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 u pper atmosphere, where they create its embedded ionosphere. Knowledge of th e Sun's chromospheric radiation is therefore essential for understanding va riability in this region, which contributes significantly to space weather. With the goal of obtaining a suitable surrogate of extreme ultraviolet irr adiance variability for atmospheric studies, we construct a composite chrom ospheric index by comparing and cross calibrating available ground and spac e-based indices. Since ground-based flux observations of the chromosphere e xist only since 1974, we construct 11-year activity cycles of the index sin ce 1950 by using a parameterization of the daily plage index and the 100-da y 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 so lar and stellar chromospheric indices suggest that long term changes in the Sun's chromospheric emission since the Maunder minimum may exceed recent s olar cycle amplitudes by as much as a factor of 2. We simulate this by addi ng to the 11-year activity cycles a speculated varying background component derived from 15-year smoothed sunspot group numbers that increases from 19 00 to 1950 and remains approximately level in the decades since then.