Uncertainty of total solar irradiance: an assessment of the last twenty years of space radiometry

Abstract

A complete set of total solar irradiance measurements from space is available dating from November 1978, yielding a time series of more than twenty years. From measurements made by different space radiometers (HF on NIMBUS-7, ACRIM I on SMM, ACRIM II on UARS and VIRGO on SOHO) a composite record of solar total irradiance can be compiled. The corrections needed for each radiometer, mainly to compensate for degradation, are based on a detailed study of the behaviour of the VIRGO (Variability of Solar Irradiance and Gravity Oscillations) radiometers. Moreover, several errors arising from operational changes or interruptions, which influence the overall uncertainty more than corrections for degradation, have to be corrected. A composite of the corrected time series is constructed in order to estimate the long-term stability, which is much better than the estimated 1 part in 105 per annum deduced as an upper limit from VIRGO radiometry. The absolute accuracy of the data, however, is more difficult to estimate.