SOLAR CYCLE DEPENDENCE OF THE SUN'S RADIUS AT = 525:0nm

The Mt. Wilson synoptic program of solar magnetic observations carried out at the 150-foot tower scans the solar disk one to twenty times per day. As part of this program, the radius is determined as an average distance between the image center and the point where the intensity in the FeI line at 525:0nm drops to 25% of its value at disk center. This record extends back to 1967. We have carried out a re-analysis of this data base correcting for such eeects as scattered light and atmospheric refraction. Radius variations based on data from this program have been previously studied with results 1] which were inconclusive due to inadequate control of the spectral pass-band. In 1982 R. Howard replaced a system in which the pass-band depended on the position and alignment of a combination of prisms by a system in which the spectral pass-band is selected by a permanent ber-optic geometry. The ber-optic system has been in use without change between 1982 and the present time and has provided a long period of instrumental stability which permits the study of radius changes. Since 1982 we nd a peak to peak change of 0.4 _ 00 which is correlated with the solar cycle. This change may play a role in the variations in the total solar irradiance. 1. SOLAR VARIABILITY Several spacecraft measurements have established that the total solar output changes with the solar cycle 2-5]. Although the dependence of the rate energy output on the level of solar magnetic activity has not yet been measured with enough continuity and precision to establish the correlation throughout the full solar cycle, it is nonetheless apparent that higher levels of solar activity coincide with greater solar energy output. This increase in total output occurs in spite of the fact that sunspot groups darken the sun's surface and block some of the radiating surface. Decreases in the sun's output are well correlated with sunspot area on the apparent solar surface. Rotational modulation of the output is readily de-tectible from this eeect. In spite of this blocking eeect, the brightenings associated with plages and faculae in active regions produce an increase in solar output that more than compensates for the blocking eeect. The variations in the total solar output have been modelled using a four component model: quiet sun, magnetic network, faculae and sunspots 6]. The last two are most important for the short …