Do Photospheric Brightness Structures Outside Magnetic Flux Tubes Contribute to Solar Luminosity Variation?

Variations in total solar irradiance (TSI) correlate well with changes in projected area of photospheric magnetic flux tubes associated with spots, faculae and network. This correlation does not, however, rule out possible TSI contributions from photospheric brightness inhomogeneities located outside flux tubes, and spatially correlated with them. Previous reconstructions report 10% amplitude agreement with radiometry that seems to rule out significant extra-flux tube contributions. We show that, while these reconstructions are insensitive to behavior of near- limb facular contrast, their sensitivity to contrasts on the disc is relatively high. Given this sensitivity, previously used observational and theoretical approximations to wide-band facular contrast are too uncertain to support claims of 10% reconstruction accuracy. Recent measurements with the Solar Bolometric Imager (SBI) provide the first observational support for the relatively high wide-band, disc-center contrasts required to produce 10% rms agreement. Longer-term bolometric imaging to measure areas and bolometric contrasts homogeneously will be required to determine whether the systematic TSI residuals we see are caused mainly by uncertainties in sunspot contrasts, or by extra-flux tube brightness structures due to bright spot rings or convective stirring.