Variation of the Diameter of the Sun as Measured by the Solar Disk Sextant (SDS)

TL;DR

The Solar Disk Sextant (SDS) measured the Sun's diameter over nearly two decades, finding real variations up to 200 mas that are not linked to solar activity, with detailed calibration procedures described.

Contribution

This study provides the first long-term, high-precision measurements of solar diameter variations using the balloon-borne SDS instrument.

Findings

Solar diameter varies by up to 200 mas over 19 years.

Diameter variation is not correlated with solar activity cycle.

Instrument calibration procedures are detailed for long-term comparison.

Abstract

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