Abstract

Context. The variability of the spectral solar irradiance (SSI) over the course of the 11-year solar cycle is one of the manifestations of solar magnetic activity. There is strong evidence that the SSI variability has an effect on the Earth’s atmosphere. The faster rotation of the Sun in the past lead to a more vigorous action of solar dynamo and thus potentially to larger amplitude of the SSI variability on the timescale of the solar activity cycle. This could lead to a stronger response of the Earth’s atmosphere as well as other solar system planets’ atmospheres to the solar activity cycle. Aims. We calculate the amplitude of the SSI and total solar irradiance (TSI) variability over the course of the solar activity cycle as a function of solar age. Methods. We employed the relationship between the stellar magnetic activity and the age based on observations of solar twins. Using this relation, we reconstructed solar magnetic activity and the corresponding solar disk area coverages by magnetic features (i.e., spots and faculae) over the last four billion years. These disk coverages were then used to calculate the amplitude of the solar-cycle SSI variability as a function of wavelength and solar age. Results. Our calculations show that the young Sun was significantly more variable than the present Sun. The amplitude of the solar-cycle TSI variability of the 600 Myr old Sun was about ten times larger than that of the present Sun. Furthermore, the variability of the young Sun was spot-dominated (the Sun being brighter at the activity minimum than in the maximum), that is, the Sun was overall brighter at activity minima than at maxima. The amplitude of the TSI variability decreased with solar age until it reached a minimum value at 2.8 Gyr. After this point, the TSI variability is faculae-dominated (the Sun is brighter at the activity maximum) and its amplitude increases with age.

Highlights

  • The action of a hydromagnetic dynamo generates a magnetic field in the solar interior (Charbonneau 2010; Brun & Browning 2017)

  • We show how the amplitude of solar irradiance variability on the activity cycle timescale depends on the solar age

  • We have employed the Spectral And Total Irradiance REconstruction (SATIRE) approach to investigate how the amplitude of solar irradiance variability on the timescale of the activity cycle might have changed with the solar age

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Summary

Introduction

The action of a hydromagnetic dynamo generates a magnetic field in the solar interior (Charbonneau 2010; Brun & Browning 2017). The slowing of the stellar rotation weakens the stellar dynamo and, stellar magnetic activity and its contribution to the stellar brightness variations (see e.g., Noyes et al 1984). Such changes of the stellar brightness caused by the spin down have been observed. Using the selected stars as proxies for the younger Sun, they concluded that the 920–1180 Å solar flux in the past was higher than the present-day level

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