The primary scientific objective for the SOLSTICE program was to make precise and accurate measurements of the solar ultraviolet (UV) spectral irradiance over the spectral range 119 nm to 420 nm. Moreover, it had the goal of measuring solar variability over arbitrarily long periods, for example, over the duration of the UARS mission. The requirement for absolute accuracy was on the order of ± 10% (2 σ value), but the requirement for absolute accuracy between any two measurements spaced throughout the UARS mission was ± 2% (2 σ value). To achieve these goals the instrument response was determined from both preflight calibrations and from in-flight calibration and validation programs. SOLSTICE was designed with the unique capability of monitoring a number of bright blue stars (those with O and B spectral type) using the same optical elements and detectors employed for the solar observations. These stars, which vary by only small fractions of a percent over long time periods, provided a stable reference for deriving the SOLSTICE instrumental degradation rates.
The primary science requirement for SOLSTICE was to provide one full solar spectrum per calendar day, and to achieve this, the data processing algorithm combined typically 15 individual observations to form the single daily spectrum, adjusted to 1 Astronomical Unit (AU: mean Sun-Earth distance). This daily SOLSTICE spectrum, called the Level 3BS product, was reported for each 1.0 nm interval (centered on the half nm) between 119 nm to 420 nm and is available from the NASA Goddard data center.
The SOLSTICE instrument made daily solar irradiance measurements between 119 and 420 nanometers approximately 15 times per day, from October 1991 until September 2001.
Key Scientific Findings
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 nm to 420 nm, steadily decreased. Changes as large as a factor of two were observed at the shortest wavelengths, whereas the changes at the longer, visible wavelengths were only fractions of one percent. Likewise, the magnitude of the 27-day variations decreased significantly as solar minimum approached.
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 did not have the requisite 0.1% long-term accuracy to address this question directly, it often recorded 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.
The validation of the SOLSTICE solar irradiances was a joint effort of four solar UV irradiance programs. The measurements of the solar ultraviolet spectral irradiance made by the two UARS solar instruments, SUSIM and SOLSTICE, were compared with same-day measurements by two other solar instruments on the Shuttle Atmospheric Laboratory for Applications and Science (ATLAS) missions, ATLAS SUSIM and Shuttle Solar Backscatter Ultraviolet (SSBUV) experiment.
Measurements from the four instruments agree to better than the 2σ uncertainty of any one instrument, which is ± 5%–10% for all wavelengths above the 160 nm, as well as for strong emission features below 160 nm. Additionally, the long-term relative accuracy of the two UARS data sets is better than the original 2% goal, especially at wavelengths greater than 160 nm. This level of agreement is credited to accurate pre-flight calibrations coupled with comprehensive in-flight calibrations to track instrument degradation.
A detailed description of the SOLSTICE instrument validation is given by Woods et al. (“Validation of the UARS Solar Ultraviolet Irradiances: Comparison with the ATLAS-1, -2 Measurements“, J. Geophys. Res., 101, 9541-9569).