Comparison of quiet-Sun radiances measured by CDS and SUMER on SOHO

Since the beginning of the SOHO (Solar and Heliospheric Observatory) missio n an intercalibration programme was carried out which included simultaneous observations of the EUV instruments CDS (Coronal Diagnostic Spectrometer) and SUMER (Solar Ultraviolet Measurements of Emitted Radiation) of common t argets on the quiet Sun. The observations in the chromospheric line of He i (584 Angstrom) and the two coronal lines of Mg x (609 Angstrom and 624 Ang strom) thus cover the long period of 4 years and provide a data set highly suitable not only for instrumental comparison but also for studies of the q uiet Sun's long term variability. Up to the SOHO accident, both instruments show a very good temporal correlation and stability. Even after the loss a nd recovery of the spacecraft, when the instruments had been exposed to ext reme temperature conditions, the performance of the CDS and SUMER instrumen ts is still good, as is the temporal correlation. However, the ratio betwee n the efficiencies of the two instruments, which remained constant with tim e until the SOHO accident seems to have changed afterwards. In the coronal lines both instruments show an increase of average radiances towards the so lar maximum.