THE SUN'S VARIABLE RADIATION AND ITS RELEVANCE FOR EARTH¹

To what extent are changes in the Earth's global environment linked with fluctuations in its primary energy source, the radiation from a variable star, the Sun? A firm scientific basis for policy making with regard to anthropogenic greenhouse warming of climate and chlorofluorocarbon depletion of ozone requires a reliable answer to this question. Reduction of the vulnerability of spacecraft operations and communications to space weather necessitates knowledge of solar induced variability in Earth's upper atmosphere. Toward these goals, solar radiation monitoring and studies of solar variability mechanisms facilitate an understanding of the sources and amplitudes of the Sun's changing radiation. Interdisciplinary studies that link these changes with a wide array of terrestrial phenomena over the longer time scales of global change and the shorter time scales of space weather address the relevance of solar radiation variability for Earth. However, although numerous associations are apparent between solar and terrestrial fluctuations, full comprehension of the physical mechanisms responsible for the many facets of radiative Sun-Earth coupling remains to be accomplished.