Variability of a composite chromospheric irradiance index during the 11-year activity cycle and over longer time periods

Chromospheric emission lines are the dominant energy input to the Earth's upper atmosphere, where they create its embedded ionosphere. Knowledge of the Sun's chromospheric radiation is therefore essential for understanding variability in this region, which contributes significantly to space weather. With the goal of obtaining a suitable surrogate of extreme ultraviolet irradiance variability for atmospheric studies, we construct a composite chromospheric index by comparing and cross calibrating available ground and space-based indices. Since ground-based flux observations of the chromosphere exist only since 1974, we construct 11-year activity cycles of the index since 1950 by using a parameterization of the daily plage index and the 100-day smoothed 10.7-cm flux, and prior to that by an analogous parameterization of the daily and 100-day smoothed sunspot group numbers. Comparisons of solar and stellar chromospheric indices suggest that long term changes in the Sun's chromospheric emission since the Maunder minimum may exceed recent solar cycle amplitudes by as much as a factor of 2. We simulate this by adding to the 11-year activity cycles a speculated varying background component derived from 15-year smoothed sunspot group numbers that increases from 1900 to 1950 and remains approximately level in the decades since then.