Variations in the Absolute Ultraviolet Intensities of Polar Coronal Holes

The well-maintained UVCS/SOHO radiometric calibration is used to determine the variations in the ultraviolet intensities in polar coronal holes between the Solar Cycle 22/23 and Cycle 23/24 minima. The radiometric calibration has been carefully monitored and updated during the mission by observing an ensemble of B stars, which, as a group, are believed to have a stable mean irradiance. These observations, along with data from the freshly calibrated Ultraviolet Coronal Spectrometer on the Spartan 201 satellite in 1998, have been used to determine the changes in the calibration. The Spartan 201 inter-calibration, together with the original laboratory calibration, was used to establish the in-flight absolute radiometric calibration scale. This paper summarizes the in-flight radiometric calibration of UVCS/SOHO and observed variations in polar coronal hole intensities as a function of heliographic height above the poles of the Sun.