Variations in the solar constant

Variations in the solar constant

. Observations with very stable and precise detectors on several Earth-orbiting satellites show that the Sunís total radiative input to the Earth, termed the solar constant or solar irradiance, is not a constant, but instead varies over time scales of days and years. Measurements from five independent space-based radiometers have been combined to produce the composite solar irradiance over two decades since 1978. They show that the Sunís output fluctuates during each 11-year sunspot cycle, changing by about 0.1 percent between maximums (1980 and 1990) and minimums (1987 and 1997) in magnetic activity. Temporary dips of up to 0.3 percent and a few daysí duration are due to the presence of large sunspots on the visible hemisphere. The larger number of sunspots near the peak in the 11-year cycle is accompanied by a rise in magnetic activity that creates an increase in luminous output that exceeds the cooling effects of sunspots. The solar constant, or irradiance, is given in units of watts per square meter, where one watt is equivalent to one joule per second. (Courtesy of Claus FrŲhlich.)

Copyright 2010, Professor Kenneth R. Lang, Tufts University