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Total Solar Irradiance Observations

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Abstract

The record of total solar irradiance (TSI) during the past 35 years has overlapping observations from space which can be merged to a composite, and three are available, namely the PMOD, the ACRIM and the IRMB composites. There are important differences between them, which are discussed in detail in order to find the best representation of solar variability during the last three cycles, for the following discussions of solar irradiance variability. Moreover, the absolute value of TSI from TIM on SORCE is 1,361 Wm−2, substantially lower than the value 1,365 Wm−2, which was observed by the classical radiometers. New results from specific experiments are now available, which are discussed in order to define the value to be used in, e.g., climate models. The most important issue regarding the recent TSI records is the low value observed during the minimum in 2009, which is 25% of a typical cycle amplitude lower than the value in 1996. The validity of this low value has been confirmed by comparing all existing TSI observations during cycle 23. On the other hand, activity indices, such as the sunspot number, the 10.7-cm radio flux (F10.7), the CaII and MgII indices and also the Ly-α irradiance or the frequency changes in low-order p modes, show a much smaller decreases relative to their respective typical cycle amplitude. It is most likely that an increasing contrast of the facular and network elements with decreasing magnetic field is responsible for this discrepancy. The value of TSI at minima is correlated with the open magnetic field of the Sun, B R, at minima. Using B R at minima, interpolated linearly in between as a fourth component of a proxy model based on the photometric sunspot index and on the MgII index improves the explanation of the variance of TSI over the full period of the last three solar cycles to 84.7%. Results from other models are also discussed.

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Notes

  1. The versions of ACRIM follow the indications of http://www.acrim.com/Data{\%}20Products.htm as YY/MM, the date of release.

  2. Daily values available at: PMOD (01/1976–06/2011): ftp://ftp.pmodwrc.ch/pub/data/irradiance/composite/DataPlots/ext_composite_d41_62_1110.dat, ACRIM (11/1978–05/2011): http://www.acrim.com/RESULTS/data/composite/composite_acrim_1105.rtf and IRMB (1985–2008): http://remotesensing.oma.be/meteo/view/en/3385925-SARR.diarad.html

  3. The VIRGO data levels are defined as follows: level-0 are the raw data as received; level-1 data are in physical units and corrected for all known effects as temperature, sun-spacecraft distance, etc.; level-1.8 are defined as corrected for degradation by comparison of the operational with a less exposed instrument of the same type; level-2 are the final results after all the possible changes during operation in space are included and, in the case of the radiometers, the PMO6V and DIARAD are harmonized.

  4. The original and corrected HF and ACRIM data used for the construction of the PMOD composite as well as the time series used for comparison are available from ftp://ftp.pmodwrc.ch/pub/data/irradiance/composite/DataPlots/org2pmod-composite.dat and for the different time series used to determine VIRGO TSI are at ftp://ftp.pmodwrc.ch/pub/data/irradiance/virgo/TSI/virgo_tsi_d_v6_002_yymm.dat with the newest update indicated as yymm.

  5. Available at ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SUNSPOT_NUMBERS/INTERNATIONAL/daily.

  6. Available at ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SOLAR_RADIO/FLUX/Penticton_Adjusted/daily.

  7. Available at http://lasp.colorado.edu/sorce/ssi_data/mgii/txt/sorce_mg_latest.txt.

  8. Available at http://lasp.colorado.edu/lisird/lya/.

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Acknowledgments

I thank the organizers of the ISSI Workshop “Observing and modelling Earth’s Energy Flows” for an invitation to present a review of TSI observations and the two referees for their very helpful comments, improving the manuscript substantially. This work is supported by the Swiss National Science Foundation by providing support for travel, which is gratefully acknowledged. Moreover, this research would not have been possible without substantial contributions by the VIRGO and SOHO teams. SOHO is a cooperative mission between ESA and NASA, which was launched in late 1995 and is still in operation (as of July 2011).

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    Claus Fröhlich

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Fröhlich, C. Total Solar Irradiance Observations. Surv Geophys 33, 453–473 (2012). https://doi.org/10.1007/s10712-011-9168-5

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