Solar UV Irradiance Measurements from SUSIM UARS

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) aboard the Upper Atmosphere Research Satellite (UARS) has measured the solar UV irradiance since October 1991. This 13 year time period includes a secondary maximum of solar cycle 22, the ensuing solar minimum, the maximum of solar cycle 23, and its declining phase. SUSIM maintains the calibration of its changing responsivity through a combination of measurements of four stable onboard deuterium calibration lamps and measurements of infrequently exposed reference channels. The wavelength-dependent UV irradiance time series exhibit two dominant periodicities, that of solar rotation (27~days) and solar cycle (11~years). The solar cycle variation of the UV irradiance was similar for solar cycles~22 and 23: about 55% at Ly-α , about 7% at 205~nm, and less than 1% above 300~nm. The SUSIM measurements are compared with other coincident measurements including those of SOLSTICE over a similar time period and with the Mg~II core-to-wing ratio index. Prospects and requirements for continued and possibly improved monitoring of the solar UV irradiance are discussed.