Solar Irradiance Variability and Luminosity Changes

This review presents a discussion of radiometric measurements of solar total irradiance variation, AS, on time scales of days, to the 15-year extent of present measurements. It does not cover variations in spectral irradi ance, particularly the relatively large solar UV and EUV flux variations of great importance in studies of trends in stratospheric ozone, and of upper atmospheric heating variations. Analysis of the pyrheliometry of AS carried out from the Solar Maxi mum Mission, Nimbus-7 and ERBS satellites has shown that a) the sun brightens by 0.05 - 0.1% around activity maximum and; b) it dims by up to approximately 0.2% when large sunspot groups cross the disc. Empirical models of AS based on areas and photometric contrasts of sunspots, and on proxies of facular area, are remarkably successful in matching the observed record of AS, on time scales between days and the full 15- year record available since 1978. These models indicate that the opposite contributions of sunspots and faculae in active regions roughly balance over time scales of years, and the net solar brightening observed around peak activity is caused by increased emission from the photospheric magnetic network. model. the when least