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
-1
spectral irradiance [Wm -2nm ]
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
NIST SSI workshop, February 2012
First Light Irradiance
VIRGO
SPM-A SPM-A SPM-A SPM-B SPM-B SPM-B
862
500
402
862
500
402
1st Light
1996/1
0.967
1.844
1.661
0.964
1.836
1.665
N&L
1984
0.995
1.900
1.705
0.995
1.900
1.705
-2.8%
-3.0%
-2.6%
-3.1%
-3.4%
-2.3%
rel.diff.
Agreement to 0.5% between A & B
Fröhlich et al., Sol.Phys. 175:267-286, 1997
NIST SSI workshop, February 2012
Variations of SPM-B:
Solar or Instrumental?
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
SSI variations are about 5 times larger than TSI variation.
Solar cycle 24 is not obvious, 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
1st Light degradation of SPM-A
VIRGO SPM-B corrected for First-Light degradation
1.010
1.008
1.006
rel. Variation
1.004
1.002
1.000
0.998
0.996
1%
1%
0.994
0.992
0.990
1995
1997
2000
2002
2005
2007
2010
 Unrealistic increase in R and G over SC23
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
Comparison with SORCE-SIM Trend
VIRGO SPM-B (First-Light degradation)
lasp.colorado.edu/sorce
1.002
862nm
1.001
1.000
0.999
0.998
2004
2005
2006
2007
2008
Slope
862
500
402
VIRGO
1.40
1.22
-0.27
SIM
-0.30
+0.83
-1.00
(ppm/day)
1.002
500nm
1.001
1.000
0.999
0.998
2004
2005
2006
2007
2008
1.002
862nm
1.001
same as TSI
1.000
SIM @ 500
< 2006: 2.72
>=2006: 0.26
0.999
0.998
2004
2005
2006
2007
2008
VIRGO SPM-B: dots; SORCE SIM: solid lines
NIST SSI workshop, February 2012
Empirical Approach
VIRGO SSI
•
•
•
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
unrealistic decline of SSI, first order
estimation of instrumental effect.
Exponential correction eliminates
most of solar cycle variation as well
0.5%
•
SPM-B500 [Wm-2nm-1]
1.830
1.825
1.820
1.815
2000
2002
2004
2006
2008
2010
2012
NIST SSI workshop, February 2012
Linear Detrending
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
Linear vs. Exponential Detrending TSI
+1.5
1368.0
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
Long-Term Correlations
Preminger et al., 2011, ApJLetters
VIRGO
Linear
Exponential
R862
+0.14 [0.06 0.22]
+0.17 [0.09 0.25]
G500
+1.20 [1.02 1.38]
+0.69 [0.54 0.84]
B402
+0.29 [0.02 0.57]
+1.01 [0.77 1.25]
TSI
+0.54 [0.47 0.62]
+0.32 [0.27 0.36]
NIST SSI workshop, February 2012
Summary
Normalization of SPM-A by SPM-B:
– Larger than expected variations of Backup channel
– Rapid initial degradation in Active channel
– ‘Early increase’ of Backup is not observed in Active channel
 Recovery of SPM-B between expositions?
 Accumulation of contamination during dark periods, activation
during exposure, Schwarzschild effect?
Empirical correction of SPM-B: fitting degradation in time
– Linear or exponential detrending yield positive correlation with
solar cycle in 3 visible channels.
NIST SSI workshop, February 2012
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
NIST SSI workshop, February 2012
Additional material
NIST SSI workshop, February 2012
Dark ageing: ‘SOHO vacations’
VIRGO/SPM Phoenix
1.5
Ratio
1.4
1.3551
0.9952
1.3486
2
Irradiance [W/m /nm]
1.3
1.2
1.1
1
0.9
0.8
935.0
1.0016
0.99218
0.99382
0.87743
935.5
0.99906
936.0
936.5
0.87661
937.0 GAP 1041.0 1041.5 1042.0 1042.5 1043.0
Day of Mission
‘Dark’ ageing of SPM-A during 104 days: -0.1% to -0.8%
NIST SSI workshop, February 2012
Degradation Rates
Ratio to 1st Light
IPHIR on Phobos2 (1988) & SOVA on EURECA (1992)
VIRGO on SOHO (1996)
1
1
0.8
0.8
0.6
0.6
0.4
0.4
335/500/862 nm
0.2
0
0
50
100
150
402/500/862 nm
0.2
200
0
0
Ratio to 1st Light
1
1
0.8
0.8
0.6
0.6
0.4
0.4
0
402/500/862 nm
0
2
4
6
Exposure time [d]
8
100
150
200
PREMOS FR-A on Picard (2010)
SOVIM on SSI (2008)
0.2
50
210/535/782 nm
0.2
10
0
0
50
100
150
Exposure time [d]
200
kkk
NIST SSI workshop, February 2012
SOVA2/EURECA
Wehrli et al., 1995, Metrologia
NIST SSI workshop, February 2012