The radiative output of the Sun is variable on different time scales, from seconds to the evolutionary scale of the star. Various processes related to solar magnetic field dynamics, turbulent convection and evolutionary mechanisms, cause these fluctuations in the Sun’s irradiance. The main component of the solar variability over the last few centuries is the well-known 11-year cycle. Total Solar Irradiance shows changes of around 0.1% with different spectral regions contributing different amounts (e.g. the UV). Solar UV variability is extremely important for the stratospheric ozone which has an impact on Earth’s atmospheric structure and dynamics through radiative heating and ozone photochemistry.
We investigate the solar Far-UV and Middle-UV during the descending phase of cycle 23 and ascending phase of cycle 24 using SOLSTICE/SORCE data. We introduce the [FUV-MUV] colour index to measure the solar UV spectral slope and study its dependence on solar activity using the Mg II index Bremen composite and time. We found a clear correlation between the [FUV-MUV] color and Mg II index. Additionally, the analysis shows a difference in the UV spectral slope during the ascending phase of cycle 23 and descending phase of cycle 24. We attribute this difference to a residual uncorrected time-dependent performance of SOLSTICE UV channels. We use the correlation between corrected [FUV-MUV] color index and Mg II index to reconstruct the solar UV spectral slope between 1978 and today.