Sep 21 – 23, 2015
Royal Observatory of Belgium
Europe/Brussels timezone

Total Solar Irradiance measurements over the last 30 years and reconstruction over the last 300 years.

Sep 22, 2015, 9:30 AM
Meridian Room (Royal Observatory of Belgium)

Meridian Room

Royal Observatory of Belgium

Avenue Circulaire - 3 - Ringlaan 1180 Brussels
Talk 4-Space era data versus historical records


Dr Steven Dewitte (RMIB)


The Total Solar Irradiance quantifies the amount of energy that the earth receives from the Sun. The quantification of the TSI variability is important to understand whether the sun can have a significant influence on climate change on earth. A carefull analysis of the TSI space measurements over the last 30 years indicates that within a measurement uncertainty of 0.1 W/m²decade the long term TSI variation can be fully explained by the 'magnetic ingredients' of sunspots and facula associated to the 11 year solar activity cycle. Further analysis of the Ca II K index measured from Mount Willson observatory for the last 100 years and of the revised sunspot time series for the last 300 years indicates that contrary to the long-held paradigm of a 'modern grand solar maximum' we are currently in the minimum of a long term modulation of the amplitude of the 11 year cycle; this amplitude modulation has a periodicity around 100 year. The determination of the absolute value of the TSI - also known as the Solar Constant - is a problem of metrology. Our new best estimate of 1362.9 +/- 0.9 W/m² at solar minimum is derived from the revised absolute value measured by the DIARAD/SOVIM instrument on the ISS in 2008. Compared to earlier versions of the DIARAD TSI evaluation we apply a new method for the determination of the so-called non-equivalence between electrical and optical power. This new evaluation method was validated during a laboratory measurement campaign at the LASP TRF facility in 2013. During this same campaign we identified an underestimation of the irradiance measured by the LASP TRF cryogenic radiometer, which after elimination of all other possible causes can only be attributed to an underestimation of the amount of scattering and diffraction occuring around the LASP TRF primary aperture. Since the TIM/SORCE space radiometer has a similar geometry and TSI evaluation method as the LASP TRF cryogenic radiometer, this suggests that the TIM/SORCE radiometer measures a too low TSI value.

Primary author

Dr Steven Dewitte (RMIB)

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