Special sessions SH41A (Oral) and SH43A (Poster), 2006 AGU Fall Meeting, 2006m. (URL to SH41A)  (URL to SH43A)

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SH41A-02 INVITED

A Critical Review of the Time Series of Total Solar Irradiance Satellite Observations

* Willson, R C (rwillson@acrim.com) , Columbia University, 12 Bahama Bend, Coronado, CA 92118, United States

Continuous time series of total solar irradiance (TSI) observations have been constructed from the set of contiguous, redundant, overlapping total solar irradiance (TSI) measurements made by satellite experiments during the past 28 years. One, the ACRIM composite time series [Willson & Mordvinov, 2003], detects a significant upward trend in TSI of 0.04 percent per decade during solar cycles 21-23. Another, the PMOD composite [Frohlich & Lean, 1998], detects no significant trend using different combinations of TSI data sets, computational philosophy and assumptions. The potential significance of the ACRIM upward trend as a climate forcing makes it important to explore the trend difference to determine which of the two composite TSI time series best represents the measurement database. Two types of experiments have provided TSI data: self-calibrating, precision TSI monitors and Earth radiation budget (ERB) experiments. TSI monitors provide much higher accuracy and precision and are capable of self- calibrating the degradation of their sensors. The ERB experiments are designed to provide less accurate and precise TSI �boundary value' results for ERB modeling and cannot self-calibrate sensor degradation. While the optimum composite TSI time series utilizes TSI monitor results to the maximum extent possible, a two year gap in the TSI monitoring record between the ACRIM1 and ACRIM2 experiments (1989 - 1991) would have prevented compilation of a continuous record over the 28 years of satellite observations were it not for the availability of ERB results during the gap. The relationship between ACRIM1 and ACRIM2 results across the ACRIM gap can be derived using the overlapping ERB data sets: the Nimbus7/ERB and/or the ERBS/ERBE. These two choices are embodied in the construction of ACRIM and PMOD composites, respectively. The philosophy of the ACRIM composite is to use the unaltered results published by the experiment science teams and the Nimbus7/ERB ACRIM gap ratio. The PMOD composite approach uses a different subset of the satellite TSI database, the ERBS/ERBE ACRIM gap ratio and modifies published Nimbus7/ERB and ACRIM1 results, based on degradation assumptions derived from linear regression TSI proxy models. There are a number of differences between the ACRIM and PMOD composites but the most important is the trend during solar cycles 21 - 23. The absence of a trend in the PMOD composite and any composite based on the ERBS/ERBE ACRIM gap ratio has been shown to be an artifact of uncorrected degradation of ERBE results during the gap. The ERBS/ERBE database was significantly affected by uncorrected degradation throughout its observational lifetime and provides a less precise ACRIM gap ratio than the Nimbus7/ERB results. TSI proxy models are not competitive in precision or accuracy with even the lowest quality satellite TSI observations. Their use in constructing the PMOD composite convolutes the relatively high uncertainty of the model with the observational data and is less likely to represent the best interpretation of the extant TSI observational database. The PMOD composite used modified published Nimbus7/ERB and ACRIM1 data. The ACRIM1 modifications were based on erroneous assumptions regarding degradation of the experiment. No effort was made to verify these assumptions using actual ACRIM1 data. The resulting PMOD composite provides better agreement with TSI proxy model predictions during the maximum of solar cycle 21 but does not provide the best representation of the actual TSI. The ACRIM TSI composite time series is the most accurate representation of the extant TSI satellite measurement database. It doesn't rely on imprecise proxy modeling or modifications of published observational results.

SH43A-1505

Total Solar Irradiance Trends During Solar Cycles 21-24

* Willson, R C (rwillson@acrim.com) , Columbia University, 12 Bahama Bend, Coronado, CA 92118, United States

Total solar irradiance (TSI) observations have been made by contiguous, redundant, overlapping satellite experiments since 1978 during solar activity cycles 21 - 23. The solar activity minimum marking the inception of cycle 24 is imminent - some sunspot magnetic polarity reversals have already been detected. The ACRIM TSI composite time series found a 0.04 percent per decade trend between the minima of cycles 21 to 23. A trend of this magnitude, sustained over many decades or centuries, could be a significant climate forcing. Great interest will be attached to the behavior of the TSI time series during the upcoming and future solar activity minima with respect to the presence or absence of a trend. An updated TSI composite will be presented to examine the trend at or near the minima between solar cycles 23 and 24.