Measurements of TSI and SSI
Werner Schmutz
PMOD/WRC, Switzerland
TOSCA Workshop
Berlin, May 14, 2012
Overview
• Total Solar Irradiance
• Absolute calibration (first light PREMOS)
• Composites (relative calibration)
• Spectral Solar Irradiance (SSI)
• SIM/SORCE
• VIRGO/SOHO
14. May 2012
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PICARD
PREMOS – SOVAP – SODISM
Filter Radiometers
14. May 2012
Total Solar Irradiance
Werner Schmutz
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TSI calibration
PREMOS A is the first and only radiometer
in space with a SI-traceable irradiance
calibration in vacuum
Traceable to the irradiance calibration
facility at LASP in Boulder (TRF)
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Traceability of PREMOS-TSI
Comparison to
cryogenic rad.
(power in vacuum)
NPL
PREMOS B
Comparison to
cryogenic rad.
(power and irradiance in
vacuum)
TRF @ LASP
PREMOS A3
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Uncertainty of the calibration
= uncertainty of TRF comparison (220 ppm)
PREMOS A
TRF radiometer
+ absolute uncertainty of TRF facility (70 ppm)
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Calibration uncertainty budget
Traceable via TRF, LASP, Boulder  to NIST
• Irradiance in vacuum
 PREMOS A uncertainty: ± 280
ppm
(± 0.4 W/m2)
Flight-spare
recalibrations
Table compiled by Greg Kopp for an ISSI workshop March 2012
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Comparison PREMOS – TIM
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Status of PREMOS-TSI
„PREMOS is in excellent health“
o
PREMOS-TSI is the most accurate
absolute measurement;
±0.4 W/m2 or ±290 ppm
o
After 2 years, PREMOS-TSI has at
most 50 ppm relative deviation to
TIM/SORCE.
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The Future of TSI observations:
Are relative observations sufficient?
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There are three TSI composites
20.12.2011
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TSI-composites normalized
PMOD, ACRIM
normalized 2004-2005
DIARAD
20.12.2011
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Composite 1996-2010
0.2 W/m2
± 0.2 W/m2 / 10-years
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Is there a long-term trend?
Fröhlich 2009, A&AL 501, L27-L30
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Could we detect a long-term
trend with a composite?
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Requirements for a TSI monitoring
„Any plan to rely on an unbroken
chain of measurements is broken“
o
o
Not only because of a potential gap;
But mainly because of the uncertainty
is continuously increasing with time !
 Accurate absolute measurements are
required !
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Requirements for a TSI monitoring
 Accurate absolute measurements are
required:
Nowadays possible !
 But we certainly also want to assess
the variations of TSI and therefore,
we still need to aim for continues and
overlapping data !
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TSI monitoring today …
Presently, 4 operational space
experiments observing TSI:
- VIRGO (launched 1995)
- ACRIM III (launched 2000)
- TIM (launched 2003)
- PREMOS (launched 2010)
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Part II:
Spectral Solar Irradiance
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The open question !
The bands 410-470 and 480-730 nm are
anti-correlated to TSI variations
Compensated by larger (than TSI) UV variations
Is the SIM observation really correct?
Or is it rather a degradation problem?
14. May 2012
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Anti-correlation in models
Contrast between active and quite Sun (SSN 150 vs SSN 0)
Black: Bright+Dark
Red: bright components
Blue: dark components
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VIRGO and PREMOS bands
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215 nm PREMOS vs SOLSTICE
 Independant correction of
PREMOS
SOLSTICE
 Strong correlation of
13.5 and 27 days
modulation
PREMOS
 PREMOS sampling is
faster
 Rotational modulation
more accurate
PREMOS
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VIRGO SSI time series 1996 - now
Christoph Wehrli & VIRGO Team
PMOD/WRC Davos
An attempt to assess instrument degradation in a self consistent way
by:
• referring operational measurements to occasional backup
operations
• correcting the backup channel by initial ageing of operational
channel
NIST SSI workshop, February 2012
VIRGO Sun Photometers
• Interference filter radiometer with 3 channels centered
at 862nm, 500nm and 402nm (R,G,B); FWHM
bandwidths 5nm; silicon PD detectors; rad-hard
windows.
• Active (SPM-A) and Backup (SPM-B) instruments
– SPM-A: exposed continuously for helioseismology application
– SPM-B: exposed rarely for solar spectral irradiance
measurements
• Calibrated by EG&G FEL lamps, NBS 1973 traceable
NIST SSI workshop, February 2012
VIRGO SPM: Level1 data
VIRGO SPM-B
VIRGO SPM-A
2
2
spectral irradiance [Wm -2nm -1]
1.6
1.4
1.2
1
0.8
70%
0.6
0.4
20%
1.8
spectral irradiance [Wm -2nm -1]
Sensitivity after 5825 days
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
Number of Backups 178
Total exposure time 2.6 days
0.2
0.2
5%
0
1996
1998
2000
2002
2004
2006
2008
2010
2012
0
1996
1998
2000
2002
2004
2006
2008
2010
2012
SPM-B:
Number of backups 178
Total exposure time 2.6 days
NIST SSI workshop, February 2012
Raw variations of SPM-B
VIRGO SPM-B and TSI
1.010
1.008
5*(TSI/<TSI>) 4
1.006
rel. Variation (TSI*5)
1.004
1.002
1.000
0.998
0.996
0.994
0.992
0.990
1996
1998
2000
2002
2004
2006
2008
2010
2012
VIRGO-SPM-B changes are about 5 times larger than the (real) solar variations in
TSI.
Solar cycle 24 SSI variation is not obvious (smaller), hidden in instrumental ageing.
NIST SSI workshop, February 2012
Initial ageing of SPM-A
VIRGO SPM-A Operational
VIRGO SPM-A First Light
1.015
1.035
1.030
rel. Variation [TSI*100]
1.020
rel. Variation
∂TSI
(*100)
1.010
1.025
1.015
1.010
1.005
1.000
1.005
1.000
0.995
0.995
0.990
17.Jan
18.Jan
19.Jan
Date 1996
Steep degradation during first hours!
20.Jan
0.990
28.Jan
04.Feb
11.Feb
18.Feb
Date 1996
25.Feb
03.Mar
Linear degradation during first month
{Commissioning activities until 29.03.1996}
NIST SSI workshop, February 2012
Ageing of SPM-A and SPM-B
versus exposure time
VIRGO SPM A & B
1.015
1.010
rel. Variation
1.005
1.000
0.995
 polynomial fit SPM-A
0.990
0.985
0
0.5
1
1.5
exposure time [d]
2
2.5
NIST SSI workshop, February 2012
SPM-B corrected by
operational degradation of SPM-A
VIRGO SPM-B corrected for SPM-A operational degradation
1.008
1.006
rel. Variation
1.004
1.002
1.000
0.998
0.996
1%
0.994
0.992
1995
1997
2000
2002
2005
2007
2010
Instrumental effects dominating over solar cycle
NIST SSI workshop, February 2012
Empirical Approach
VIRGO SSI
•
•
0.5%
-1
•
SSI timeseries represent a
mixture of Solar Cycle and
instrumental effects
Active & Backup SPM degrade
differently in time or exposure
time
Linear correction accounts for
probable decline of SSI, i.e. first
order estimation of instrumental
effect.
Exponential correction eliminates
most of solar cycle variation as
well
1.825
-2
•
SPM-B500 [Wm nm ]
1.830
1.820
1.815
2000
2002
2004
2006
2008
2010
2012
NIST SSI workshop, February 2012
Linear vs. Exponential Detrending:
 what does it to TSI ?
1368.0
+1.5
1367.0
+1.0
1365.0
+0.5
1363.0
1000
2000
3000 4000
MissionDay
5000
6000
+2.0
+1.0
+0.0
TSI (residuals)
1364.0
TSI (residuals)
TSI
1366.0
+0.0
-0.5
-1.0
-1.5
-1.0
-2.0
Lslope 0.54 [ 0.47 0.62]
Eslope 0.32 [ 0.27 0.36]
-2.0
-3.0
1000
2000
3000 4000
MissionDay
5000
6000
-2.5
1363
1364
1365 1366
TSI
1367
1368
NIST SSI workshop, February 2012
Linear Detrending 2000-2012
slope 1.20 [ 1.02 1.38] [1/µm]
3
1366
2
TSI
1368
1
1362
1000
2000
3000 4000
MissionDay
5000
6000
SSI500 (res)
5
SSI500 (residuals)
1364
0
-1
-2
-3
0
-4
-5
1000
2000
3000 4000
MissionDay
5000
6000
-5
1363
1364
1365 1366
TSI
1367
1368
NIST SSI workshop, February 2012
Summary
Normalization of SPM-A by SPM-B:
– Larger than expected variations of Backup channel
 “instrumental effects”
• None
Rapid initial
degradation in wavelength
Active channel bands
of VIRGOS’s
versus
862 nm, 500 nm, 402 nm
• ‘Early increase’ of Backup (not observed in operational channel)
is
anti-correlated
Empirical correction
of SPM-B:to TSI variations
fitting degradation in time with:
– Linear or exponential detrending yields positive correlation
with solar cycle (TSI) in
 all 3 visible channels !!!
NIST SSI workshop, February 2012
Thank you for your attention
PREMOS
14. May 2012
PICARD
Werner Schmutz
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Alternative analysis including proxies
(C. Fröhlich, EGU 2011)
SORCE
Empiric correction versus Time (double exponential),
Temperature (linear + Boltzmann), TSI and Mg-II
Index.
NIST SSI workshop, February 2012