Solar UV variations during the decline of Cycle 23

Previous satellite measurements of solar UV variability show consistent solar cycle irradiance changes within instrumental uncertainties, and also show consistent spectral dependence for both rotational and solar cycle variations. Empirical solar irradiance models produce solar UV variations that agree well with observational data. Recent UV irradiance data from the Solar Radiation and Climate Experiment (SORCE) Spectral Irradiance Monitor (SIM) and Solar Stellar Irradiance Comparison Experiment (SOLSTICE) instruments longward of 170 nm covering the declining phase of Cycle 23 show solar variations that greatly exceed both previous measurements and predicted irradiance changes over this period. The spectral dependence of the SIM and SOLSTICE variations differs from previous results. However, short-term solar variability derived from SIM and SOLSTICE UV irradiance data agrees with other concurrent solar UV measurements and previous results, suggesting no change in solar physics. The SORCE long-term UV results can be explained by undercorrection of instrument response changes during early on-orbit measurements.

► Previous solar cycle UV irradiance changes agree with one another and model results.
► Solar UV proxy data do not show significant changes between cycle minima.
► SORCE irradiance data in Cycle 23 show large UV changes, anomalous spectral behavior.
► Short-term irradiance variations in Cycle 23 are consistent for all instruments.
► SORCE UV irradiance data appear to have uncorrected calibration changes.