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The Sun's net radiative output varies on timescales of minutes to gigayears.Direct measurements of the total solar irradiance (TSI) show changes in thespatially- and spectrally-integrated radiant energy on timescales as short asminutes to as long as a solar cycle. Variations of ~0.01 % over a few minutesare caused by the ever-present superposition of convection and oscillationswith very large solar flares on rare occasion causing slightly-largermeasureable signals. On timescales of days to weeks, changing photosphericmagnetic activity affects solar brightness at the ~0.1 % level. The 11-yearsolar cycle shows variations of comparable magnitude with irradiances peakingnear solar maximum. Secular variations are more difficult to discern, beinglimited by instrument stability and the relatively short duration of thespace-borne record. Historical reconstructions of the Sun's irradiance based onindicators of solar-surface magnetic activity, such as sunspots, faculae, andcosmogenic isotope records, suggest solar brightness changes over decades tomillennia, although the magnitudes of these variations have high uncertaintiesdue to the indirect historical records on which they rely. Stellar evolutionaffects yet longer timescales and is responsible for the greatest solarvariabilities. In this manuscript I summarize the Sun's variability magnitudesover different temporal regimes and discuss the irradiance record's relevancefor solar and climate studies as well as for detections of exo-solar planetstransiting Sun-like stars.

Magnitudes and Timescales of Total Solar Irradiance Variability