Magnitudes and Timescales of Total Solar Irradiance Variability (1606.05258v2)

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.

Overview of Total Solar Irradiance Variability

The paper authored by Greg Kopp investigates the magnitudes and timescales of variability in total solar irradiance (TSI). It provides a detailed analysis of how the Sun's radiative output changes on various temporal scales—from minutes to gigayears—based on data collected primarily from space-borne instruments.

Measurements and Observations

Since 1978, TSI has been consistently measured using space-based instruments, which offer the most reliable recordings free from atmospheric distortions influencing ground-based measurements. The TSI averages around 1361 W/m², yet exhibits fluctuations linked to solar activities such as sunspots and faculae. Measurements indicate TSI can increase by approximately 0.1% due to solar activity cycles, with sunspot formations particularly contributing to TSI variability on 27-day solar rotational scales. It is noteworthy that in extreme cases, large solar flares can further impact TSI readings by measurable but transient increments.

Secular Trends and Composites

Kopp's analysis explores composite records created by combining data from various instruments, revealing differences in trends which are suggestive of potential secular variability in solar irradiance. Among the composite records analyzed, the PMOD composite, which incorporates several corrections for suspected instrumental artifacts, emerges as the most solar-representative based on historical validation and agreement with solar irradiance reconstruction models. However, secular trends over extended periods are challenging to definitively accredit due to instrument stability uncertainties and limited duration of the space-borne record.

Proxy Models and Historical Reconstructions

To extend the understanding of solar irradiance variability beyond the current measurement era, the paper examines multiple proxy models. Principal among these are the empirical NRLTSI model, which utilizes historical sunspot records, and the semi-empirical SATIRE model, which incorporates solar-disk-area coverage to reconstruct past total solar irradiance. These reconstructions point towards a time during the Maunder Minimum with lower solar activity indicating possible reductions in TSI by as much as 0.5 to 1 W/m² compared to current average values.

Implications for Climate Studies

The significance of TSI variability is underscored in its connection to terrestrial climate changes primarily during pre-industrial times when natural forces were more dominant. Despite anthropogenic factors playing a substantial role recently, understanding solar variability remains critical for historical climate studies. The paper emphasizes the necessity for enhanced instrument stability to offer more precise insights into long-term solar irradiance trends, which bear implications on estimates of Earth's climate sensitivity to solar forcing.

Conclusions and Future Directions

In conclusion, while the existing TSI measurement record has offered valuable insights into solar variability, it faces limitations in defining secular trends due to stability constraints. Future efforts must focus on improving instrument robustness to achieve lower measurement uncertainties necessary for adequately understanding solar variability's broader and longer-term impacts. Such advancements promise refinements in solar modeling, enhancing historical reconstructions and providing a firmer basis for climate sensitivity analyses tied to solar irradiance changes.

Overall, Kopp's paper contributes a comprehensive understanding of total solar irradiance variability while outlining the complexities and limitations inherent in measuring such variability, key to advancing research in solar physics and climate science.