Peter Foukal, Heliophysics, Inc., Nahant, MA
The irradiance variation caused by spots and faculae is known to be proportional (but not necessarily equal) to changes in their projected areas. The first wide band facular contrasts obtained recently during the balloon flight of the Solar Bolometric Imager (SBI) should soon provide a more accurate test whether other mechanisms besides spots and faculae contribute to the rotational and 11-year variation, and might dominate on longer time scales. Previous evidence for multi-decadal variations based on stellar photometry has recently been retracted, and increasingly sensitive searches for such variations using solar photometry, heliometry, and helioseismology, have also so far failed to reveal any widely convincing evidence.
However this test turns out, the existence of an irradiance variation correlated with spot and facular areas is remarkable. The presence of localized dark and bright areas does not guarantee an effect on irradiance, since the global heat flow could quickly adjust to compensate. Analysis of this irradiance variation draws attention to the high thermal diffusivity of solar convection, and to the severe damping of deeper-lying thermal perturbations by the 100,000 year radiative relaxation time of the Sun’s convection zone. Contrary to recent claims, no consistent model of “structural” irradiance variations has been able to escape this robust constraint. Ironically then, it is the very presence of the short-term irradiance fluctuations that teaches us why the Sun seems to be so remarkably quiet photometrically.
The correlations between climate and solar activity now reported through the Holocene provide motivation for more sensitive solar measurements, which may yet reveal e.g. global irradiance variations arising in relatively superficial layers. But as we learn more about solar luminosity variation, it seems increasingly likely that Sun-climate influences are driven by changes of solar outputs of amplitude similar to those encountered over recent solar cycles.