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 tau = 390 days and a l/e amplitude of 528 x 10(-6), whereas DIARAD-L exhibits a general increase in its sensitivity of about 0.25 x 10(-6)/day combined with an early exponential decrease of tau = 405 days and an l/e amplitude of 65 x 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).