Far-ultraviolet intensities and center-to-limb variations of active regions and quiet sun using UARS SOLSTICE irradiance measurements and ground-based spectroheliograms

The average intensity and center-to-limb variation of active regions, enhan ced network (large-scale active region fragments), and quiet Sun are estima ted by minimizing the variance between time series of the Solar-Stellar Irr adiance Comparison Experiment (SOLSTICE) far-ultraviolet (FUV) irradiance m easurements and a model of the solar irradiance that depends on the distrib ution of solar surface features. The model is constrained by grouping the b road range of solar features into four components that contribute the most to solar irradiance variability over the solar activity cycle. The areas of these components arc identified on approximately 800 ground-based Ca II K spectroheliograms with an algorithm that uses criteria of size, filling fac tor, intensity, and contiguity. These 800 maps are the "known" parameters t o the solar irradiance model. The unknown parameters to this model are the average intensities and center-to-limb variation of these four components. Because of systematic uncertainties associated with the SOLSTICE measuremen ts and the Ca II K maps we are limited to reporting the computed intensitie s and center-to-limb variation of active regions, enhanced network, and qui et Sun for wavelengths between 120.0 and 170.0 nm. We find good spectral ag reement between active region center-to-limb variations and image-based mea surements of quiet-Sun center-to-limb variations. Intensities for plage, en hanced network, and quiet Sun are reported with a typical uncertainty of ab out 8%. This uncertainty is primarily associated with the SOLSTICE FUV cali bration.