Solar VUV measurements obtained by SOHO instruments and their radiometric calibration

The Solar and Heliospheric Observatory (SOHO) of ESA and NASA is equipped with a suite of instruments capable of observing the Sun from the core to the outer corona. Several of these instruments detect radiation in the vacuum-ultraviolet (VUV) wavelength range (shortwards of 200 nm), where precise and accurate radiometric measurements are of extreme significance for solar and terrestrial investigations, but, at the same time, are very difficult to obtain due to degradation effects of most optical systems under solar ultraviolet irradiation. The SOHO instruments (built under strict particulate and chemical cleanliness conditions) have been carefully calibrated before launch (traceable to primary source standards) and cross-calibrated during the mission operations. The solar observations obtained since the solar sunspot minimum in 1996 to the present time allowed us to make a major step forward in solar XUV radiometry, both as far as spectral radiance and irradiance measurements are concerned.