Historical Solar Irradiance Using the Updated Sunspot Record

Abstract:

The total solar irradiance (TSI), which is by far the largest contributor to the energy input to the Earth’s climate system, has been measured from space for the last four decades. Earth-climate studies rely on long-duration records, for which this is only beginning to qualify. To extend this record back in time and enable lengthier correlations with other Earth-climate data records, solar-irradiance models are used. One such is the NRLTSI model, which uses TSI measurement-era correlations with solar proxies to estimate historical TSI values via the temporal extensions enabled by those proxies. The longest-duration direct-measurement solar proxy is the sunspot record, which spans over 400 years now. Reevaluations of that record via reanalyzes of the original data records, the discovery of new sunspot records, and different combinations of observers’ records via new analytical methods have led to the Solar Index and Long-term Solar Observations (SILSO) V.2 sunspot record. This new record, shown in red in the figure, shows much higher sunspot counts and different temporal variability than the prior (V.1) record (shown in green in the figure).

As the primary input to many historical solar-variability studies, this new sunspot record has profound implications on solar-irradiance models. Our Solar Irradiance Science Team (SIST) is reevaluating the NRLTSI2 model using results from the Advective Flux Transport (AFT) model, which can simulate the solar-surface magnetic fields over the 400-year sunspot record and estimate the closed magnetic flux that is associated with the contributors to TSI variability. This NRLTSI2 model is the version currently used in the NOAA Solar Irradiance Climate Data Record, which will be updated with the results of this SIST study. We present an overview of and current results from this effort.