Variations in solar radiative forcing
The first precise data on solar irradiance measured from satellites were not available until the late 1970s. The "solar constant", introduced by French physicist Claude Pouillet in 1838, fluctuates in the short term, and its 11-year cycle is characterized by a variation in total irradiance of around 0.1%. Paradoxically, irradiance increases with the number of sunspots: sunspots darken the Sun, but their effect is more than offset by that of the bright zones, or faculae, associated with them.
Compiling the raw data measured since 1978 by the various satellites presents a number of difficulties, as the series do not cover the entire last three cycles. Recent work is converging towards a synthetic curve showing that the eleven-year cycle is superimposed on a base line that is evolving slightly, probably downwards. Satellite data also show that variations in solar irradiance are not homogeneous across the entire spectrum, with ultra-violet rays in particular showing the greatest relative amplitude (with important consequences for stratospheric heating and the formation of stratospheric ozone through photochemical reactions).
To go beyond the first satellite measurements, we need to study other data such as sunspot observations, geomagnetic variability and cosmogenic nuclides. Over the last 20 years, many authors have shown that sunspot numbers and other characteristics can be used to reconstruct solar irradiance over the last 3-4 centuries. It is also possible to reconstruct solar activity by studying the abundance of cosmonucleides on Earth. These isotopes are formed by the interaction of galactic cosmic rays with atmospheric molecules. As protons in primary radiation are charged particles, cosmonucleide production is strongly modulated by the intensity of the Sun's and Earth's magnetic fields. Geochemists measure the abundance of these isotopes in natural archives such as polar ice (for beryllium-10 and chlorine-36), tree rings and coral (for carbon-14). The main readings from these studies are that solar minima are numerous, and that the Sun has spent a significant part of the last millennia in quiet phases (from several decades to a century), when it presented magnetic activity and therefore lower irradiance.
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Article published in La Lettre du Collège de France n°31, June 2011
