Composite TSI Time Series

• TSI time series

Continuous time series of total solar irradiance (TSI) observations have been constructed from the set of redundant, overlapping total solar irradiance (TSI) measurements made by satellite experiments during the past 34 Years. One, the ACRIM composite [Willson & Mordvinov, 2003 (Fig. 1)], displays a significant upward trend in TSI of 0.04 percent per decade during solar cycles 21-23. Another, the PMOD composite [Frohlich & Lean, 1998 (Fig. 2)], displays no significant trend over this period using different combinations of TSI data sets, computational philosophy and assumptions. Both time series demonstrate no significant trend over the two decade period separating the first and third solar activity minima.

• Quality of TSI observations

The potential significance of ACRIM's upward trend during solar cycles 21-23 as a climate forcing makes it important to explore the ACRIM-PMOD trend difference to determine which of the two composites best represents the TSI measurement database. Two types of experiment have provided TSI data: self-calibrating, precision TSI monitors and Earth radiation budget (ERB) experiments.

TSI monitoring experiments are designed to provide state of the art accuracy and precision. The ERB experiments (both Nimbus7/ERB and ERBS/ERBE) are designed to lower standards for TSI observations appropriate to the requirements of earth radiation budget investigations. TSI monitors are solar pointed so they can observe continuously (or during the sunlit part of every orbit) and are capable of self-calibrating the degradation of their sensors. Nimbus7/ERB viewed the sun while it passed through its field of view every orbit three days of every four. The ERBS/ERBE viewed the sun as it passed through its field of view during one orbit every two weeks. Neither ERB experiment could self calibrate the degradation of their sensors. Because of their multi-sensor design, TSI monitoring experiments are able to self-calibrate their sensor degradation and provide significantly greater  precision, accuracy and  traceability for their observations than the ERB experiments. Both the Nimbus7/ERB and ERBS/ERBE met their observational requirements but but their results are not competitive in traceability with those of TSI monitoring experiments.

• The TSI observational record

An optimum composite TSI time series would utilize the results of monitoring experiments to the maximum extent possible because of the smaller uncertainties of their data. Some ERB results must be used, however, since there are two periods during the past 34 years during which only ERB observations are available.

The first ERB period extended from November 1978 to early 1980 during which time the Nimbus7/ERB TSI observations were the only measurements made. This period ended in February 1980 with the launch of ACRIM1 on the Solar Maximum Mission. ACRIM1 was the prototype TSI monitoring experiment that established the standards and approach used by succeeding experiments.

The second ERB period was during a two year gap in the TSI monitoring record between the ACRIM1 and ACRIM2 experiments (1989 - 1991). ACRIM2 was originally intended to be launched by the space shuttle in 1989 on the NASA Upper Atmosphere Research Satellite. The launch was delayed more than two years by the Challenger disaster. Compilation of a continuous record over the entire 34 years of satellite observations would not be possible were it not for the availability of ERB results during the gap. The relationship between ACRIM results across the gap can be derived using the overlapping ERB data sets: the Nimbus7/ERB or the ERBS/ERBE. These two choices are embodied in the construction of ACRIM and PMOD composites, respectively, shown in Fig.'s 1 and 2.

• ACRIM time series approach

The philosophy of the ACRIM composite approach is to use the results published by the experiment science teams and the Nimbus7/ERB ACRIM gap ratio. The original science teams have (of had) the most thorough understanding of their experiments and are therefore most likely to produce results that best represent the performance of the instrumentation. Their published results should always be used unless there are compelling reasons for changing the data and/or results by updating the algorithm and reprocessing. ACRIM uses the Nimbus7/ERB 'gap' ratio to relate ACRIM1 & 2 results because the quality and quantity of ERB data is far superior to the ERBE data.

• PMOD time series approach

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, to conform them to the predictions of TSI proxy models [Frohlich & Lean, 1998]. The  sparse ERBS/ERBE data required the PMOD model to use ~ 90% interpolated ERBE data to compute their ACRIM gap ratio. The PMOD result is more of a model of TSI than a composite of actual measured satellite values.

• Discussion of composites

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 (See Fig. 4).

TSI proxy models are not competitive in precision or accuracy with even the lowest quality satellite TSI observations. Most are regression models of TSI against one or more proxy solar spectral features or records of solar magnetic activity. Use of such models in constructing the PMOD composite convolutes their large uncertainties with the satellite TSI  observational data. The resulting TSI composite is more likely to represent the modeler's preconception of what the TSI time series should look like than the reality conveyed by satellite TSI observational results published by the experiment science teams.

The PMOD composite approach modified results published by the science teams of the Nimbus7/ERB and ACRIM1. No original computations or algorithm developments were made by the PMOD fabricators. Modifications made to both Nimbus7/ERB and ACRIM1 results were based on erroneous assumptions about instrument operation and sensor degradation  that conflicted with the experiment science teams' published analyses. The resulting PMOD composite conforms their TSI time series with proxy model predictions during the maxima of solar cycles 21 and 22 and the solar minima between them. The differences between the ACRIM1 and PMOD composites are clearly evident in Figure 3 where the measured shape of the solar cycles 21 and 22 maxima and the intervening solar activity minima slope have been altered by the PMOD model.

• Discussion of the ACRIM Gap

The ERBS/ERBE database was affected by significant uncorrected degradation during its observational lifetime. During the ACRIM Gap ERBE uncorrected degradation was likely caused by the excessive UV solar fluxes generated by the high level of solar magnetic activity approaching the solar cycle 22 maximum. The difference in trends between ACRIM and PMOD Composites is a direct consequence of the uncorrected degradation of the ERBE results during the ACRIM Gap in which the paradigm of positive correlation between TSI and solar magnetic activity is followed by the results of Nimbus7/ERB but not by those of ERBS/ERBE. (See Fig. 4)

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