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

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

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

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

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

Greg Kopp, p. 6ASIC 3 – 16 May 2006TSI Accuracy Two primary TSI composites differ by 40 ppm/yr. Caused by 2 years of marginal quality data – not even a gap! Climate Record Needs:... or 10 ppm/yr Stability and Overlap ?

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

Greg Kopp, p. 8ASIC 3 – 16 May 2006TSI Accuracy TSI Accuracy Workshop Organizer: Jim Butler, NASA/GSFC Location: NIST Gaithersburg, MD Dates: 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!”

TSI Accuracy Workshop Questions Multiple Radiometers Track Changes and Indicate Consistency With Stated Uncertainties 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?

Agenda Absolute Accuracy Stability Calibrations

Greg Kopp, p. 11ASIC 3 – 16 May 2006TSI Accuracy Summary of Instruments InstrumentCav # Cavity Layout Precision Aperture Position V Mon I/R Mon BlackBlack Type Active Therm Control Dark Meas. Freq. Comments ERB1?internal??paintspecularnoevery meas. ACRIM I3cones, front to back internalyes paintspecularnolow ACRIM II3cones, front to back internalyes paintspecularnolow ACRIM III3cones, front to back internalyes paintspecularnolow ERBE1cones, front to back internalyes paintspecularnoevery meas. one 3-min meas every 2 wks VIRGO-PMO2inverted cones, front to back internalyes paintspecularno (at L1) nonelow-freq. shutter VIRGO- DIARAD 2cylinders, side by side internalyes paintdiffuseno (at L1) none TIM4cones, side by side front of instrum. no NiPdiffuseyesevery orbit pulse-width modulation

Greg Kopp, p. 12ASIC 3 – 16 May 2006TSI Accuracy Reviewed Uncertainties SORCE/TIM RelativeW/m2 Area Thermal efficiency Eectrical. Power Cavity absorption Total RSS VIRGO/DIARAD L ACRIM VIRGO/PMO

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

Greg Kopp, p. 14ASIC 3 – 16 May 2006TSI Accuracy 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

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

Greg Kopp, p. 16ASIC 3 – 16 May 2006TSI Accuracy Scatter Can Erroneously Increase Signal Baffles Cavity Precision Aperture View-Limiting Aperture Precision Aperture View-Limiting Aperture TIM Cutaway ACRIM III Cutaway View-Limiting Aperture Precision Aperture View-Limiting Aperture Precision Aperture Sunlight Additional light allowed into instrument can scatter into cavity All instruments except TIM put primary aperture close to the cavity –Could cause systematically higher TSI values reported Majority of light is blocked before entering instrument

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

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

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

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

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

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