SOLAR-CYCLE-22 UV SPECTRAL IRRADIANCE VARIABILITY - CURRENT MEASUREMENTS BY SUSIM UARS

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) has measured the solar spectral irradiance for wavelengths 115-410 nm on a daily b asis since October 11, 1991. The absolutely calibrated solar UV irradi ances through January 8, 1996 have been produced. Their time-dependent behavior is similar to that of the Mg II index as measured both by NO AA-9 SBUV and by SUSIM itself The maximum long-term variation observed by SUSIM is at L alpha and is measured to be in excess of a factor of 2. This maximum variation decreases with increasing wavelength until about 300 nm where no significant long-term variation is directly meas ured above SUSIM's estimated 1-2% relative accuracy. The wavelength de pendence of the measured UV variability is found to roughly correspond to the mean emission height given by solar atmospheric radiative tran sfer models. Because SUSIM observations began when solar activity was near its peak and now extend to very near its minimum, estimates of th e solar cycle 22 UV variability are generated from a combination of th ese measurements and solar activity proxy indices.