A technique for estimating long-term variations of solar total irradiance: Preliminary estimates based on observations of the Sun and solar-type stars

Summary

We analyze concurrent observations of Ca II H and K chromospheric emission fluxes (which are associated with the disk-averaged surface magnetic fluxes) and variability in the b and y bands of the Strömgren uvby photometric system (which are associated with the integrated total irradiance or brightness variation) in a group of stars, including the Sun. Those solar-type stars have a narrow range of mass and age, and similar observed activity variability. Their combined variability over several years reveals a range for the ratio of brightness change to chromospheric activity amplitude. We assume that the scatter in the ratios of brightness change to chromospheric activity change measured over a decade represents a distribution of variability that is intrinsic to the activity of a typical solar-type star (i.e., the Sun) over centuries. Under that assumption, we infer an empirical bound of possible solar brightness variations over centuries (Zhang et al., 1994). The estimated increase in solar brightness accompanying the net increase in average activity from the Maunder Minimum interval (ca. A.D. 1660–1710) to the decade of the 1980’s ranges from as low as 0.2 % to as high as 0.7 %; the indeterminacy of the brightness-chromospheric activity change ratio produces the large range of estimates. In addition, we estimate the timing of possible solar brightness change over the past three centuries. We also discuss the observed relation between the activity cycle amplitude and the activity cycle length, P _cyc , for the Sun and solar-type stars. The activity cycle amplitude is inversely related to the parameterized quantity, (P _cyc /P _rot )², where P _rot is the rotation period.