The dependence of the [FUV-MUV] colour on solar cycle

Abstract

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.

The Correlation of Synthetic UV Color versus Mg ii Index along the Solar Cycle

“…5, as obtained by our reconstructions and SOLSTICE measurements. Following Lovric et al (2017) the scatter plot shows monthly averaged data and the differences are computed with respect to the average value over the period analyzed, that is…”

“…We provide here a basic description of the datasets, their treatment and a definition of the color index. An extensive description of the datasets, data analysis and correlations with solar magnetic activity of the computed indices is provided in Lovric et al (2017). Both SOLSTICE FUV and MUV spectral irradiance observations were interpolated using a Piecewise Cubic Hermite Interpolating Polynomial scheme to take into account of missing data.…”

“…Fluxes in each band were then integrated using a Riemann integration algorithm, and finally the monthly averages of FUV and MUV spectral fluxes were computed. The fluxes were used to derive the FUV and MUV magnitudes necessary to compute the [FUV-MUV] color index, defined as: Assuming the same degradation process for all wavelengths in the FUV an average reduction of about 0.0002% efficiency per month is enough to remove the observed slope difference (Lovric et al 2017). Under this hypothesis the [FUV-MUV] color index shows the same linear relation over the whole analyzed temporal range.…”

“…The purposes of this paper are to answer this question and to investigate the physical mechanisms determining the high linear relation between the UV spectral color index and solar activity measured with the Mg II index. To this aim we compare the results of Lovric et al (2017) with synthetic indices obtained with an irradiance reconstruction technique based on the use of semi-empirical atmosphere models and full-disk observations. The paper is organized as follows.…”

“…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.…”