Number 151 (Story #4), November 15, 1993 by Phillip F. Schewe and Ben Stein SOLAR RADIATION VARIES only a few tenths of a percent at visible wavelengths over the sun's activity cycle, but varies by as much as a factor of 10 at extreme-ultraviolet and x-ray wavelengths. Solar UV radiation has an important role in Earth's atmospheric chemistry, particularly in the formation of stratospheric ozone. UV rays help to make ozone (from atmospheric oxygen) in the first place; then the ozone protects living organisms from harmful effects of the UV. The sun's UV flux, monitored by the Upper Atmospheric Research Satellite (UARS), is declining now as the sun passes into the less-active part of its 11-year cycle. By studying the solar UV spectrum over short and long periods of time, UARS hopes to provide information that can lead to a differentiation between natural and anthropogenic sources of ozone variability. (Physics World, Oct. 1993.)