Solar UV variability is extremely relevant for the stratospheric ozone. It has an impact on Earth's atmospheric structure and dynamics through radiative heating and ozone photochemistry. Our goal is to study the slope of the solar UV spectrum in two UV bands important to the stratospheric ozone production. In order to investigate the solar spectral variability, we use data from SOLSTICE (the Solar Stellar Irradiance Comparison Experiment) on board the Solar Radiation and Climate Experiment (SORCE) satellite. Datasets used are far UV (115-180 nm) and middle UV (180-310 nm), as well as the Mg II index (the Bremen composite). We introduce the SOLSTICE [FUV-MUV] colour to study the solar spectral characteristics, as well as to analyse the colour versus Mg II index. To isolate the 11-year scale variation, we used Empirical Mode Decomposition (EMD) on the datasets. The [FUV-MUV] colour strongly correlates with the Mg II index. The [FUV-MUV] colour shows a time-dependent behaviour when plotted versus the Mg II index. To explain this dependence we hypothesize an efficiency reduction of SOLSTICE FUV irradiance using an exponential ageing law.
UV solar irradiance strongly affects the chemical and physical properties of the Earth's atmosphere. UV radiation is also a fundamental input for modeling the habitable zones of stars and the atmospheres of their exo-planets. Unfortunately, measurements of solar irradiance are affected by instrumental degradation and are not available before 1978. For other stars, the situation is worsened by interstellar medium absorption. Therefore, estimates of solar and stellar UV radiation and variability often rely on modeling. Recently, Lovric et al. (2017) used SORCE/SOLSTICE data to investigate the variability of a color-index that is a descriptor of the UV radiation that modulates the photochemistry of planets atmospheres. After correcting the SOLSTICE data for residual instrumental effects, the authors found the color-index to be strongly correlated with the Mg II index, a solar activity proxy. In this paper we employ an irradiance reconstruction to synthetize the UV color and Mg II index with the purpose of investigating the physical mechanisms that produce the strong correlation between the color-index and the solar activity. Our reconstruction, which extends back to 1989, reproduces very well the observations, and shows that the two indices can be described by the same linear relation for almost three cycles, thus ruling out an overcompensation of SORCE/SOLTICE data in the analysis of Lovric et al. (2017). We suggest that the strong correlation between the indices results from the UV radiation analyzed originating in the chromosphere, where atmosphere models of quiet and magnetic features present similar temperature and density gradients.
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