Comparison of ACRIM and PMOD Total Solar Irradiance composites time series during solar cycles 21 - 23

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1600 Global Change (New Category), 1650 Solar Variability, 3359 Radiative Processes, 7538 Solar Irradiance

Scientific paper

The set of contiguous, redundant and overlapping total solar irradiance (TSI) observations made by satellite experiments since late 1978 enables construction of continuous composite TSI time series. A multi-decadal TSI composite requires establishment of the relationship between ACRIM1 and ACRIM2 results across the two year gap between them using one of two overlapping data sets: the Nimbus7/ERB or ERBS/ERBE. Two TSI composites have been published, the ACRIM using the Nimbus7/ERB comparisons [Willson & Mordvinov, 2003] and the PMOD using the ERBS/ERBE comparisons [Frohlich & Lean, 1998]. The ACRIM composite uses unaltered published results from a subset of the satellite TSI data. The most significant feature of the ACRIM composite for climate change is an upward trend of 0.04 (+/- 0.01) % per decade between activity minima during solar cycles 21-23. The PMOD composite uses a different subset of the same satellite TSI data, modifying some results using TSI proxy models as a guide. There are a number of differences between the PMOD and ACRIM composites but the most important is the absence of a significant trend between minima in the PMOD. The trend difference is the direct result of the choice of Nimbus7/ERB or ERBS/ERBE to relate ACRIM1 and ACRIM2 results across the ACRIM gap. The lack of a minima-to-minima trend in composites based on the ERBS/ERBE can be shown to be an artifact of uncorrected degradation during the `gap'. The significance of the ACRIM composite trend will be explored in the context of the traceability of the constituent TSI data sets. Other differences between the ACRIM and PMOD composites result from PMOD's use of TSI proxy models to justify altering published results of ACRIM1 and Nimbus7/ERB. TSI regression (proxy) models based on chromospheric spectral features are not competitive in accuracy, precision or traceability with satellite TSI observations and are therefore may be expected to cause spurious effects when used in the construction of TSI composites. The TSI record has been sustained by overlapping, redundant experiments using their level of measurement precision to sustain longer term traceability. This TSI monitoring strategy is essential for continuity in the future because the uncertainty of current satellite sensors (~ 0.1 %) is an order of magnitude too large to detect subtle long term TSI variations of potential climate change significance. [Willson, R.C., A. V. Mordvinov, JGRL 30, pp. 1199-1202, 2003, Frohlich C., J. Lean, JGRL 25, pp. 4377-4380, 1998]

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