ON THE CONSTANCY OF THE SOLAR RADIUS. III

The Michelson Doppler Imager on board the Solar and Heliospheric Observatory satellite has operated for over a sunspot cycle. This instrument is now relatively well understood and provides a nearly continuous record of the solar radius in combination with previously developed algorithms. Because these data are obtained from above Earth's atmosphere, they are uniquely sensitive to possible long-term changes of the Sun's size. We report here on the first homogeneous, highly precise, and complete solar-cycle measurement of the Sun's radius variability. Our results show that any intrinsic changes in the solar radius that are synchronous with the sunspot cycle must be smaller than 23 mas peak to peak. In addition, we find that the average solar radius must not be changing (on average) by more than 1.2 mas yr-1. If ground- and space-based measurements are both correct, the pervasive difference between the constancy of the solar radius seen from space and the apparent ground-based solar astrometric variability can only be accounted for by long-term changes in the terrestrial atmosphere.