Sunspot cycle and associated variation of the solar spectral irradiance

Precise monitoring of the solar absorption-line spectrum in the period 1976-80 has revealed systematic changes of the strength of photospheric lines during the ascending part of the current solar cycle. The most likely cause is a slight cooling of the lower photosphere and a concomitant heating of higher layers. The total energy radiated into space thereby remains unchanged, nevertheless a flattening of the photospheric temperature gradient will be accompanied by a redistribution of energy in the solar spectrum. Its magnitude is predicted here using stellar model-atmosphere techniques. This 'change of colour' is characterized by a 0.2% decrease of the irradiance in the blue, and an increase of the same order in the red and near IR. In this way solar activity may modulate terrestrial climate even in the absence of perceptible changes of the solar constant. (author)