A Century of Polar Faculae Variations

Abstract

The numbers of faculae at the poles of the Sun have been estimated from white-light images obtained at the Mount Wilson Observatory during 1985-2006 and combined with prior estimates extending back to 1906, when the observations began. The combined data show an 11 yr cyclic variation with faculae maxima occurring during sunspot minima in each of the past 10 sunspot cycles. Also, these numbers of polar faculae are well correlated with the line-of-sight component of the polar magnetic field measured at the Wilcox Solar Observatory since 1976. The numbers of polar faculae show a secular decrease since 1986, suggesting that the polar fields are now weaker than they have been during any cycle in the past century.