Overview

Solar radiation is the energy source that not only maintains the warmth of the earth but alsoenergises the atmospheric and ocean circulations. It is not surprising, therefore, that the sun andthe strength of solar radiation (the solar irradiance) have been studied in great detail. It hasbeen only since the availability of satellites that direct measurements of solar irradiance outsideof the earth s atmosphere have been possible. Prior to satellites, any measurement of the solarbeam has had to contend with the effects of the atmosphere and its modification of radiation. Evenballoon-borne instruments could not reach sufficient altitude to eliminate all effects of the upperatmosphere.

Despite the increasing precision of instruments developed during the 19th and 20th centuriesthe variance in the measurements was relatively large due to scattering of solar radiation byaerosols and cloud particles, and by the selected absorption by greenhouse gases and atmosphericparticles. The variations in the measurements could not be attributed to variability or trends insolar irradiance with any certainty. For practical purposes, a universally agreed value of solarirradiance was treated as the Solar Constant. However, it was suspected for a long time that solarirradiance is variable. With the development of satellite-borne instruments, it has been possibleover the past two decades to monitor the sun s irradiance outside the atmosphere s influence andestimate more accurately the magnitude of solar variability.

In spite of their better observing position the satellite instruments have their own set ofproblems. The instruments have great precision and are able to detect the magnitude of significantshort period fluctuations...