Measuring cycle variations in the solar limb darkening

Abstract

The Precision Solar Photometric Telescope (PSPT) at Mauna Loa Solar Observatory (MLSO) in Hawaii yields full disk solar images with high (approximately 0.1%) photometric precision. Determination of the solar limb darkening function and known image defects to the same level of precision presents fundamental conceptual and practical difficulties, but promises synoptic measurement of small variations in the mean thermodynamic stratification of the solar atmosphere. We describe a procedure which identifies "quiet sun" in PSPT images and uniquely determines a limb darkening function describing its radial intensity variation. A series of activity masks, based on CaIIK intensity variations, are applied to the continuum images in order to understand the magnetic contribution to the center to limb variation. We present preliminary results and discuss future plans.