Cooling of the Sun's photosphere coincident with increased sunspot activity

Abstract

IN summer 1975, I began to monitor the spectroscopic temperature of integrated sunlight (‘the Sun viewed as a star’). The first 1.5 yr of observations were taken at a time of minimum activity and the full disk temperature was apparently constant at ±1–2 K. However, 1977 produced a marked increase of activity with the onset of cycle no. 21. In response to this activity the Sun seems to have cooled by ∼ 6 K. To the extent that the Sun is a black body, a 6 K change in temperature corresponds to a ∼0.5% reduction in luminosity. The inverse relation between activity and temperature is a new discovery, and may prove helpful in explaining certain aspects of the Sun–Earth climate relationship.