SOLAR UV ACTIVITY AT SOLAR-CYCLE-21 AND SOLAR-CYCLE-22 MINIMUM FROM NOAA-9 SBUV 2 DATA/

Although solar ultraviolet (UV) irradiance measurements have been made regularly from satellite instruments for almost 20 years, only one co mplete solar cycle minimum has been observed during this period. Solar activity is currently moving through the minimum phase between cycles 22 and 23, so it is of interest to compare recent data taken from the NOAA-9 SBUV/2 instrument with data taken by the same instrument durin g the previous solar minimum in 1985-1986. NOAA-9 SBUV/2 is the first instrument to make continuous solar UV measurements for a complete sol ar cycle. Direct irradiance measurements (e.g., 205 nm) from NOAA-9 ar e currently useful for examining short-term variations, but have not b een corrected for long-term instrument sensitivity changes. We use the Mg II proxy index to illustrate variability on solar cycle time scale s, and to provide complementary information on short-term variability Comparisons with contemporaneous data from Nimbus-7 SBUV (1985-1986) a nd UARS SUSIM (1994-1995) are used to validate the results obtained fr om the NOAA-9 data. Current short-term UV activity differs from the cy cle 21-22 minimum. Continuous 13-day periodicity was observed from Sep tember 1994 to March 1995, a condition which has only been seen previo usly for shorter intervals during rising or maximum activity levels. T he 205 nm irradiance and Mg II index are expected to track very closel y on short time scales, but show differences in behavior during the mi nimum between cycles 22 and 23.