Measurements of the Solar Ultraviolet Irradiance

Solar radiation at wavelengths shorter than 300 nm is almost completely absorbed by the Earth's atmosphere-between 120 and 300 nm it plays a major role in O 2 and O 3 photolysis within the stratosphere and mesosphere. Although this ultraviolet portion of solar radiation comprises less than 1% of the total solar irradiance, it directly influences the physical and chemical processes in the atmosphere, and likely has important indirect influences on climate as well. In the past it has been difficult to measure the solar ultraviolet; not only because instruments must be taken above the atmosphere to make observations, but also because the very energetic photons damage optical systems, causing loss in instrument sensitivity, which leads to measurement uncertainties and ambiguity that exceed solar variations. In the past twenty years instrument capabilities have steadily improved, and current data sets now provide a reasonably clear understanding of solar variability. During the two solar cycles for which measurements are in hand, the Sun has apparently varied by roughly a factor of two at Lyman-a, 5 to 10% out to 200 nm, and only a few percent between 200 to 300 nm. At wavelengths beyond 300 nm the spectral observations set an upper limit of no more than 1% solar variation, with the additional constraint that the variation of total solar irradiance (TSI) is on the order of 0.1%.