Magnitudes and Timescales of Total Solar Irradiance Variability

TL;DR

This paper reviews the variability of the Sun's total solar irradiance across multiple timescales, from minutes to millennia, highlighting measurement challenges, historical reconstructions, and implications for climate and exoplanet studies.

Contribution

It provides a comprehensive summary of the magnitudes and timescales of solar irradiance variability, integrating recent measurements, historical data, and stellar evolution effects.

Findings

Short-term variations (~0.01%) caused by convection, oscillations, and flares.

Solar cycle variations (~0.1%) linked to magnetic activity.

Long-term secular changes are uncertain due to measurement limitations.

Abstract

The Sun's net radiative output varies on timescales of minutes to gigayears. Direct measurements of the total solar irradiance (TSI) show changes in the spatially- and spectrally-integrated radiant energy on timescales as short as minutes to as long as a solar cycle. Variations of ~0.01 % over a few minutes are caused by the ever-present superposition of convection and oscillations with very large solar flares on rare occasion causing slightly-larger measureable signals. On timescales of days to weeks, changing photospheric magnetic activity affects solar brightness at the ~0.1 % level. The 11-year solar cycle shows variations of comparable magnitude with irradiances peaking near solar maximum. Secular variations are more difficult to discern, being limited by instrument stability and the relatively short duration of the space-borne record. Historical reconstructions of the Sun's irradiance based on indicators of solar-surface magnetic activity, such as sunspots, faculae, and cosmogenic isotope records, suggest solar brightness changes over decades to millennia, although the magnitudes of these variations have high uncertainties due to the indirect historical records on which they rely. Stellar evolution affects yet longer timescales and is responsible for the greatest solar variabilities. In this manuscript I summarize the Sun's variability magnitudes over different temporal regimes and discuss the irradiance record's relevance for solar and climate studies as well as for detections of exo-solar planets transiting Sun-like stars.