Physikalisch-Metorologisches Observatorium Davos -- World Radiation Center pmod-wrc

VIRGO Data Products

VIRGO is an investigation for solar irradiance variability and helio-seismology on the ESA/NASA SOHO Mission

Table of Contents

  1. Instrumentation and Scientific Objectives
  2. VIRGO Data
  3. VIRGO Radiometry and Determination of TSI
  4. Detrending of SPM
  5. References

For further information see the official virgo web site.

1. Instrumentation and Scientific Objectives

The VIRGO Experiment on the ESA/NASA SOHO Mission has three types of instruments to measure total solar irradiance (TSI) with the absolute radiometers DIARAD and PMO6V, spectral solar irradiance at 402, 500 and 862 nm with a 3-channel sun photometer (SPM) and spectral radiance at 500 nm with a luminosity oscillation imager (LOI).

VIRGO provides

  • continuous high-precision, high-stability and high-accuracy measurements of the solar total and spectral irradiance and spectral radiance variation;
  • frequencies, amplitudes and phases of oscillation modes in the frequency range of 0.001 to 8 mHz.

These data will be utilized to achieve the main scientific objectives of VIRGO summarized in the following list:

  • detect and classify low-degree g modes of solar oscillations;
  • determine the sound speed, density stratification and rotation in the solar interior, specifically determine the physical and dynamical properties of the solar core;
  • study the solar atmosphere through comparison of amplitudes and phases of the p modes with these from velocity observations e.g. GOLF, SOI/MDI;
  • search for the long periodicities or quasiperiodicities that have been found in other solar parameters;
  • utilize the solar 'noise' signal to develop models for the global signature of stellar surface parameters;
  • determine properties of the solar asphericity and its variation in time;
  • study the relation between p-mode frequency changes and irradiance variations;
  • study the influence of solar active regions and other large-scale structures on total and spectral irradiance;
  • study the solar energy budget;
  • provide accurate total and spectral irradiance data for input in terrestrial climate modelling.

The official virgo web site describes in detail the experiment, its instruments and science objectives. In the following we concentrate on data products and their evaluation .

2. VIRGO Data

Data Products and Evaluation

All VIRGO data are sent after reception on ground to the VIRGO Data Center which is operated by and located at the Instituto Astrofisica de Canarias in La Laguna on Tenerife. After reception the raw data are separated into instrument files forming the level-0 data sets organized in daily files. During this process possible gaps in the science data due to transmission problems are filled from the 12 and 24-hour delayed data packets, which are stored on board and transmitted together with real-time packets. Level-1 data are those that have been converted to physical units; they include the calibrations and contain all the corrections known a priori for instrument-related effects, such as the influence of temperature variation. The signals are also reduced to 1 AU distance and to zero radial velocity. The processing is based on the algorithms developed and maintained by the CoI responsible for the instrument concerned (Instrument CoIs: Th.Appourchaux for LOI, Seven Dewitte (D. Crommelynck until 1998) for DIARAD, I. Ruedi (J.Romero and M. Anklin until 1996 and 1999, respectively) for PMO6-V and Ch. Wehrli for SPM). These Level-1 data are publically available at the SOHO archive and are still raw in the sense that they do not contain corrections for, e.g. degradation, which can be calculated only from a posteriori analysis.

In a next step corrections for exposure and non-exposure dependent changes have to be determined. In the case of TSI the former ones can be deduced for each radiometer individually and yields two level-1.8 time series, whereas the latter is only accessible by comparison of the two. A combination of the two yields then the final VIRGO total solar irradiance (TSI) as explained in Section VIRGO Radiometry below. For the SPM data a similar procedure for the correction of the degradation is not possible, mainly because these instruments have much larger overall changes and the results from the rarely exposed back-up SPM do not provide a reliable source for the assessment of the long-term changes. Nevertheless the time series can be detrended and allow to assess periods of up to 1 year; they are labelled as level-2. These data (level-1.8 and 2) are available from our ftp server as described below.

Description and Availability of Level-1 Data Products

The level-1 data are organized in daily fits files with the data in ascii extensions. The data and format are described in the corresponding headers. As it is a standard FITS format, the files can be read with the FITS library for the programming language you use. An example of the FITS headers of SPM, DIARAD, PMO6V and LOI files for 1 January 2003 can be retrieved as FITS_Headers.txt from our ftp server; and there is also a collection of IDL routines for reading the VIRGO daily FITS files. The SPM, PMO6V and LOI data are 1-minute samples (1440 values per day) and the DIARAD data 3-minute samples (480 values per day). As MDI and GOLF, VIRGO should have had its one-minute values centered at 00:00:00 TAI; but early in the mission the midnight pulse which is used by VIRGO for synchronzation of the timing chain was set to 00:00:00 UTC instead of TAI, which yields a delay of 32 seconds (the sum of leap seconds since start of TAI). Thus the 1-minute averages of SPM and LOI are centered at 32s after the TAI minute (1st measurment @ 00:00:32 TAI). The 20-second averages of PMO6V every minute are centered at 22s after the TAI minute (1st measurment @ 00:00:18 TAI) and the 10-second averages of DIARAD every 3 minute at 87 seconds after the start of a 3 minute period (1st measurment @ 00:00:87 TAI).

From the data files the following information is retrieved by the idl routines:

  • yymmdd_1.P00: 1440 values of PMO6V-A TSI in Wm-2 and a quality flag as 8-bit unsigned integer with the following meaning of the bit values: 64: cover closed; 32: SOHO off-pointing; 16: DIARAD calibration on (increases the noise of PMO6V-A, TSI values could be outliers), 8: period between reference values is more than the nominal 8 hours, values are less reliable, 4: smoothness condition failed, values are less reliable; 2: some temperatures from the HK packets are missing, values interpolated and essentially no problem for the data quality; 1: no distance correction. Contact: Wolfgang Finsterle
  • yymmdd_1.D01: 480 values of DIARAD-L TSI in Wm-2 . The quality flag is not read as it contains status information and not the quality, the quality is reflected is either good if a value of TSI is available or bad if the value returned is NaN. Contact: Steven Dewitte
  • yymmdd_1.S02: 1440 values of the red, green and blue channels in Wm-2nm-1 and one quality flag as 3-bit unsigned integer with the following meaning of the bit values: 4: no depointing correction could be applied (because of lack of the corresponding ancillary S/C data, this bit is always set, and the de-pointing angle set to zero); 2: no distance correction could be applied; 1: no temperature correction could be applied. Contact: Christoph Wehrli
  • yymmdd_1.L00: 1440 values of the 12 main pixels (microamp) with one quality flag, 4 guiding pixels (microamp) with one quality flag, and the EW and NS diameter in arcmin. The quality flag is given as ascii 'GOOD' or 'POOR'. The following figure identifies the numbering for the LOI pixels. Contact: Thierry Appourchaux

The level-1 data products are normally updated on a weekly basis and transferred from VDC to the SOHO archive, from where they can be downloaded.

Description and Availability of Level-1.8 and 2 Data Products

The level-1.8 and 2 TSI time series for VIRGO, PMO6V and DIARAD are available as hourly and daily values. For a detailed description of the evaluation see VIRGO Radiometry below. Note that since version 5_005 the date of the update is added to the version. Since version 6_000 the algorithms for the evaluation have changed for the PMO6V evaluation from level 1 to 1.8 and for the corrections of DIARAD level 1.8 to 2.0 (see below). The actual VIRGO TSI is shown in Figure 2.1 which is also part of the composite TSI.

Figure 2.1: VIRGO level-2 time series. (pdf-Figure )

The level-2 data of the SPM are also available as hourly and daily values. A section describing the evaluation will be added in future; in the mean time the paper presented at SOHO-11 may be used as reference. The actual time series for the red, green and blue channels are plotted on Figure 2.2 together with TSI detrended with the same polynomials.

Figure 2.2: Detrended SPM Level-2 data . ( pdf-Figure )

Description and Availability of Data Products for Helioseismic Studies

Detrended and cleaned 1-minute time series for helioseismic studies are available for the SPM red, green and blue channel, the PMO6V TSI and the 12 pixels of LOI as idl save files. The SPM and LOI time series are detrended with a triangular filter with a 2-day base. The PMO6A time series is corrected for degradation (with the corresponding correction table ) and gaps shorter than 8 hours are filled with ARIMA extrapolations from both sides. These time series span somewhat different periods and are updated sporadically.

3. VIRGO Radiometry and Determination of TSI

From raw measurements to a reliable record of Total Solar Irradiance (TSI)


The VIRGO Experiment has two types of radiometers to measure total solar irradiance (TSI): DIARAD and PMO6V. The former has been developed and built by IRMB, the latter by PMOD/WRC. For the VIRGO radiometers detailed descriptions of the principle and performance can be found in Fröhlich et al. (1995), Fröhlich et al. (1997). The analysis of the data in order to assess and characterize the long-term behaviour is described in Anklin et al. (1998), Fröhlich & Finsterle (2001), Crommelynck et al (2003), Fröhlich (2003). Most of these publications can be found as reprints or pre-prints on our anonymous ftp server.
The scientific objective of VIRGO related the total solar irradiance, as stated in the original proposal and in Fröhlich et al. (1995), is to provide a reliable time series of TSI, which takes full advantage of the two types of radiometers. This final product was termed data level-2 and includes corrections for exposure dependent changes which can be determined for each type of radiometer separately - yielding data level-1.8 - and corrections for exposure independent changes which can only be determined from a comparison of the two level-1.8 time series. Obviously, the final objective is to provide a time series of TSI which is as close as possible representing the output of the Sun and does not depend on the radiometer used.

From Level-0 to Level-1

The raw data (level-0) as transmitted from the spacecraft to ground are transformed into physical units using the radiometric factor which is determined for each individual radiometer by characterization experiments in the laboratory. Furthermore corrections for all a-priory known effects, are applied such as temperature, distance and radial velocity to the Sun. These data are labelled level-1. The algorithms for the calculation of level-1 data were developed by the corresponding institutes: by IRMB for DIARAD and by PMOD/WRC for the PMO6V radiometers. These algorithms are described in Fröhlich et al. (1997).
From Fig.3.1 it is obvious that these time series do not show the variations of the solar irradiance alone, but also changes of the radiometric sensitivity which are quite different for the different types radiometers. It is also obvious that DIARAD shows less sensitivity changes than PMO6V. It is interesting to note that for the latter a linear fit to the decrease of sensitivity, normally termed as degradation, corresponds almost exactly to the one observed during the EURECA mission. In contrast to this behaviour DIARAD, which flew also on EURECA shows on SOHO much less change than then.

Figure 3.1: Level 1 data of the VIRGO radiometer: DIARAD-L, DIARAD-R, PMO6V-A and PMO6V-B. (pdf-Figure )

From Level-1 to Level-1.8

It is normal practice to use comparison of radiometers of the same type - but with different exposure times - to determine exposure dependent changes. The incorporation of these changes as corrections leads then to level-1.8 for DIARAD and PMO6V separately.
In general the exposure time for the rarely used instruments is much lower than for the operational ones so that only a very small , and possibly negligible sensitivity change is observed for the former ones. In the case of DIARAD the exposure times of the L and R channels have reached until end of 2003 1382 and 1.9, and for PMO6V-A and B 2810 and 17.2 days. Thus for DIARAD-R the exposure time is low enough to assume no change. For the PMO6V radiometers, however, this is not the case because of the more frequent exposure of PMO6V-B during the first 190 days, the test phase for a new operational procedure which was needed due to a failure of the electronic circuit of the shutters of the PMO6V and the necessary change to use the covers instead, but only every 8 hours (instead of the 1-minute-open-1-minute-closed phases). During this period PMO6V-B was exposed every 8 h around the closure of PMO6V-A, which led to an exposure of 10.7 days. After that time PMO6V-B was exposed once per week.
In order to understand the long-term behaviour of the radiometers a model for such changes has been developed, which is based on hyperbolic functions taking the effective dose of radiation received by each radiometer into account (see e.g. Fröhlich, 2003 and references therein). From Fig. 3.1 it is quite obvious that the PMO6V radiometer show two different effects: an early increase of the sensitivity and a long-term degradation. The latter is similar to what is observed for all radiometers with specular paints with a rate of the order of 2 ppm per exposure day (e.g. HF, ACRIMs, ERBS; PMO6 on SOVA) . A recent re-investigation of the effect of the heating of the primary aperture by absorption of the sun light which illuminates the aperture through the larger view-limiting aperture shows definitively hat this effect was under-estimated in the characterization of the PMO6 radiometers. The results suggest that the early increase is related to a change of the absorptance of the primary aperture and as a consequence a change of its temperature which in turn radiates more power into the cavity and is recorded as an increase of irradiance. The early behaviour of PMO6V was always characterized by a rapid increase and a so-called short-term decrease of the sensitivity which is now interpreted as the result of a rapid darkening and a slower bleaching of the aperture surface. This interpretation is confirmed by inspection of the apertures of the SOVA radiometers which were in space during the EURECA mission and brought back afterwards: the aperture of the less-exposed back-up instrument is dark and the operational one is bleached., but still darker than the part which was not illuminated. This seems to be specific to the stainless steel used for the apertures as other radiometer show this effect also as e.g. HF, ERBS, and possibly the ACRIMs. DIARAD does not show an early increase, but its apertures are highly polished and electrolytically coated with nickel which seem to prevent this effect. The analysis of the behaviour of the PMO6V is further complicated by the fact that the early increase influences also the data of PMO6V-B as shown in Fig.3.1. Ideally the same corrections should be applied for both radiometers A and B with the only difference being their exposure time. Indeed, the re-analysis of the PMO6V shows that this can be done and the result are internally consistent corrections which are based on the constants of the early increase determined for PMO6V-A by comparison with DIARAD and on an adjustment of the short-term degradation parameters in order to eliminate the non-exposure correction of PMO6V-A of the earlier versions. The result is that both radiometers are corrected within a couple percent with the same parameters, but the corresponding dose. This confirms earlier statements that the non-exposure corrections of PMO6V-A were reminiscent of an incomplete correction of the early increase behaviour. The ratio of the individually corrected PMO6V is then fitted with a hyperbolic function for the period before and after the SOHO vacations and a possible slip over the gap is introduced. The results of this analysis is shown in Fig.3.2. For DIARAD this is sufficient; for the PMO6V radiometers the two functions are supplemented by a short term decrease at the beginning of the mission.

Figure 3.2: The top panel shows the deviation of the ratio of PMO6V-A/B from a linear trend of about 2 ppm/day. The middle panel shows the deviation after applying the corrections for the early increase and short-term decrease and the long-term degradation which are shown in the bottom panel. (pdf-Figure )

The same type of analysis is applied for DIARAD with the assumption that DIARAD-R is exposed so little that no exposure-dependent changes can be assumed. Already from Fig.3.1 it is clear that DIARAD does not show an important degradation as e.g. a similar instrument during the EURECA mission. Nevertheless a similar approach to fit hyperbolic function was applied, but the results do not help to explain the behaviour and a straightforward determination of the correction from the ratio DIARAD-L/R without fitting functions would obviously yield the same result. The results are shown in Fig.3.3 and the process fitting does not influence the determined corrections and the final level-1.8 result. The level-1.8 presented here for DIARAD were essentially identical to the ones presented by the IRMB group before they started to ignore the DIARAD-R data from the start after the SOHO vacations up to the end of 2000 and assumed no change during this period (see Crommelynck et al. (2004) and the DIARAD page).

Figure 3.3: The top panel shows the ratio of DIARAD-L/R, the middle panel the deviation after applying the final corrections which are shown in the bottom panel. The red points in the top panel are discarded as outliers for the further evaluation. (pdf-Figure)

Time series of DIARAD and PMO6V at level-1.8 as described here are included in the final VIRGO hourly and daily data sets (columns 4 and 5) .

From Level-1.8 to Level-2

Up to this point the procedure to determine the corrections for the-long term behaviour is clear, although not necessarily straightforward. Now we are faced with the problem of deriving a best estimate for TSI from the two time-series which are obviously different. The ratio A/L of the level-1.8 data are shown in the top panel of Fig.3.4 and covers a range of more than 600ppm which is most likely incompatible with the uncertainty of the corrections derived to get level-1.8. Thus, this is a strong indication that there must be other effects influencing the radiometry than exposure dependent changes. It is the first time that we have such direct evidence, thanks to (i) the fact that VIRGO comprises two type of radiometers which measure side-by-side the same Sun and (ii) the very quiet thermal environment we have on SOHO. There is obviously no way that metrological arguments can resolve this issue, all arguments have been exhausted in preparing level-1.8; nor can we leave a user of TSI with the decision which of both time series to use, if the experts are unable to propose a solution. On the other hand, this is obviously a great opportunity for the advancement of room-temperature radiometry and is a real challenge.

An easy solution would be to take the average of both time-series, but from Fig.3.4 it is obvious that there are significant changes of this ratio with time. The magenta points show the ratio as observed, and the blue ones when DIARAD is corrected for an effect after a switch-off /on of the experiment. This effect was first observed after an accidental switch-off of VIRGO in September 1996 and can be unambiguously attributed to DIARAD. This behaviour of DIARAD was 'confirmed' in February 2002 when all instruments on SoHO were switched off due to a power re-configuration of SoHO. This effect is not understood, but a correction can be applied by fitting a exponential function which turns out to be have a time constant of 64 days and an amplitude of 0.21 Wm-2 for these two interruptions. If this a typical behaviour of DIARAD we may assume a similar change after the SOHO vacations and after the first switch-on. For the former we assume the same time constant and determine the amplitude by fitting; the result is - as expected - higher with 0.39 Wm-2. The remaining ratio (blue diamonds in Fig. 3.4.) can be fitted by an exponential function (time constant of 1005 days and an amplitude of 0.80 Wm-2) with a jump over the SOHO gap of 0.38 Wm-2. This may be due to an increase of DIARAD or a decrease of PMO6V. The decision cannot be taken from internal arguments, we need some guidelines from other sources. There are two possibilities: comparison with measurement by ACRIM and ERBE and comparison with empirical models. Before the availability of the version-2 ACRIM II data in spring 2002, both ACRIM II and ERBE data sets were too noisy to reach a sound conclusion. The improved version-2 ACRIM-II data show a more or less constant TSI over the period of two years (1996-1997) of solar minimum. The same is indicated by the empirical models based on the influence of sunspots, faculae and magnetic network. As the PMO6V level-1.8 data show the same behaviour it is concluded that DIARAD increased during that period. Thus, the determined exponential function is used to correct for the behaviour of DIARAD. The result is shown in the middle panel of Fig.3.4. The remaining corrections have to be distributed among PMO6V and DIARAD. This is done by using the absolute deviation of the ratios to ACRIM II filteres with a boxcar running mean of 365 days. If one of the ratios exceeds a certain limit, that radiometer gets the correction, if both are below the limit the correction is equally distributed. It is interesting to note that the larger deviations are mainly due to PMO6V (probably due to the way its operated and due to the uncertainty in the corrections needed for this operation). Until version 5_005 the corrections after the vacations were limited to the exponential increase of DIARAD which is important during the first 2 to 3 years. After that period the comparison PMO6V/DIARAD began to show a slight decrease, which was for version 5_006 attributed to DIARAD as a linear trend with -0.068 ppm/day after mid 2000. The present corrections with the short-term switch-off/on corrections and long-term exponential function over the full observational period explains also this trend, so that no extra treatment is needed. This also supports the new approach as a whole

This yields to the final TSI record. The difference of the corrected DIARAD and PMO6V is now essentially constant in time and corresponds to the difference in absolute values. Although the noise of PMO6V is slightly higher than the one of DIARAD the plain average of the corrected values is presented as the final VIRGO hourly and daily TSI. The increasing sensitivity of DIARAD for short-term variations during the first 3 years (see Fröhlich and Finsterle, 2001) is not corrected because it may not be significant relative to the higher noise of PMO6V.

Figure 3.4: The top panel shows the ratio of PMO6V-A/DIARAD-L, the middle panel the deviation after applying the final corrections which are shown in the bottom panel. (pdf-Figure)

Figure 3.5: DIARAD, PMO6V,. and VIRGO level-1.8 and level-2 time series. (
pdf-Figure )

If you want to use the 1-minute data of PMO6V or the 3-minute data for DIARAD, you can fetch them from the SOHO archive and then you can use the correction table to get a level-2 product .

5. References

Anklin, M., Fröhlich, C., Finsterle, W. Crommelynck, D. A. and Dewitte, S.; 1998, Assessment of Degradation of VIRGO Radiometers Onboard SOHO, Metrologia 35, pp. 685-688, 1999.

Crommelinck, D., Dewitte, S. and Chevalier, A.: 2004, Total Solar Irradiance from VIRGO on SOHO: 5 Years of Operation of DIARAD, submitted to JGR

Fröhlich, C., Romero, J., Roth, H., Wehrli, C., Andersen, B. N., Appourchaux, T., Domingo, V., Telljohann, U., Berthomieu, B., Delache, P., Provost, J., Toutain, T., Crommelynck, D., Chevalier, A., Fichot, A., Däppen, W., Gough, D. O., Hoeksema, T., Jiménez, Gómez, M., Herreros, J., Roca-Cortés, T., Jones, A. R., Pap, J.and Willson, R. C.: 1995, VIRGO: Experiment for Helioseismology and Solar Irradiance Monitoring, Sol. Phys. 162, 101-128.

Fröhlich, C., Crommelynck, D., Wehrli, C., Anklin, M., Dewitte, S., Fichot, A., Finsterle, W., Jiménez, A., Chevalier, A. and Roth, H. J.; 1997, In-Flight Performances of VIRGO Solar Irradiance Instruments on SOHO,  Sol. Phys. 175, 267-286.

Fröhlich, C. and Finsterle, W.: 2001, VIRGO Radiometry and Total Solar Irradiance 1996-2000 Revised, in: Recent Insights into the Physics of the Sun and Heliosphere: Highlights from SOHO and other Space Missions, Eds: P. Brekke, B. Fleck, J.B. Gurman, Astr.Soc.Pacific, IAU Symp. 203, 105-110

Fröhlich, C.: 2003, Long-Term Behaviour of Space Radiometers, Metrologia, 40, 60-65.

Responsible for this page is Claus Fröhlich ; last update of text: 19 January 2004

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