Understanding the Role of Small-Scale Flux in Solar Spectral Irradiance Variation

Global solar spectral irradiance variations depend on changes in magnetic flux concentrations at the smallest scales. Modeling has focused on the contributions of magnetic structures in full disk images as those contributions have strong center-to-limb dependencies, but these dependencies have never been determined radiometrically; only the photometric intensity relative to some reference 'quiet-sun'(1), the magnetic structure contrast, is measurable with ground based imagery. This is problematic because unresolved inhomogeneities influence not only the full-disk structure intensities themselves, but also the quiet-sun background against which their contrast is measured. We thus argue that, to understand the physical causes underlying solar spectral irradiance variations, two fundamental questions must be addressed: What is the real I-lambda(mu) as a function of B in full-disk images? This can only be answered by imaging the Sun radiometrically from space, and we propose a Radiometric Solar Imager design. What governs spectral irradiance changes at sub arc-second scales? This can be addressed by a combination of high resolution ground based imaging (ATST-VBI) and three dimensional radiative magnetohydrodynamic modeling, and we propose a synoptic approach. Finally, a way to account for the variance introduced by unresolved substructure in spectral irradiance modeling must be devised. This is critical, as imaging and modeling at the highest resolutions but over the full solar disk will likely remain unattainable for some time.