Solar irradiance variability

Abstract

The solar “constant” varies over time scales from minutes to years and decades. For the influence on the atmospheric energy budget time scales longer than a few days become important, for climate research information over years to decades is needed. From direct measurements of the solar “constant” during the last 10 years, by e.g. the ACRIM experiment on the Solar Maximum Mission satellite (SMM), one indeed becomes aware of substantial changes on time scales longer than days. The most important influences seem to be related to solar activity. Results from a multi-variate spectral analysis to investigate the influence of sunspots and bright magnetic elements on the total solar irradiance during 1980 and in 1984/85 show that for periods up to about 100 days, most of the power in the spectrum of the ACRIM irradiance variance is explained by the effect of the complex sunspot groups showing new activity. The correlation with old sunspots is much lower. In 1984/85, near the solar minimum, around the 27 days rotational period of the sun the greatest part of the ACRIM irradiance power spectrum is related to the enhanced UV flux, which indicates that during the solar minimum, when only a few active regions are on the sun, the main contribution to the irradiance variation arises from the active network. After eliminating the effect of sunspots and bright magnetic elements from the irradiance power spectrum, there are still peaks around 9 and 27 days. This indicates that global effects not related to the solar activity produce irradiance variations, which may be modulated by the same mechanism as that producing solar activity. The same record of the total solar irradiance shows long-term variations over the last 8 years, which are related to the activity cycle and show a difference between the maximum in 1980 and the minimum in 1986 of 0.08% with the irradiance being higher during the solar maximum. Model calculations using these directly measured values as calibration and taking into account the influence of the excess radiation from bright magnetic elements and of the dark spots are used to extend the record back to 1954. The results are also compared with measurements in the late sixties, which were obviously of much less accuracy, but can be compared at least qualitatively with the model predictions. They show also how variable the influence of the activity on the irradiance is from the cycle to cycle.