Total Solar Irradiance Variability From 1978 to Present

Author:  Claus Fröhlich, Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Switzerland


            Since February 1996 VIRGO radiometers observe total solar irradiance from SOHO at L1, allowing continuous and uninterrupted observations of the Sun.  Besides the 3-month gap during the SOHO vacation in summer 1998 and a few other minor gaps the record covers more than 99% of the almost 8 years of operation.  This allows a very detailed assessment of the long-term behavior of the two VIRGO radiometers, PMO6V and DIARAD. The results of this analysis are presented and from comparison with simultaneous measurement from other missions their implications for other space radiometers discussed.  With these data and those from HF/NIMBUS7, ACRIM-I/SMM and ACRIM-II/UARS a composite of total solar irradiance (TSI) is presented (updated and improved from Fröhlich and Lean, 1998), which covers now almost three solar cycles starting in November 1978, just before the maximum of solar cycle 21.  Radiometrically the composite is based on ACRIM-I and ACRIM-II, with the latter shifted to the scale of ACRIM-I, and finally referred to SARR via ACRIM-II.  During the gap between ACRIM-I and II the results from the HF radiometer are used.  These data need to be corrected for a change, which was first detected by comparison with ERBS.  The inclusion or omittance of this change is the main reason for the discrepancy between this composite and the one presented by Willson and Mordvinov (2003). The discussion concentrates on this change and its determination by comparison of HF with ERBS and a proxy model for interpolation between its 14-day observations. Comparison of the composite with ERBS over the period from 1984-2003 supports very strongly the validity and need of the HF correction and confirms that TSI had no significant trend over the last 25 years of observation from space.


C. Fröhlich and J. Lean, 1998, The Sun's Total Irradiance: Cycles and Trends in the Past Two Decades and Associated Climate Change Uncertainties, Geophys. Res. Let., {25}, 4377—4380

R. C. Willson and A.~V. Mordvinov, 2003, Secular Total Solar Irradiance trend during solar cycles 21-23, Geophys. Res. Let., {30}, 1199—1202