Long-term total solar irradiance (TSI) variability based upon 1980-2000, spacecraft measurements

1980-2000, long-term spacecraft total solar irradiance (TSI) data sets were compared to verify TSI variability trends and to identify radiometer response shifts or drifts which may be misinterpreted as TSI variability. In addition, an empirical TSI model fit, based upon 10.7-cm solar radio fluxes and prompt photometric sunspot indices, was used to identify possible radiometer response changes. The spacecraft data sets indicate a long-term TSI variability component of approximately 0.1% (1.4 Watts-per-squared-meter) with a 10-year period. During the 1986 and 1996 periods of minimum solar magnetic activity, the most likely TSI minimum magnitude was found to lie in the 1365-1366 Watts-per-squared-meter range. The 1999-2000, spacecraft TSI measurements and the empirical model fit suggest that the 1999-2000, TSI peak magnitudes, corresponding to maximum solar magnetic activity, may be lower than the corresponding TSI maximum magnitudes which were measured during the 1979-1981, and 1989-1991 periods of maximum solar activity. This result suggests that there may exist another long-term variability component with a period longer than 10 years.