Solar ultraviolet irradiance and its temporal variation

Solar radiation at wavelengths below 300 nm is almost completely absorbed b y the Earth's atmosphere, becoming the dominant direct energy source and pl aying a major role in the chemistry and dynamics. Even small changes in thi s incoming radiation held will have both direct and indirect influences on atmospheric processes, and perhaps will affect the Earth's climate as well. Some of the very earliest space missions included devices to measure solar ultraviolet irradiance, but for the most part they lacked the necessary pre cision and accuracy to record true solar variability over long time periods . The technology has continued to improve, and today reliable measurements over time scales up to, and including, the 11-year solar cycle, an being ob tained. This review provides a summary of measurements made during the most recent solar cycle (number 22 extending from 1986-1996), with emphasis on the spec tral range 120-300 nm. Comparisons and validations of recent data sets are considered, together with an assessment of the present understanding of the solar variations. There is now general agreement that for solar cycle 22 t he variation is as large as a factor of two at the shortest wavelengths, de creasing to roughly 10% near 200 nm. Proceeding to wavelengths above 200 nm the solar variability continues to decrease, and at about 300 nm it become s smaller than the present measurement capability of about 1%. (C) 1999 Els evier Science Ltd. All rights reserved.