Spectrum Line Intensity as a Surrogate for Solar Irradiance Variations

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Abstract
Active Cavity Radiometer Irradiance Monitor (ACRIM) solar constant measurements from 1980 to 1986 are compared with ground-based, irradiance spectrophotometry of selected Fraunhofer lines. Both data sets were identically sampled and smoothed with an 85-day running mean, and the ACRIM total solar irradiance ( S ) values were corrected for sunspot blocking ( S c ). The strength of the mid-photospheric manganese 539.4-nanometer line tracks almost perfectly with ACRIM S c , Other spectral features formed high in the photosphere and chromosphere also track well. These comparisons independently confirm the variability in the ACRIM S c , signal, indicate that the source of irradiance is faculae, and indicate that ACRIM S c , follows the 11-year activity cycle.