Solar Variability in the Mg <sc>ii</sc> h and k Lines

Solar irradiance and its variations in the ultraviolet (UV) control the photochemistry in Earth's atmosphere and influence Earth's climate. The variability in the 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, the 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 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 Spectral And Total Irradiance REconstruction (SATIRE) 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 the Mg ii index time series on climate-relevant timescales of decades and longer. Here we present the time series of the Mg ii index spanning nearly three centuries.