Phys.org's Solar energy tracker powers down after 17 years says:

"The big surprise with TSI was that the amount of irradiance it measured was 4.6 watts per square meter less than what was expected," said Tom Woods, SORCE's principal investigator and senior research associate at the University of Colorado's Laboratory for Atmospheric and Space Physics (LASP) in Boulder, Colorado. "That started a whole scientific discussion and the development of a new calibration laboratory for TSI instruments. It turned out that the TIM was correct, and all the past irradiance measurements were erroneously high."

"It's not often in climate studies that you make a quantum leap in measurement capability, but the tenfold improvement in accuracy by the SORCE / TIM was exactly that," said Greg Kopp, TIM instrument scientist for SORCE and TSIS at LASP.

Question: What was it about either the past measurements or their analysis or calibration that made their measurements of the Sun's output about 0.3% high? What TSI's improvement primarily instrumental, or due to better calibration?


From the abstract of Kopp & Lean (2011) "A new, lower value of total solar irradiance: Evidence and climate significance":

Scattered light is a primary cause of the higher irradiance values measured by the earlier generation of solar radiometers in which the precision aperture defining the measured solar beam is located behind a larger, view‐limiting aperture. In the TIM, the opposite order of these apertures precludes this spurious signal by limiting the light entering the instrument.

(emphasis mine)

For example, in the case of the ACRIM instruments used on the Solar Maximum Mission, the Upper Atmosphere Research Satellite, and ACRIMSat:

Notably, for the ACRIM instrument NIST determined that diffraction from the view‐limiting aperture contributes a 0.13% signal not accounted for in data from the three ACRIM instruments [Butler et al., 2008]. This correction lowers the reported ACRIM values, resolving part of ACRIM's difference with TIM. In ACRIM and all other instruments, the precision aperture used to define the measured solar beam is deep inside the instrument with a larger view‐limiting aperture at the front, which, depending on edge imperfections, in addition to diffraction can directly scatter light into the absorbing cavity. Additionally, this design allows into the instrument interior two to three times the amount of light intended to be measured; if not completely absorbed or scattered back out, this additional light produces erroneously high signals. In contrast, the TIM's optical design places the precision aperture at the front so only light intended to be measured enters (Figure 4a).

The paper goes into a bit more detail about the testing done to determine the reasons for the instrumental differences.

  • $\begingroup$ Thanks for the answer! I'll have a look at the references this weekend and see what I can find about those systems. It would have to be quite a tiny aperture for diffraction proper to add a few parts per thousand, to the transmitted light so I think it must be mostly scattered light, but I may be getting ahead of myself. (It doesn't affect your answer, I'm just curious why they use the word) $\endgroup$ – uhoh Sep 12 '20 at 15:31

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