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

TL;DR

This study investigates how the solar UV spectrum's color index varies with the solar cycle, using satellite data and a new color metric, revealing a strong correlation with the Mg II index and a time-dependent behavior.

Contribution

Introduces the SOLSTICE [FUV - MUV] colour as a new metric to analyze solar UV spectral variability and its relation to the Mg II index over the solar cycle.

Findings

Strong correlation between [FUV - MUV] colour and Mg II index.

Time-dependent behaviour of the colour versus Mg II index.

Proposed exponential aging law to explain irradiance reduction.

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 for the stratospheric ozone production. In order to investigate the solar spectral variability, we use SOLSTICE (the Solar Stellar Irradiance Comparison Experiment) data onboard Solar Radiation and Climate Experiment (SORCE) satellite. Data sets used are far UV (115-180nm) and middle UV (180-310nm), 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 analysis of the colour versus Mg II index. To isolate the 11-year scale variation, we used the Empirical Mode decomposition (EMD) on the data sets. The [FUV - MUV] colour strongly correlates with the Mg II index. More in detail, the [FUV - MUV] colour shows a time dependent behavior when plotted versus Mg II index. To explain this dependence we hypothesize an efficiency reduction of SOLSTICE FUV irradiance using an exponential aging law.