The SoHO CELIAS/SEM data base

Abstract The CELIAS/SEM spectrometer aboard SoHO continuously monitors the full disk solar EUV and XUV irradiance. The instrumental noise level of the SEM is less than 0.5% of the signal in all channels, hence the short term signal variations larger than this in the SEM data base represent true solar variations in the EUV/XUV flux. Thrree sounding rocket calibration underflights since the SoHO launch have provided absolute EUV/soft X-ray solar flux values with which the long term stability of the SoHO SEM can be determined. In each of those flights the SEM “clone” has been calibrated before and after launch at the SURF II and SURF III synchrotron radiation facility of NIST in Gaithersburg, Maryland. The relative uncertainty of the SEM clone over a three year period is only ± 5.5% (1σ) and yields an absolute uncertainty of only 8.5% in transfering the absolute flux calibration to the SoHO CELIAS/SEM instrument.