Variation of the diameter of the Sun as measured by the Solar Disk Sextant (SDS)

Abstract

The balloon-borne Solar Disk Sextant (SDS) experiment has measured the\nangular size of the Sun on seven occasions spanning the years 1992 to 2011. The\nsolar half-diameter -- observed in a 100-nm wide passband centred at 615 nm --\nis found to vary over that period by up to 200 mas, while the typical estimated\nuncertainty of each measure is 20 mas. The diameter variation is not in phase\nwith the solar activity cycle; thus, the measured diameter variation cannot be\nexplained as an observational artefact of surface activity. Other possible\ninstrument-related explanations for the observed variation are considered but\nfound unlikely, leading us to conclude that the variation is real. The SDS is\ndescribed here in detail, as is the complete analysis procedure necessary to\ncalibrate the instrument and allow comparison of diameter measures across\ndecades.\n