Sun‐as‐a‐Star Spectrum Variations 1974–2006

Abstract

We have observed selected Fraunhofer lines, both integrated over the full disk and for a small circular region near the center of the solar disk, on 1215 days over the past 30 years. Our full disk results for the chromosphere show that Ca II K 3933 Å nicely tracks the 11 yr magnetic cycle based on sunspot number, with a peak amplitude in central intensity of ~37%. The wavelength of the midline core absorption feature, called K3, referenced to nearby photospheric Fe, displays an activity cycle variation with an amplitude of 3 mÅ (6 mÅ center disk). Other chromospheric lines, such as He I 10830 Å, Ca II 8542 Å, Hα, and the CN 3883 Å bandhead, track Ca II K intensity with lower relative amplitudes. In the low photosphere, temperature-sensitive C I 5380 Å appears constant in intensity to 0.2%. In the high photosphere, the cores of strong Fe I lines, Na D1 and D2, and the Mg I b lines, present a puzzling signal, perhaps indicating a role for the 22 yr Hale cycle. Solar minimum around 1985 was clearly seen, but the following minimum in 1996 was missing. Our center disk results show that both Ca II K and C I 5380 Å intensities are constant, indicating that the basal quiet atmosphere is unaffected by cycle magnetism within our observational error. A lower limit to the Ca II K central intensity atmosphere is 0.040. This possibly represents conditions as they were during the Maunder minimum. Converted to the Mount Wilson S-index (H+K index), the Sun center disk is at the lower activity limit for solar-type stars. An appendix provides instructions for URL access to both the raw and reduced data.