LONG-TERM TOTAL SOLAR IRRADIANCE VARIABILITY DURING SUNSPOT CYCLE-22

Total solar irradiance measurements from the 1984-1993 Earth Radiation Budget Satellite (ERBS) active cavity radiometer and 1978-1993 Nimbus 7 transfer cavity radiometer spacecraft experiments are analyzed to d etect the presences of 11-, 22-, and 80-year irradiance variability co mponents, The analyses confirmed the existence of a significant 11-yea r irradiance variability component, associated with solar magnetic act ivity and the sunspot cycle. The analyses also suggest the presence of a 22- or 80-year variability component. The earlier Nimbus 7 and Sola r Maximum Mission (SMM) spacecraft irradiance measurements decreased a pproximately 1.2 and 1.3 Wm(-2), respectively, between 1980 and 1986. The Nimbus 7 values increased 1.2 Wm(-2) between 1986 and 1989. The ER BS irradiance measurements increased 1.3 W-m-2 during 1986-1989, and t hen decreased 0.4 Wm(-2) (at an annual rate of 0.14 Wm(-2)yr(-1)) duri ng 1990-1993. Considering the correlations between ERBS, Nimbus 7, and SMM irradiance trends and solar magnetic activity, the total solar ir radiance should decrease to minimum levels by 1997 as solar activity d ecreases to minimum levels, and then increase to maximum levels by the year 2000 as solar activity rises. The ERBS measurements yielded 1365 .4 +/- 0.7 Wm(-2) as the mean irradiance value with measurement accura cies and precisions of 0.2% and 0.02%, respectively. The ERBS mean irr adiance value is within 0.2% of the 1367.4, 1365.9, and 1366.9 Wm(-2) mean values for the SMM, Upper Atmosphere Research Satellite (UARS), a nd Space Shuttle Atmospheric Laboratory for Applications and Science ( ATLAS 1) Solar Constant (SOLCON) active cavity radiometer spacecraft e xperiments, respectively. The Nimbus 7 measurements yielded 1372.1 Wm( -2) as the mean value with a measurement accuracy of 0.5%. Empirical i rradiance model fits, based upon 10.7-cm solar radio flux (F10) and ph otometric sunspot index (PSI), were used to assess the quality of the ERBS, Nimbus 7, SMM, and the UARS irradiance data sets and to identify irradiance variability trends which may be caused by drifts or shifts in the spacecraft sensor responses. Comparisons among the fits and me asured irradiances indicate that the Nimbus 7 radiometer response shif ted by a total of 0.8 Wm(-2) between September 1989 and April 1990 and that the ERBS and UARS radiometers each drifted approximately 0.5 Wm( -2) during the first 5 months in orbit.