34th Solar Physics Division Meeting, June 2003
Session 20 Instrumentation
Poster, Wednesday, June 18, 2003, 3:30-5:00pm, Mezzanine

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[20.01] A Cryogenic Pyrheliometer for More Accurate Solar Irradiance Measurements

P. Foukal (Heliophysics, Inc.)

Space-borne pyrheliometry over the past two solar cycles has demonstrated the variability of total solar irradiance in response to photospheric magnetic structures such as sunspots or faculae, over the 11-yr activity cycle.But the reproducibility of the measurements remains marginal to detect or rule out possible trends in irradiance below the 0.05-0.1% variation over the 11-yr cycle, but conceivably dominant over multi-decadal time scales of greatest relevance to climate.

In metrology laboratories,conventional radiometers similar to those presently flown by NASA and ESA have been superseded in the past ten years by cryogenic radiometers of ten times higher absolute accuracy and long term reproducibility.But their helium cooling makes them difficult to use in space. Recently, advances in superconducting transition thermometry at NIST, and in high-temperature superconducting materials,have presented the opportunity to reach cryogenic radiometer performance at LN2 temperatures attainable with space qualified single stage cryocoolers.

We report here on our results with a prototype SCT-based radiometer, developed to investigate this opportunity to improve the accuracy of space borne pyrheliometry.We show that the sensitivity achieved is an order of magnitude better than with conventional radiometers, although the noise threshold falls short of values attainable with LHe cooling.The measured non-equivalence errors, and results of monochromatic intercomparisons against trap detectors, are both consistent with absolute accuracy at the 0.01% level, thus comparable to LHe cooled radiometers. Improved thermal and mechanical design will be required to reduce slow drifts, to test this accuracy conclusively.

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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.