SunRISE Proposal: Precision Solar Disk Photometry--A Tool for Understanding Changes in the Sun's Luminosity

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This proposal will continue an ongoing effort to precisely measure large-scale solar surface brightness variations across the solar disk. A unique and specialized instrument to make these measurements has been constructed at Big Bear Solar Observatory, and preliminary data have been acquired. With a timeseries of precision photometric data, the PI will conduct a number of investigations: 1) The PI will compare measured latitude- and time-dependent brightness variations with time-dependent solar constant measurements. The more complete spatial information gotten from the photometer will be used to better understand the physical mechanisms responsible for short- and long-term solar luminosity variations. For example, measured average latitude- and >-dependent brightness structure (here > is the usual heliocentric angle), together with the measured facular contrast function, can be used to directly measure the contribution of faculae to solar luminosity changes. Further, if the Sun's luminosity variations are determined (using our photometric data) to be confined to the active latitudes, then precision ground-based measurements such as those proposed here could make an excellent substitute for space- based solar constant measurements, when the latter are unavailable. 2) The PI will make a better measurement of the pole-equator temperature difference, making some effort to separate the cycle- dependent magnetic signals from the presumably time-independent) differential rotation signal. Such a measurement will constrain models of the Sun's differential rotation. 3) The PI will accurately measure brightness variations near the solar pole, in order to get a better understanding of the polar magnetic fields and polar faculae. Making precision measurements of the large-scale brightness structure of the Sun is to date a largely unexplored area of research. The data set of measurements could contribute significantly to the understanding and monitoring of the Sun's luminosity variations. #