Solar irradiance variability: Progress in measurement and empirical analysis

Here we report the progress in both measurements and analysis of total solar irradiance (TSI) during the last 24 years. Recent TSI measurements made by ACRIM III and VIRGO in the last two years agree to within 0.5 W m ^-2 and show the same pattern of short-term variability. A 24-year composite record of TSI measurements gives estimates of its variation for two solar cycles. Such composites give the first estimates of secular variation of the solar output. Our analysis of TSI data from solar minimum to maximum for cycles 22 and 23 gives nearly identical regression equations because of improvement in VIRGO degradation corrections, thus, resolving the empirical issue raised by de Toma et al. [Astrophys. J. Lett. 549 (2001) L131]. This agreement occurs despite a decrease in cycle 23 of sunspot number by ≈33% below solar maximum values for cycles 21 and 22.