A Comparison among Solar Diameter Measurements Carried Out from the Ground and outside Earth's Atmosphere

Abstract

The solar diameter has been measured since the 17th century, using different methods and instruments, and without a clear strategy of measurement, which could explain the lack of coherence between the results obtained. The present study confirms that the discrepancies between solar diameter measurements are mostly due to differences in instrumental characteristics, the spectral domain of observation of each investigation, and atmospheric turbulence for ground measurements. We show that correcting the measurements for the effects introduced by the properties of each instrument reduces the differences between the radius measurements to the level of uncertainty of each instrument. This study makes use of simulated and real data. For the simulated data, we use an empirical model of the solar limb shape, and for observations, we use measurements carried out with the Solar Disk Sextant experiment, the CCD solar astrolabe of Calern Observatory, and the Michelson Doppler Imager on board SOHO

Solar change and climate: an update in the light of the current exceptional solar minimum

“…The best observations place an upper limit of approximately 0.01 per cent on variations in R over decades (Kuhn et al 2004;Lefebvre et al 2006;Djafer et al 2008), and reports of larger changes remain controversial (e.g. Lefebvre & Kosovichev 2005;Noël 2005).…”

Processing Method Effect on Sun Diameter Measurement With CCD Solar Astrolabe

“…The difference of 138 mas in the radius of the Sun between the compact wavelet and the smoothing methods partly explains, in addition to the experimental effects (Djafer et al 2008a), the difference between the mean radius of the Sun measured during 1996 with the SDS and at Calern during the same period. The mean value of the radius of the Sun with the SDS is 959.788 ± 0.091 mas and 959.628 ± 0.075 mas at Calern (Djafer et al 2008b;.…”

“…Most measurements yield different values of the diameter of the Sun. Thuillier et al (2005) and Djafer et al (2008a) have emphasized that no unique strategy exists to measure the solar diameter and that different methods are used, mostly based on photometric recordings of the solar-limb profile using CCDs as detectors. One of these widely used instruments is the solar astrolabe.…”

“…All the methods necessitate a reliable noise filtering. Djafer et al (2008aDjafer et al ( , 2008b have presented in detail the filtering methods and their effects on the measurement of the diameter of the Sun using the solar disk sextant (SDS) data. With a CCD solar astrolabe, the filtering method currently being used is based on a smoothing process of 1, 3, 5, and 7 pixel windows.…”

Book of Proceedings: Ninth Workshop, Sunny Beach, Bulgaria, May 30 - June 3, 2017

“…The right panel shows the solar radius measurements made since 1970 and leads to the same conclusion. Note that data can be found in [Wittmann, 1977], from 1836to 1975, or [Djafer et al, 2008 [Kuhn et al, 2004]; the survey can be completed by the data listed in [Rozelot et al, 2016]. Figure 1 exhibits a great dispersion, spanning several tenths of Proceedings of Ninth Workshop "Solar Influences on the Magnetosphere, Ionosphere and Atmosphere" Sunny Beach, Bulgaria, May 30 -June 3, 2016 Topic: Sun and Solar Activity 72 arc-second, meaning that even with gradually increasing accuracy, by different means or techniques, including space measurements, the results obtained so far do not allow to determine without ambiguity an estimation in absolute value of the solar diameter, as well as its variation or its constancy as a function of solar activity.…”