Comparison of quiet-Sun radiances measured by CDS and SUMER on SOHO
Solanki, S. K.; Wilhelm, K.; Schühle, U.; Thompson, W. T.; Huber, M. C. E.; Rüedi, I.; Lang, J.; Hollandt, J.; Pauluhn, A.
Switzerland, Germany, United Kingdom, United States, Netherlands
Abstract
Since the beginning of the SOHO (Solar and Heliospheric Observatory) mission 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 targets on the quiet Sun. The observations in the chromospheric line of He i (584 Å) and the two coronal lines of Mg x (609 Å and 624 Å) 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 quiet Sun's long term variability. Up to the SOHO accident, both instruments show a very good temporal correlation and stability. Even after the loss and recovery of the spacecraft, when the instruments had been exposed to extreme temperature conditions, the performance of the CDS and SUMER instruments is still good, as is the temporal correlation. However, the ratio between the efficiencies of the two instruments, which remained constant with time until the SOHO accident seems to have changed afterwards. In the coronal lines both instruments show an increase of average radiances towards the solar maximum.