Global variations of the chromospheric irradiance

The radiation emitted by chromospheric spectral lines integrated over the whole solar disk (i.e. the solar chromospheric irradiance) has been observed to vary at almost all time scales accessible to observations. These variations show some similarities with those of the Sun's total irradiance (dominated by contributions from the photosphere), but also significant differences. A brief overview of the observations of chromospheric irradiance variations will be given, followed by a discussion of possible mechanisms causing such variations and a presentation of models that aim to quantitatively reconstructed the observed as well as longer term variations.