Solar variability in the Mg II h and k lines

TL;DR

This paper introduces a model based on SATIRE to reconstruct the Mg II index over nearly three centuries, providing insights into solar UV variability's impact on Earth's climate beyond direct measurements.

Contribution

The novel model accurately reproduces observed Mg II index variations and extends the timeseries to climate-relevant timescales of several centuries.

Findings

Model agrees well with observed Mg II index variations

Reconstructed Mg II index covers nearly three centuries

Provides a tool for studying long-term solar UV variability effects

Abstract

Solar irradiance and its variations in the ultraviolet (UV) control the photochemistry in Earth's atmosphere and influence Earth's climate. The variability of Mg II h and k core-to-wing ratio, also known as the Mg II index, is highly correlated with the solar UV irradiance variability. Because of this, Mg II index is routinely used as a proxy for solar UV irradiance variability, which can help to get insights into the influence of solar UV irradiance variability on Earth's climate. Measurements of the Mg II index, however, have only been carried out since 1978 and do not cover the climate relevant timescales longer than a few decades. Here we present a model to calculate the Mg II index and its variability based on the well-established SATIRE (Spectral And Total Irradiance REconstruction) model. We demonstrate that our model calculations yield an excellent agreement with the observed Mg II index variations, both on the solar activity cycle and on the solar rotation timescales. Using this model, we synthesize Mg II index timeseries on climate relevant timescales of decades and longer. Here we present the timeseries of the Mg II index spanning nearly three centuries.