SOLAR UV IRRADIANCE VARIABILITY DURING THE DECLINING PHASE OF THE SOLAR-CYCLE-22

The SUSIM (Solar Ultraviolet Spectral Irradiance Monitor) and the SOLSTICE (Solar Stellar Irradiance Comparison Experiment) instruments on the UARS (Upper Atmosphere Research Satellite) have been making continuous measurements of the solar UV flux in the spectral range 115-420 nm since October 1991. This period, characterized as the declining phase of solar cycle 22, shows a transition from near maximum to near minimum solar activity levels. During this period, the solar UV flux at Lyman alpha decreased by about 45% from a mean solar maximum value of about 9 mW/m(2), and the integrated solar flux between 200-205 nm decreased by about 5 % from a mean value of about 47 mW/m(2). Using the MgII index as a proxy of solar UV irradiance variability, it is shown that the temporal relationship of the UARS solar Lyman alpha irradiance and the MgII index during solar cycle 22 is significantly different than during solar cycle 21, inferred from the SME (Solar Mesosphere Explorer) Lyman alpha measurements. Moreover, during solar cycle 22, the scale factor for solar Lyman alpha irradiance (% change for 1% change in MgII index) is about 1.5 times larger for long term changes than for changes over the time scale of a solar rotation. Unlike Lyman alpha, the scale factor for the UV flux in the 200-205 nm wavelength range, is close to unity both for the rotational and longer time scales. The spectral dependence of the two scale factors in the 120-200 nm range is derived from the SOLSTICE data which may be used with the MgII index to estimate the UV irradiance variability in this spectral range for both short and long time scales.