 ± 0.19 arcsec (696113 ± 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.

Or maybe not!  🙂

Measurement usually seems a trivial thing until you try it, hence the field Metrology, the science of measurement, best known for Standards bodies such as NPL, NIST and so on around the world.  In a case like this I doubt whether metrology professionals are involved nevertheless science doing it’s best. Both fields go to extraordinary lengths, build equipment as necessary.

You might recall the series of posts on TSI where in 2008 NIST had declared the instrumentation was not as good as supposed.

A measurement within 1% is in most fields taken as good and 0.1% as the start of precision, now get down to ppm and better. We don’t know solar parameters to 0.1%, still too poor for AGW detection purposes where I recall a figure of 0.03% was wanted, still woefully poor for safety.

In this case the figure given is +-200ppm. Would a metrologist accept it?
I suspect not for the simple reason this is a severely qualified figure, there is no physical body, measuring substance in various excited states. Some consider we live within the sun, inside it’s outer atmosphere.

In addition this paper necessarily deals with atmospheric optical properties, turbidity, clarity generally.

Post by Tim