The Accuracy of Total Solar Irradiance Measurements Greg

The Accuracy of Total Solar Irradiance Measurements Greg Kopp, CU/LASP, Boulder, CO ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 1

Impacts of Solar Irradiance on Earth Secular Trends (a proxy of TSI) Sun - Climate Correlations . . . Across All Time Scales 27 -Day Solar Rotation 11 -Year Solar Cycle Sultan Hameed, Stony Brook SORCE Meeting 2004 L. Hood, Univ. Arizona SORCE Meeting 2003 ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 2

27 -Year TSI Data Record ASIC 3 – 16 May 2006 TSI Accuracy None of these instruments is calibrated end-toend for irradiance. Greg Kopp, p. 3

Summary of Stated Instrument Accuracies Uncertainties are 1 - ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 4

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. ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 5

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! ? ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 6

27 -Year TSI Record Relies on Continuity Current climate record plan relies on continuity and mission overlap. But why the offsets? ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 7

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!” ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 8

TSI Accuracy Workshop Questions • Review Instrument Designs – Are there systematic differences that could cause TSI offsets? • Review Calibrations & Uncertainties Multiple Radiometers Track Changes and Indicate Consistency With Stated 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?

Agenda Absolute Accuracy Stability Calibrations

Summary of Instruments Instrument Cav # Cavity Layout Precision Aperture Position V Mon I/R Mon Black Type Active Therm Control Dark Meas. Freq. ERB 1 ? internal ? ? paint specular no every meas. ACRIM I 3 cones, front to back internal yes yes paint specular no low ACRIM III 3 cones, front to back internal yes paint specular no low ERBE 1 cones, front to back internal yes paint specular no every meas. one 3 -min meas every 2 wks VIRGO-PMO 2 inverted cones, front to back internal yes paint specular no (at L 1) none low-freq. shutter VIRGODIARAD 2 cylinders, side by side internal yes paint diffuse no (at L 1) none TIM 4 cones, side by side front of instrum. no no Ni. P diffuse yes every orbit ASIC 3 – 16 May 2006 TSI Accuracy Comments pulse-width modulation Greg Kopp, p. 11

Reviewed Uncertainties SORCE/TIM ACRIM VIRGO/PMO VIRGO/DIARAD L Relative ASIC 3 – 16 May 2006 W/m 2 Area 0. 000425 0. 58 Thermal efficiency 0. 000130 0. 18 Eectrical. Power 0. 000150 0. 20 Cavity absorption 0. 000030 0. 04 Total 0. 000735 1. 00 RSS 0. 000470 0. 64 TSI Accuracy Greg Kopp, p. 12

TSI Instrument Uncertainties & Intra-Instrument Variations Uncertainties are 1 - ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 13

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. – – • Applied Power: The TIM uses DSP-controlled pulse width modulation while other radiometers apply DC power. – • 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. – – • Very unlikely to have 0. 3% difference Optical – Scatter Prior to Limiting Aperture: Instruments with oversized (nonlimiting) aperture near front of instrument allow much more sunlight into instrument. – – • • Does not account for 0. 3% TSI differences Does not explain inter-cavity variations within single instrument 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 ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 14

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 ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 15

Scatter Can Erroneously Increase Signal • All instruments except TIM put primary aperture close to the cavity – Could cause systematically higher TSI values reported Sunlight ACRIM III Cutaway 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 ASIC 3 – 16 May 2006 View-Limiting Aperture TSI Accuracy Greg Kopp, p. 16

Diffraction Can Erroneously Change Signal • All instruments except TIM put primary aperture close to the cavity Sunlight View-Limiting Aperture Precision Aperture View-Limiting Aperture Failure to correct for light diffracted into cavity out of cavity erroneously increases signal decreases signal ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 17

TSI Instrument Uncertainties & Intra-Instrument Variations NIST calculates diffraction should lower these results. ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 18

TSI Instrument Uncertainties – With Diffraction Correction not yet approved or applied by ACRIM or ERBE Teams ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 19

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 vacuum window beamsplitter (1%) TSI instrument stabilized laser trap diode vacuum chamber ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 20

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] ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 21

TSI Radiometer Facility Layout Radiometer Requirements TSI Instrument in Vacuum Tank 1 -D Stage Cryogenic Radiometer Light Source ASIC 3 – 16 May 2006 TSI Accuracy Greg Kopp, p. 22