Precision Full Disk Solar Photometry

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Recent precision photometric observations of the sun have shown it to be a variable star. Satellite observations of integrated sunlight, and ground-based solar limb photometry suggest that the luminosity varies by about 0.1% during a solar cycle, being brighter when there are more sunspots. The ground- based photometry suggests that the latitudinal effective surface temperature distribution of the sun varies over a solar cycle, and this change is not accounted for by the time dependent flux excess and deficit of sunspots and ordinary faculae. How does the solar luminosity affect terrestrial climate? There is growing statistical evidence that some local measures of weather are correlated with solar activity. Since we have just noted that solar activity (i.e., sunspot number) is correlated with the solar flux, it is appealing to speculate that some weather variations are caused by the small flux changes. The Maunder minimum, a time of few sunspots and thus perhaps a smaller solar flux, was coincident with a period of colder mean temperatures in much of Europe. The program funded here will 1) extend the precision limb photometry measurements to disk center with the comparable precision and higher spatial resolution, 2) measure the solar cycle variation of the faculae, sunspot, and background thermal flux contributions for two years during a period near solar maximum. The overall aim is to elucidate the mechanism behind solar luminosity variations and relate these to solar activity. It might then be possible to reconstruct solar variability over recent history and relate that to climate change.