Solar luminosity variations in solar cycle 21

Abstract

The ACRIM I experiment (Active Cavity Radiometer Irradiance Monitor) on the solar maximum Mission (SMM) satellite has provided a nearly continuous record of solar total irradiance variations since early 1980¹. It has detected variations on time scales ranging from minutes to SMM's lifetime. The long-term variations have revealed a downward trend during the declining phase of solar cycle 21 (ref. 2) of the sunspot cycle, a flat period between mid-1985 and mid-1987, and an upturn in late 1987 which suggests a direct correlation of luminosity and solar active region population. If the upturn continues into the activity maximum of solar cycle 22, a relation between solar activity and luminosity of possible climatological significance could have been discovered. The sense of the correlation agrees with what has been predicted from the coincidence of the 'little ice age' climate anomaly in the sixteenth and seventeenth centuries and the Maunder Minimum of solar activity³. The best-fit relationship for the variation of total irradiance S, with sunspot number R_z, and 10–cm flux F₁₀, are S=1,366.82+7.7l×10⁻³ R_z and S=1,366.27+8.98×10⁻³ F₁₀ (W m⁻²). These could be used to approximate total irradiance variations over the periods for which these indices have been compiled.