Solar Radius Measurements at Mount Wilson Observatory

Possible temporal variations of the solar radius are important as an indicator of internal energy storage and as a mechanism for changes in the total solar irradiance. Variations in the total solar irradiance with an amplitude of 0.1% have been observed from space for more than two decades. Although the variability of this solar output has been definitely established, the detailed dependence of the rate of energy output on the level of solar magnetic activity has not yet been measured with enough continuity and precision to determine the correlation throughout the full solar cycle. While a large fraction of the irradiance variability can be explained by the distribution of solar magnetic activity at the surface, small changes in the solar radius (i.e., contributing to the global variability of the solar envelope) could account for a significant fraction of the remaining variations. Studies of the apparent solar radius variation have reported contradictory results, in the form of both correlations and anticorrelations between the solar radius and, for example, the cycle of sunspot numbers. We present results from more than 30 yr of solar radius measurements obtained from the Mount Wilson synoptic program of solar magnetic observations carried out at the 150 foot (45.72 m) tower. We have used an improved definition of the solar radius that also allows us to study the heliolatitude dependence of the radius measurements. We find that the variations of the average radius are not significantly correlated with the solar cycle over the last three decades. We also compare the heliolatitude dependence of these radius measurements with recent results obtained at the Pic du Midi Observatory in France.