The center-to-limb behavior of solar active regions at ultraviolet wavelengths

The time series of solar ultraviolet irradiances measured by the Solar Ultraviolet Spectral Irradiance Monitor on the Upper Atmosphere Research Satellite have been analyzed to describe the center-to-limb behavior of the excess surface brightness of solar active regions over the wavelength range 142–265 nm. Comparison of these results with the analysis by Worden, Woods, and Bowman (2001, ApJ, 560, 1020) of the time series produced by the Solar-Stellar Irradiance Comparison Experiment on the same spacecraft over the wavelength range 120–170 nm shows excellent agreement. For 170–265 nm, we present new results on the center-to-limb behavior of the surface brightness of solar active regions. Comparisons with previous results for the quiet disk show roughly similar behaviors at wavelengths below 168 nm and above 210 nm, where both exhibit weak limb brightening and darkening, respectively. At intermediate wavelengths, 168–210 nm, active regions exhibit much stronger limb darkening than does the quiet disk. Our Fourier analysis and the multi-component modeling of Worden et al. (2001, ApJ, 560, 1020) are found to be complementary and could be productively combined in future work. We also compare our results with a similar analysis based upon the semi-empirical model atmospheres of Fontenla et al. (1999, ApJ, 518, 480), further improved by Avrett. We compare the measurement- and model-based analyses and suggest the direction of improvements needed in the model atmospheres.