The radiation flux from the Sun received by the Earth at its average distance, 1 au. When measured at a specific wavelength or wavelengths, it is called spectral irradiance, while over all wavelengths it is referred to as the total solar irradiance (TSI). At visible and near-infrared wavelengths the solar irradiance shows very little variation (a fraction of a percent) with time. The ultraviolet and X-ray irradiances show much larger variations over a solar cycle due to active regions and solar flares. Although much smaller than the visible and near-infrared irradiance, this high-energy radiation is absorbed by the Earth’s atmosphere and is very important for atmospheric chemistry.