Assessment of degradation of VIRGO radiometers on board SOHO

The determination of the total solar irradiance (TSI) from the SOHO/VIRGO experiment is made by first correcting the time series for all a priori known influences, and then correcting the data for instrumental degradation, using the back-up instruments PMO6-VB and DIARAD-R. The long-term behaviour of PMO6-VA shows an exponential decrease in its sensitivity with a time constant of τ = 390 days and a 1/e amplitude of 528 × 10 -6 , whereas DIARAD-L exhibits a general increase in its sensitivity of about 0.25 × 10 -6 /day combined with an early exponential decrease of 405 days and an 1/e amplitude of 65 × 10 -6 . After correcting PMO6- VA and DIARAD-L for their degradation, the VIRGO TSI shows a minimum around July 1996 and has been increasing since then by about 0.4 W m -2 .