SUSIMʹS 11-year observational record of the solar UV irradiance Original Research Article

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM), a wavelength-scanning, dual-dispersion, dual-spectrometer instrument aboard the Upper Atmosphere Research Satellite (UARS), has measured the solar ultraviolet (UV) spectral irradiance (115–410 nm) since October 1991. This 11-year period, the duration of a solar activity cycle, extends from a late secondary maximum of solar cycle 22 through the intervening solar minimum and the maximum of solar cycle 23. Accordingly, SUSIM observed nearly the entire maximum-to-minimum variation of the solar UV irradiance of both solar cycles. The UV irradiance variations during the two solar cycles are compared. Apart from solar rotation effects and to within experimental accuracy, they show similar variation in the UV spectral irradiance. Solar cycle amplitudes calculated after removing solar rotation effects were ∼50% for the strong O I, C II, and Si IV emission features below 145 run, ∼8–18% between the A1 edge and 145 nm, respectively, and ∼4% between the Al edge and 263 nm. The amplitude of the solar cycle periodicity was not detected above ∼300 nm.