Assessment of degradation of VIRGO radiometers on board SOHO

The determination of the total solar irradiance (TSI) from the SOHO/VIRGO e xperiment is made by first correcting the time series for all a priori know n influences, and then correcting the data for instrumental degradation, us ing the back-up instruments PMO6-VB and DIARAD-R. The long-term behaviour o f PMO6-VA shows an exponential decrease in its sensitivity with a time cons tant 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 a mplitude of 65 x 10(-6). After correcting PMO6-VA and DIARAD-L for their de gradation, the VIRGO TSI shows a minimum around July 1996 and has been incr easing since then by about 0.4 W m(-2).