The Variability and Absolute Magnitude of Solar Spectral Irradiance

Abstract

It has been hypothesized that the sun’s irradiance may fluctuate at near UV and blue wavelengths due to changes in solar line blanketing associated with solar activity. Preliminary determinations of the solar spectral irradiance in the visible spectrum, made over a 6 mo period at the Mauna Loa Observatory, indicated that the solar radiation was constant to at least 1%. There were indications, however, that the blue light of the sun (λ ∿ 400 nm) may have varied by a few tenths of one percent. It is recommended that solar spectral irradiance be measured in absolute (SI) units so that long-term (decades) changes, that may be related to terrestrial weather, can be quantized. It is now possible to determine spectral irradiance to + 1% accuracy by incorporating a new dye laser-active cavity radiometer calibration method.