he SOLSTICE has made continuous, daily observations of the Sun since October 3, 1991. The instrument response has been very stable, and moreover, all changes in sensitivity have been accurately and precisely tracked using the instrument's unique capability of observing bright blue stars as reference standards. The knowledge and removal of instrument effects thereby produce reliable time series of true solar variability.
The stellar calibration technique employed by SOLSTICE has placed the ultraviolet irradiance from the Sun on a "standard" reference scale. At any future time, the Sun can be calibrated against this same stellar reference, and thereby directly related to these UARS observations. In this way solar variability over arbitrarily long time periods can be measured.
Early in the UARS mission the Sun was very active, and conditions at that time well represent "solar maximum" conditions. There is a very strong and clear signal of the 27-day (rotation period of the Sun) variability, caused by the passage of large active regions across the face of the Sun.
During the intervening three and a half years, the output of the Sun at ultraviolet wavelengths, 120 to 420 nm, has steadily decreased. Changes as large as a factor of two are observed at the shortest wavelengths, whereas the changes at the longer, visible wavelengths are only fractions of one percent. Likewise, the magnitude of the 27-day variations has decreased significantly as we near solar minimum.
The total radiation from the Sun varies by only small fractions of one percent over its 11-year activity cycle, but even such small changes can have important climate implications. Little is known about how these small changes are distributed in wavelength, and, although SOLSTICE does not have the requisite 0.1% long-term accuracy to address this question directly, it often records shorter term variations with exceptional precision. Studying the ultraviolet contribution to variations in the "solar constant" over these limited periods will improve our understanding of the long-term variations.