TSI: The Incoming Side of the Equation
Greg Kopp
Laboratory for Atmospheric and Space Physics
University of Colorado
1234 Innovation Dr., Boulder, CO 80303, USA
Greg.Kopp@LASP.Colorado.edu
SORCE Science Meeting
TIM Update
20-22 Sept. 2006
Greg Kopp, Page 1
TIM Very Accurately Measures TSI and Its Variability
SORCE Science Meeting
TIM Update
20-22 Sept. 2006
Greg Kopp, Page 2
The Earth's Radiation and Energy Balance
(from Kiehl and Trenberth, 1997)
TIM measures
incoming solar
radiation
SORCE Science Meeting
TIM Update
20-22 Sept. 2006
Greg Kopp, Page 4
What Is the Correct Absolute Value?
updated regularly at
http://spot.colorado.edu/~koppg/TSI
SORCE Science Meeting
TIM Update
20-22 Sept. 2006
Greg Kopp, Page 5
28-Year TSI Record Relies on Continuity
Current climate record plan
relies on continuity and
mission overlap.
But why the offsets?
SORCE Science Meeting
TIM Update
20-22 Sept. 2006
Greg Kopp, Page 6
Summary of Stated Instrument Accuracies
Instrument
ERB (NIMBUS 7)
ACRIM I
ACRIM II
ACRIM III
ERBE
VIRGO
VIRGO-PMO
VIRGO-DIARAD
DIARAD-like
SORCE/TIM
TSI Value
[W/m^2]
1371.9
1367.5
1364.2
1366.1
1365.2
1365.7
1365.7
1366.4
1366.4
1361.0
Stated
Uncertainty
[ppm]
5000
1000
1000
1000
833
1000
1204
470
600
350
Uncertainties are 1-σ
SORCE Science Meeting
TIM Update
20-22 Sept. 2006
Greg Kopp, Page 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!”
SORCE Science Meeting
TIM Update
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Greg Kopp, Page 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?
TSI Instrument Uncertainties & Intra-Instrument Variations
Instrument
ERB (NIMBUS 7)
ACRIM I
ACRIM II
ACRIM III
ERBE
VIRGO
VIRGO-PMO
VIRGO-DIARAD
DIARAD-like
SORCE/TIM
TSI Value
[W/m^2]
1371.9
1367.5
1364.2
1366.1
1365.2
1365.7
1365.7
1366.4
1366.4
1361.0
Stated
Uncertainty
Uncertainty
[ppm]
5000
1000
1000
1000
833
1000
1204
470
600
350
Cavity
Variations
! [ppm]
511
2046
1036
2271
299
2858
1612
301
Uncertainties are 1-σ
SORCE Science Meeting
20-22 Sept. 2006
TIM Update
Greg Kopp, Page 10
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
SORCE Science Meeting
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TIM Update
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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
SORCE Science Meeting
20-22 Sept. 2006
TIM Update
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What Is the Correct Absolute Value? Work in Progress...
updated regularly at
http://spot.colorado.edu/~koppg/TSI
SORCE Science Meeting
20-22 Sept. 2006
TIM Update
Greg Kopp, Page 13
DeToma and White – Empirical Modeling of TSI
•
DeToma and White fit TSI time series to empirical models
– Best fits with MgII and SFO photometric sum Σr
G. DeToma and O.
White, “Empirical
Modeling of TSI: A
Critical View,” Solar
Physics, 2006 (in
press)
SORCE Science Meeting
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TIM Update
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DeToma and White – Empirical Modeling of TSI
“The model for the TIM TSI time series gives the best fit
with an rms of only 73 ppm during the 2.5 years of the
measurements.”
“VIRGO and TIM TSI time series agree very well, so their
empirical models are nearly the same. In contrast, VIRGO
and DIARAD TSI data show significant differences...”
SORCE Science Meeting
20-22 Sept. 2006
TIM Update
Greg Kopp, Page 15
DeToma and White Conclusions
•
TSI measurements from new experiments show good agreement in
variations seen on time scales of days to months.
– (Relative) Uncertainties of the order of 50 ppm are now common.
– On longer time scales, ACRIM3 and DIARAD time series show differences from
TIM and VIRGO measurements with amplitudes from 0.2 to 0.4 Wm−2 .
• Lower frequency differences appear to come from problems in correction
for degradation in-flight and failure to track radiometer stability well.
•
Models using Mg II 280 nm and Σr indices now reproduce the VIRGO time
series from 1996 to 2005 with an rms residual of 104 ppm.
– Insight into the stability and usefulness of empirical models within a single time
series comes from regression analysis of intervals within the nine- year VIRGO
TSI record.
•
Extension of this empirical model study to other TSI time series reveals
properties of these time series
– The model for the TIM TSI time series gives the best fit with an rms of only 73
ppm during the 2.5 years of the measurements. TIM TSI measurements are the
most precise because of its new instrumental design. As a result, TIM
measurements are the most consistent with the indices we use. However, the
TIM record is still quite short for an accurate assessment of its long-term
stability relative to the indices and other TSI time series.
SORCE Science Meeting
20-22 Sept. 2006
TIM Update
Greg Kopp, Page 16
SSI Model Based on TSI
•
How well does SSI correlate with TSI?
– Correlations as a function of wavelength could allow TSI-based adjustments to
account for solar variability of a mean solar spectrum
SORCE Science Meeting
20-22 Sept. 2006
TIM Update
Greg Kopp, Page 17
Correlations Between TSI and SSI
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Correlations Between TSI and SSI – Movie
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TIM Update
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SSI Model Based on TSI
•
TSI correction applied to SSI mean provides some correction
– But not all
•
Regions of applicability (based on 600 nm)
– >0.1% accuracy - Use a constant solar spectrum
– ~0.05-0.1% - Better than an irradiance model with no solar spectral variability
– <0.05% - Use real solar spectral irradiance
Sensitivity
to TSI
Relative
[ W/m2/nm Sensitivity
per
Wavelength Ave. SSI
to TSI
W/m2]
[nm]
[W/m2/nm]
[unitless]
350
0.964
0.003396
4.795
400
1.603
0.004845
4.114
450
2.037
0.004733
3.161
500
1.955
0.003240
2.255
550
1.884
0.002593
1.873
600
1.766
0.002110
1.626
700
1.410
0.001427
1.378
800
1.123
0.001038
1.259
900
0.908
0.000798
1.196
1000
0.739
0.001281
2.357
1100
0.602
0.000493
1.115
1200
0.496
0.000349
0.957
1300
0.420
0.000253
0.821
1400
0.357
0.000198
0.756
1500
0.301
0.000147
0.663
1600
0.253
0.000120
0.647
SORCE Science Meeting
20-22 Sept. 2006
TIM Update
Greg Kopp, Page 20
TIM Will Next Be Flying on Glory...
S/C TSI Observations
TIM Instrument on Glory
TIM Instrument
TPS
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TIM Update
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...but perhaps no longer on NPOESS
TIM
S/C TSI Observations
SIM
?
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TIM Update
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Summary
•
TIM TSI values remain ~4.5 W/m2 lower than other S/C TSI instruments.
– Testing and future calibration facilities underway to resolve such differences
– This lower TSI value decreases the difference between incoming and outgoing
radiation
•
The TIM shows very little optical degradation from solar exposure, and the
electronics show no signs of change since launch.
– TIM degradation is low at 120 ppm for 3 yrs of operation
• This is tracked and corrected in data analysis
•
Results from 1 March 2003 to the present are released daily and are
available 7 days after acquisition.
SORCE Science Meeting
20-22 Sept. 2006
TIM Update
Greg Kopp, Page 23
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
ACRIM III Cutaway
View-Limiting
Aperture
Precision
Aperture
Precision
Aperture
View-Limiting
Aperture
Precision
Aperture
View-Limiting
Aperture
Precision
Aperture
TIM Cutaway
Baffles
Cavity
Additional light
allowed into
instrument can
scatter into cavity
Majority of light is
blocked before
entering instrument
SORCE Science Meeting
TIM Update
View-Limiting
Aperture
20-22 Sept. 2006
Greg Kopp, Page 24
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
SORCE Science Meeting
20-22 Sept. 2006
TIM Update
Greg Kopp, Page 25
TSI Instrument Uncertainties & Intra-Instrument Variations
NIST calculates diffraction should
lower these results.
SORCE Science Meeting
20-22 Sept. 2006
TIM Update
Greg Kopp, Page 26
TSI Instrument Uncertainties – With Diffraction Correction
Correction not yet approved or applied by ACRIM or ERBE Teams
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TIM Update
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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
SORCE Science Meeting
20-22 Sept. 2006
TIM Update
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Glory is Creating the TSI Radiometer Facility (TRF)
Position 1: Cryogenic Radiometer under Test
TRF Capabilities
• Compare to reference
cryogenic radiometer
• Measure irradiance
• Input solar power levels
• Operate in vacuum
Have strong community support for a
NIST-traceable cryogenic radiometer
facility to calibrate TSI instruments to
~100 ppm accuracy (TSI Accuracy
Workshop 2005; NEWRAD 2005)
Position 2: TSI Instrument under Test
Cryogenic
Radiometer
TSI Instrument in
Vacuum Tank
Vacuum Arms
with Flexible
Bellows
“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]
Common
Entrance
Window
Incident Light
Incident Light
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