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The Accuracy of Total Solar Irradiance Measurements

The Accuracy of Total Solar Irradiance Measurements. Greg Kopp, CU/LASP, Boulder, CO. Impacts of Solar Irradiance on Earth. Sun - Climate Correlations . . . Across All Time Scales . 11-Year Solar Cycle. Secular Trends. (a proxy of TSI). 27-Day Solar Rotation.

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The Accuracy of Total Solar Irradiance Measurements

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  1. The Accuracy of Total Solar Irradiance Measurements Greg Kopp, CU/LASP, Boulder, CO

  2. Impacts of Solar Irradiance on Earth Sun - Climate Correlations . . . Across All Time Scales 11-Year Solar Cycle Secular Trends (a proxy of TSI) 27-Day Solar Rotation Sultan Hameed, Stony Brook SORCE Meeting 2004 L. Hood, Univ. Arizona SORCE Meeting 2003

  3. None of these instruments is calibrated end-to-end for irradiance. 27-Year TSI Data Record

  4. Summary of Stated Instrument Accuracies Uncertainties are 1-

  5. Climate Record Needs: 100 ppm Absolute Accuracy... • Dick Willson: “...uncertainties of 100 ppm or less would be required to produce a TSI record with sufficient traceability over the multi-decadal to centennial time scales for climate change and solar physics investigations without employing and overlapping, redundant measurement...” No TSI instrument has yet achieved this level of accuracy.

  6. Climate Record Needs: ... or 10 ppm/yr Stability and Overlap Two primary TSI composites differ by 40 ppm/yr. Caused by 2 years of marginal quality data – not even a gap! ?

  7. 27-Year TSI Record Relies on Continuity Current climate record plan relies on continuity and mission overlap. But why the offsets?

  8. TSI Accuracy Workshop • Organizer: Jim Butler, NASA/GSFC • Location: NIST Gaithersburg, MD • Dates: 18-20 July 2005 • Attendees • Representatives of several TSI instruments • ACRIM I, II, and III • ERBS/ERBE • SORCE/TIM • VIRGO/PMO • VIRGO/DIARAD & SOLCON • NIST, NASA • Approach • Day 1: Accuracy (“the Day 1 Problem”) • Day 2: Stability • Day 3: Improved or current calibration facilities • Dick Willson: “We haven’t had a meeting like this in 20 years!”

  9. Review Instrument Designs • Are there systematic differences that could cause TSI offsets? • Review Calibrations & Uncertainties • How accurately is each instrument calibrated? How well are uncertainties estimated? • How well are degradation and stability understood? • What were goals and actuals? • Intra-instrument Consistency • Do intra-instrument cavity comparisons agree with stated uncertainties? • What ground calibrations or facilities would improve the future TSI record? Multiple Radiometers Track Changes and Indicate Consistency With Stated Uncertainties TSI Accuracy Workshop Questions

  10. Agenda Absolute Accuracy Stability Calibrations

  11. Summary of Instruments

  12. Reviewed Uncertainties SORCE/TIM ACRIM VIRGO/PMO VIRGO/DIARAD L

  13. TSI Instrument Uncertainties & Intra-Instrument Variations Uncertainties are 1-

  14. Possible Causes of Differences in Absolute Values • Underestimated Uncertainties: Is this simply the state of the art in these radiometric measurements, with all uncertainties being underestimated? • Apertures: Measurements from different facilities have greater variations than stated aperture measurement uncertainties. • Does not account for 0.3% TSI differences • Does not explain inter-cavity variations within single instrument • Applied Power: The TIM uses DSP-controlled pulse width modulation while other radiometers apply DC power. • Very unlikely to have 0.3% difference • Optical – Scatter Prior to Limiting Aperture: Instruments with oversized (non-limiting) aperture near front of instrument allow much more sunlight into instrument. • The TIM precision aperture and shutter are at the front of the instrument, so this is a difference. • Scatter will erroneously increase the signal through the limiting aperture. • Optical – Diffraction: This is a 0.12% effect in ACRIM and is not corrected • Darks: Uncertainties in dark corrections are large. • These are large corrections, depend on FOV, and vary with temperature. • Darks are not measured regularly on several instruments. • Aperture Heating: Uncertainties in heating due to different aperture materials, conduction, mounting, emissivities

  15. Roadmap for Future Improvements • Complete aperture comparison measurements • Include ACRIM apertures in NIST aperture comparison • Power comparison • NIST power comparison to trapped diode transfer standard • NPL power trap comparison • 0.05% accuracy • Scatter/diffraction measurement • NIST to monitor changes in signal as beam expands to overfill entrance aperture • Would like an irradiance comparison against an absolute reference radiometer with 0.01% accuracy • JPL Table Mountain Observatory inter-comparisons are merely relative • PMOD World Radiation Reference is linked to an absolute scale but without desired accuracy • NASA’s Glory program is creating the TSI Radiometer Facility to compare TSI instruments on an absolute scale

  16. Scatter Can Erroneously Increase Signal • All instruments except TIM put primary aperture close to the cavity • Could cause systematically higher TSI values reported Sunlight Sunlight Precision Aperture ACRIM III Cutaway View-Limiting Aperture Precision Aperture View-Limiting Aperture Precision Aperture View-Limiting Aperture TIM Cutaway Precision Aperture Baffles Cavity Additional light allowed into instrument can scatter into cavity Majority of light is blocked before entering instrument View-Limiting Aperture

  17. Diffraction Can Erroneously Change Signal • All instruments except TIM put primary aperture close to the cavity Sunlight Sunlight View-Limiting Aperture Precision Aperture Precision Aperture View-Limiting Aperture Failure to correct for light diffracted into cavity erroneously increases signal Failure to correct for light diffracted out of cavity erroneously decreases signal

  18. NIST calculates diffraction should lower these results. TSI Instrument Uncertainties & Intra-Instrument Variations

  19. TSI Instrument Uncertainties – With Diffraction Correction Correction not yet approved or applied by ACRIM or ERBE Teams

  20. Address Applied Power: Trap Diode Power Comparison • NIST and LASP are preparing to do optical power comparisons between a transfer standard and ground TSI instruments • NPL has done similar power comparisons before, but in air TSI instrument vacuum window beamsplitter (1%) stabilized laser trap diode vacuum chamber

  21. Intend to Create Facility for TSI Calibrations • Have strong community support for a NIST-traceable cryogenic radiometer facility to calibrate TSI instruments to ~100 ppm absolute accuracy (TSI Accuracy Workshop 2005, NEWRAD 2005) • Glory Science will create this facility to enable TSI instrument comparisons against an absolute standard “Benchmark observations of total solar irradiance and spectrally resolved solar irradiance to an accuracy of 0.03 percent referenced to NIST standards are required to elucidate the origin of climate change.” [NRC Committee on Earth Science and Applications from Space]

  22. TSI Radiometer Facility Layout Radiometer Requirements TSI Instrument in Vacuum Tank 1-D Stage Cryogenic Radiometer Light Source

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