The Mg 280‐nm doublet as a monitor of changes in solar ultraviolet irradiance

Five years of 160‐ to 400‐nm solar flux measurements by the Solar Backscattered Ultraviolet experiment on Nimbus 7 have been analyzed. The flux in the center of strong lines and at shorter wavelengths varies with periods that correspond to modulation by the rotation of active regions. The modulation is greater at the centers of strong lines and at shorter wavelengths, corresponding to radiation that originates at higher levels in the solar atmosphere. The ratio of the irradiance in the core of the Mg 280‐nm line to the irradiance at neighboring wavelengths is used as an index of solar variation. A scaling factor is derived by comparing rotational modulation at other wavelengths with the rotational modulation of the index. The scaled Mg II 280‐nm strength successfully represents both rotational and long‐term variations across the Al absorption edge near 210 nm. This ratio can therefore provide an empirical representation of long‐term ultraviolet solar variability. Scaling factors are derived and changes estimated at several ultraviolet wavelengths. At 204 nm, in the wavelength region that drives atmospheric photochemistry, the solar irradiance drops about 4% from its average level for 1979–1980 to late 1983. The total estimated range of variation of the 27‐day averaged (one rotation) 204‐nm irradiance is 6%, over the 5 years of measurements. A least squares fit shows that over the 5 years, 27‐day averages of 10.7‐cm radio flux and of the Mg II index follow a linear relation. The radio flux can therefore be used to estimate changes in the solar ultraviolet for times before the launch of Nimbus 7.