Solar luminosity variation. III - Calcium K variation from solar minimum to maximum in cycle 21

Abstract

Completion of measurements of the full disk Ca II H and K profiles from minimum to maximum, in the current solar cycle 21, shows that while the central intensity of the K line increases by 30% on the average, a peak change of +40% was recorded near the maximum of solar activity in late 1979. The 1 A K index shows a smaller, corresponding increase of 18%, and it is suggested that these changes are due to the occurrence of solar plages on the visible solar hemisphere. Spectral features formed above the temperature minimum are closely related in their variability, but not as well related to changes in the strength of narrow photospheric lines and the K<SUB>1</SUB> wings. It is found that Ca II variability correlates closely with the plage index, the Zurich sunspot number, and the Ottawa 10 cm flux measurements.