Monitoring the solar temperature: Spectroscopic temperature variations of the sun

The C I lambda 5380 line in the solar flux spectrum was measured over the 1978-1992 interval. Analysis of the data shows seasonal and instrumental effects, but after allowance for these, the ratios of spectral line depths, C I lambda 5380 to Fe I lambda 5379 and to Ti II lambda 5381, are shown to be robust indicators of effective temperature. These data show the solar temperature to have varied systematically during the activity cycle and nearly in phase with other indicators of the cycle. The amplitude of the variation is 1.5 K +/- 0.2 K, similar to but slightly less than the range implied by the variations of the sunspot-corrected irradiance. There is also evidence for a secular trend amounting to approximate to +0.014 K per year.