Total Solar Irradiance from VIRGO Radiometry, an Update to Version 6.4
Claus Fröhlich
VIRGO Team, CH-7265 Davos Wolfgang Switzerland
PMOB − DIAL (Wm−2)
For the analysis of the long-term behaviour of the radimeter an appropriate model is needed. It is based on the sunburn of quartz by dissociation of SiO2 at the surface and described in detail in Fröhlich (2014). It depends on the dose
during exposure for which the MgII index is used and on
temperature. Moreover, it includes also a term for a possible recovery during non-exposure periods. The model used
in the following analysis has a total of 6 coefficients.
The early behaviour of PMO6V is characterized by a
rapid increase which is the result of a darkening of the illuminated part of the precision aperture, and hence a reduction of the reflected light into the baffle and an increase of a
possible aperture heating. At first the explanation of this effect was difficult mainly because of its rather large value of
about 500 ppm for PMO6V-A, 350 ppm for B, 590 ppm for
ACRIM-I and about 1000 ppm for HF (Fröhlich, 2006). So,
the scattered light from the precision aperture into the baffle
and back to the receiver could be an explanation, the amount
of which obviously changes with the reflectivity of the precision aperture. This effect was also suspected to explain
the large difference in absolute values of about 0.3 % between the classical radiometers HF, ACRIM, PMO6V and
DIARAD and TIM on SORCE. However, the corresponding correction was measured by Brusa and Fröhlich (1986)
with a laser illuminating the precision aperture and measuring the radiation scattered back through the aperture with a
Silicon detector; the result was of the order of 350–500 ppm
- no way to explain a 0.3 % influence. With a reflectivity of
about 60% the baffle receives about 40 mW which is mostly
absorbed. Hence, the infrared radiation from the heated baffle could certainly be important and can possibly explain the
effect. Comparison at TRF of spare radiometers of PMO6V
and ACRIM with a cryogenic radiometer have shown that,
indeed, the amount of scattered and infrared radiation from
the baffle is large enough to explain the scale difference (e.g.
Kopp et al., 2012). It is interesting, that DIARAD does not
show an early increase and the recent TRF comparison confirm also that it has no effect of ’scattered’ radiation. The
fact that DIARAD has a curved precision aperture which focuses the radiation falling on it back into the view-limiting
aperture and thus out of the radiometer, explains its nonsensitivity to ‘scattered’ radiation. The scale correction for
DIARAD has a completely different reason due to the substantial difference in area covered by the electrical heater
and the irradiance.
The level-1 VIRGO data are corrected for all
known effects, such as electrical calibration, temperature,
instrument-sun distance and radial velocity. The normal
0.4
PMO6V−B − DIARAD−L
310−day filtered
Result of fit
0.2
0.0
−0.2
Version: 6_004_1312
−0.4
2002
2004
2006
2008
2010
2012
2014
100
0
−100
1000
SoHO Mission Day
Version 6_004_1312
2000
1200
Exponential correction for DIARAD−L
Correction including ∆
Early increase correction for PMO6V−B
Correction including ∆
−0.2
−0.4
2002
2004
2006
2008
2010
2012
0.0
Claus Fröhlich. Degradation of radimeters in space: Application to VIRGO TASI and SSI. Technical report,
VIRGO Team, CH-7265 Davos Wolfgang, Switzerland,
2014. This is a working paper and the present version
is available from ftp://ftp.pmodwrc.ch/pub/
Claus/VIRGO-TSI/VIRGO_TSI_6004.pdf.
G. Kopp, A. Fehlmann, W. Finsterle, D. Harber, K. Heuerman, and R. Willson. Total solar irradiance data record
accuracy and consistency improvements. Metrologia, 49:
29, 2012. doi: 10.1088/0026-1394/49/2/S29.
2004
1.0
0
2000
3000
4000
SoHO Mission Day
5000
6000
−2
Difference from shuttered to covered operation: 0.280 Wm
200
3000
4000
SoHO Mission Day
5000
6000
Figure 2: Corrections of DIARAD-L from comparision with
DIARAD-R for degradation and with PMO6V-A for switch-offs
for both versions, the ’old’ 6.002 and the ’new’ 6.004.
PMOA (Wm−2)
2000
PMO6V−A as measured
Fit with dose etc
PMO6V−A corrected
Change: −192.0 ± 2.1 ppm
−200
700
800
900
1000
SoHO Mission Day
1100
1200
Version: 6_004_1312
4.0
new PMO6V−B − PMO6V−A
old PMO6V−A corrections
new PMO6V−A corrections
new including shift
2.0
1000
2000
3000
4000
SoHO Mission Day
5000
0
Start of cover
and PMO6V−B operation
−200
−400
Degradation fixed at −2.0 ppm/expday
PM6V−A as measured
with new correction
with old correction
new corrected and shifted
1366.0
1365.5
1365.0
60
80
100
SOHO Misson Day
120
1998
2000
2002
2004
2006
2008
2010
2012
DiffAL <−
PMO6V−A −>
DIARAD−L −>
0.0
−1.0
slope: −13.074 ± 0.187 ppm/a
−1.5
0.4
0.2
0.0
−0.2
0.4
0.2
0.0
−0.2
PMO6V
2004
2006
slope: −12.928 ± 0.211 ppm/a
2010
2012
2014
DIARAD 2006
2004
slope: −13.662 ± 0.281 ppm/a
2008
2010
2012
2014
2008
2010
2012
2014
0.0
0.4
2002
2004
2006
2008
2010
2012
0.0
2014
1364
0.8
1996
0.7
VIRGO TSI
PMO6V TSI
DIARAD TSI
Version: 6_004_1312
1998
2000
2002
2004
2006
2008
2010
2012
2014
0.6
0.5
2000
2002
−2
2000
2006
2008
2010
2012
2014
ACRIM−II V101001 − 1362.2 Wm−2
ACRIM−III V1311 − 1362.2 Wm−2
TIM V9−1308 − 1360.3 Wm−2
VIRGO V6_004_1312 − 1366.0 Wm−2
Difference between Minima:
−2
VIRGO: 0.1693 Wm−2
ACRIM: 0.3676 Wm
ACRIM + TIM: 0.1947 Wm−2
1998
2000
2002
2002
2004
2006
2008
2010
2012
2014
Figure 6: The top panel shows the variance of DIA-L and
PMO-A as used to weight each for the average VIRGO value.
The middle panel shows the three final time series and the bottom panel the difference between the new version 6 004 and
the old one 6 002.
2004
2006
2008
2010
2012
2014
A/V all: −188.57 ± 2.09 ppm/dec
A/V desc: −102.38 ± 6.24 ppm/dec
A/V TIM: −204.18 ± 3.31 ppm/dec
200
0
−200
T/V all: − 150ppm: 131.04 ± 1.87 ppm/dec
T/V min: − 150ppm: 159.17 ± 5.26 ppm/dec
A/V asc: −48.21 ± 22.39 ppm/dec
Daily difference 6_004_1312 − 6_002_1302
365−day filtered difference
1998
2004
0
−400
1996
0.4
0.3
0.2
1996
1998
a)
1996b)
400
1365
1363
2006
−1
−3
1366
Figure 4: Top panel: Ratio of the irradiance of PMO6V-A to
a combination of the proxy model and DIARAD. The change
of absolute values due to change from shuttered to covered
operation of -0.280 Wm−2 is also determined. Bottom panel:
Corrected PMO6V-A values ’with new corrections’ compared
to the former ones as ’with old corrections’.
1996
1
0.2
2000
2008
TIM and their difference and the bottom panels the difference
to PMO6V and DIARAD respectively. DIARAD shows less
short-term noise than PMO6V, on the other hand PMO6V has
less longer term deviations relative to TIM.
2014
1.0
0.6
1998
2014
0.8
−0.2
−0.4
−0.6
1368
1996
1367
2012
Figure 7: The top panel shows the time series of VIRGO and
versions, the ’old’ 6.002 and the ’new’ 6.004 of PMO-A from
comparison with the corrected PMO-B and the top panel the
comparison with ACRIM for the determination of the change
over the gap.
0.2
2010
−0.5
6000
Figure 5: The bottom panel shows the corrections for both
1996
0.6
0.4
2008
VIRGO 6_004_1312
TIM V0014_1308 + 0.1Wm−2
Difference to TIM (daily values)
Difference to TIM (81−day filtered)
Linear fit
0.5
−100
6.0
2006
VIRGO
2004
1000
−0.5
1000
Claus Fröhlich. Total solar irradiance observations. Surveys in Geophysics, 33:453–473, 2011. doi: 10.1007/
s10712-011-9168-5.
100
0
PMO6V-B and DIARAD level-1.8 used to fit the early increase
and the exponential model. The middle panel shows the comparison of the corrected PMO6V-B with ACRIM-II to determine the change over the SoHO vacations, and the bottom
panel the corrected data as dashed lines.
PMOA/Model − 1 (ppm)
Correction (Wm−2)
new R/L
new R/L outliers
old from R/L
new from R/L
old switch−off − 0.3
new switch−off −0.2
0.5
0
C. Fröhlich, D. Crommelynck, C. Wehrli, M. Anklin, S. Dewitte, A. Fichot, W. Finsterle, A. Jiménez, A. Chevalier,
and H. J. Roth. In-flight performances of VIRGO solar
irradiance instruments on SOHO. Sol. Phys., 175:267–
286, 1997. doi: {10.1023/A:1004929108864}.
0.0
2014
radiometers, PMO6V-A and DIARAD-L and their back-ups,
PMO6V-B and DIARAD-R.
1.0
C. Fröhlich. Evidence of a long-term trend in total solar
irradiance. A&A, 501:L27–L30, 2009. doi: {10.1051/
0004-6361/200912318}.
−0.8
0
Figure 1: Measurements (Level 1) of the two operational
1.5
C. Fröhlich. Solar irradiance variability since 1978: Revision of the PMOD composite during solar cycle 21.
Space Sci. Rev., 125:53–65, 2006. doi: 10.1007/
s11214-006-9046-5.
Ratio PMO6V−A/ACRIM, used
Ratio PMO6V−A/ACRIM, outliers
Mean before/after
200
−0.6
Figure 3: The top panel shows the difference between
2.0
R. W. Brusa and C. Fröhlich. Absolute radiometers (PMO6)
and their experimental characterization. Appl. Opt., 25:
4173–4180, 1986. doi: {10.1364/AO.25.004173}.
−1.0
DIARAD−R − 4.5 Wm2
DIARAD−L
PMO6V−A − 0.3 Wm22
PMO6V−B − 1.7 Wm
1998
1100
References
TSI (Wm−2)
1996
900
I would like to thank W Finsterle and A. Fehlmann for many
helpful discussions. Thanks are extended to the VIRGO
and SOHO teams, without their continuous efforts this work
would never have been possible. SOHO is a cooperative
mission of ESA and NASA, launched on 2nd December
1995, more than 18 years ago - and is still going strong.
Ratio − 1.0 (ppm)
1358
800
Acknowledgements
weights
1360
Change: −220.3 ± 7.7 ppm
−200
0.0
1362
6000
300
Ratio PMO6V−B/ACRIM, used
Ratio PMO6V−B/ACRIM, outliers
Mean before/after
200
1366
1364
5000
300
700
Correction (Wm−2)
Total Solar Irradiance (Wm−2)
1368
3000
4000
SoHO Mission Day
The new VIRGO TSI is certainly more reliable from a
VIRGO-only point of view. No other time series are used
Var(A)−Var(L)
2000
2000
• VIRGO TSI has still the artefact during the keyholes
of SOHO which are coming from the PMO6V data
due to some inadequate temperature interpolation of
missing values in the present level 0-to-1 evaluation.
In general the PMO6V values have less medium- to
long-term deviations relative to TIM than DIARAD,
but the noise of PMO6V is higher. This is already
seen in the comparison of DIARAD with PMO-B
(see Fig. 3).
VIRGO TSI (Wm−2)
1998
1000
• Relative to ACRIM 3 VIRGO TSI has during the period of TIM an upward trend of 20.2 ± 1.9 ppm/a.
New−old (Wm−2)
1996
1370
PMOB/ACRIM −1 (ppm)
0
• Relative to TIM VIRGO TSI has a downward trend
of 13.1 ± 1.9 ppm/a which would yield a difference
of about 150 ppm over a solar cycle.
besides ACRIM 2 to bridge the gap during the summer vacations of SOHO. However, we found that both PMO6V do
not show a significant change - so a posteriori we could use
this fact in the evaluation and leave out all the tests with
ACRIM 2.
From the final result we can determine the change of
PMO6V over the SOHO gap as 13.3 ± 27.6 ppm and for
DIARAD as 288.7 ± 15.7 ppm. These results can be used
to determine the 1-σ uncertainty as less than 30 ppm due to
the SOHO gap. Together with the uncertainty of the slope
to TIM over cycle 24 of 12×1.9 = 23 ppm (from Fig. 7), an
estimate of the uncertainty of the difference between the last
two minima in 1996 and 2008 by summing the two components amounts to 50 ppm or if we use the rms value to
35 ppm. This value is lower than the 92 ppm reported in
Fröhlich (2009) and makes the change between the minima of now 124 ppm more significant. However, the present
value is smaller than the one reported in (Fröhlich, 2011) of
168 ppm for version 6 002 1110).
Deviation from Mean (Wm−2)
Corrections for the VIRGO PMO6V and
DIARAD time series
by electrical substitution heater which is larger than the irradiated area by a factor of about 2.5. Normally these two
area are the same size and coincident by design and hence,
there is no influence of a change of the thermal resistance
for ’normal’ radiometers. With the coefficients of the exponential the final DIARAD level 2 values can now also be
determined.
Before we can determine the level 2 PMO6V-A we need
to correct the A values at the beginning for the early increase. For this early increase correction we need TSI values to compare with. As DIARAD starts at mission day 68
only, we expand these data with the proxy model back to
mission day 48. For fitting the model a further parameter
is added, representing the typical degradation of 2 ppm/day.
The result is shown in Fig. 4. With the level 2 PMO-B series the corrections for PMO-A over the whole mission can
be determined and interpolated by the method of splines to
the hourly values as shown in Fig. 5 and the change over
the SoHO interruption is determined by comparison with
ACRIM (top plot). Again, the PMO-A has not changed significantly during SOHO vacations as PMO-B.
VIRGO TSI is defined as the average of the PMO6V
and DIARAD values and is determined by weighting. The
weights are deduced from the difference of the variances,
the standard deviation squared, of PMO6V minus the one of
DIARAD, each determined from a 81-day-running period
and shown by the green line on the top panel of Fig. 6, again
smoothed with a 131-day boxcar. It is normalized in such a
way that the absolute maximum is set to 0.5 and a positive
difference means that the noise of PMO6V is higher and its
weight correspondingly lower and a negative one that the
noise of DIARAD is higher. So, the derived weights 0 . . . 1
are shown as red and blue dashed lines (on the left hand
scale). DIARAD has in general less noise than PMO6V
with e.g. the spikes during the keyholes due to the fact that
still the original level 0-to-1 algorithm. For the moment, it
seemed, however, more important to have an internally consistent way to produce level 2 data, than improving on the
noise of PMO6V.
As far as the absolute value is concerned VIRGO TSI
is still on its original scale, because the detailed analysis of
the absolute scale of PMO6V and DIARAD data is not yet
completed, but new values will be available soon. Besides
the absolute value, the major results can be summarized as
follows:
PMOA/ACRIM −1 (ppm)
This poster is a summary of Fröhlich (2014) which is a
working paper updated when ever new material becomes
available. At present this new version of TSI is based on
the level 1 data as available from the VDC. Especially the
data of the PMO6V radiometer are still using the original procedure to produce level 1 data and thus show still
the noise from inadequate interpolation of missing temperatures, such as the noise during the SOHO keyholes. Furthermore, the scale is still on the original WRR related value
for PMO6V and on the original for DIARAD.
practice to correct level-1 data (Fig. 1) for degradation and
other changes in space is based on the comparison of the
measurements from the operational radiometers with those
of the less exposed ones, the back-ups. For DIARAD this
procedure is straightforward because the backup is exposed
very little during the mission (about 10 days during the now
18 years in space) and can be assumed as having no degradation. The correction depends, however, on how the ratio
DIARAD-R/L is determined. The present evaluation is directly based on the DIARAD level 1 files from the VDC
(VIRGO Data Center at Tenerife, operated by the IAC).
As the left and right channels of DIARAD cannot be operated at the same time, the algorithm calculates the ratio (or difference) from the linearly interpolated DIARADL values during that day and the average of DIARAD-R,
shown as ’new R/L and ’new R/L outliers on Fig. 2. For the
PMO6V radiometers this procedure is complicated by the
fact that the shutters of both radiometers could no longer operated after about 70 days in space and had to be replaced
by opening and closing the covers at 8-hour intervals (see
e.g. Fröhlich et al., 1997). At the beginning of this new
operational mode PMO6V-B was quite frequently exposed
and thus these measurements need to be corrected for the
early increase before they can be used to correct PMO6V-A.
From mission days 83-218 the cover was normally closed
and opened every 8 hours for 30 minutes, up to the SoHO
vacation every 7 days and afterwards every 10 days.
For version 6.4 a new algorithm has be developed for the
interpolation between the rarely measured back-up data. It
is based on splines (in IDL use the functions spl init
and spl interp) which yields values for the final corrections of L or A which now show also some variations
related to temperature because they are less smoothed. For
DIARAD the result is shown on Fig. 2 as ’new from R/L.
DIARAD has also some changes related to the switch-off
of the experiment during the mission, which extended for
2 to 3 days six times during the 18-year operation due to
Emergency Sun Re-acquisition (ECR) of SoHO and once
due to a latch-up-induced switch-off of the VIRGO power
supply. After the event in September 1996 it became clear
that DIARAD showed a slow recovery after the experiment
was switched on again and a similar recovery was also identified after the SoHO vacations. The later events, however,
were no longer as clear as the ones before. Both the corrections for versions up to 6.002 and for 6.004 are shown in
Fig. 2 as ’old’ and ’new switch-off’. With all these corrections the DIARAD level 1.8 values can be determined. In
the course of the mission it became clear that the DIARAD
must have also a non-exposure-dependent change of its sensitivity, both from comparison with PMO6V and other radiometers in space, and it was likely that this change can
be modeled by an exponential. In the present version the
difference between the level 1.8 DIARAD and the level 1
PMO6V-B is fitted to the model for the early-increase for
PMO6V-B and to an exponential for DIARAD. The coefficients for PMO6V-B are then used to correct it and by comparison with ACRIM the change over the SoHO vacations
is determined. The procedure and the results are shown in
Fig. 3 and it is quite interesting that PMO6V-B does not
show any significant change over the SoHO interruption.
These changes may have the same reason as the long-term
sensitivity change, namely a change of the thermal contact
of the cavity and the heat-flux meter (probably due to exposure to the space environment), and hence a change in
the non-equivalence due to the difference in area covered
Correction (Wm−2)
Introduction
1998
2000
2002
2004
2006
2008
2010
2012
2014
Figure 8: VIRGO TSI is compared to ACRIM and TIM. The
ACRIM values are on the scale of ACRIM 3 (version 1311).
It is interesting to note that the slope relative to ACRIM for
the whole period and for the TIM period only are very similar,
although the ACRIM time series is composed of ACRIM 2 and
3.