ISS-SOLAR: Total (TSI) and spectral (SSI) irradiance measurements

Advances in Space Research 37 (2006) 255–264 www.elsevier.com/locate/asr

ISS-SOLAR: Total (TSI) and spectral (SSI) irradiance measurements G. Schmidtke b

a,*

, C. Fro¨hlich b, G. Thuillier

c

a Fraunhofer-Institut fu¨r Physikalische Messtechnik, Heidenhofstrasse. 8, D-79110 Freiburg, Germany Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, CH-7260 Davos Dorf, Switzerland c Service dÕAeronomie du CNRS, Bp 3, F-91371Verrie`res le Buisson, France

Received 14 September 2004; received in revised form 22 December 2004; accepted 4 January 2005

Abstract The primary objective of the ISS-SOLAR mission on Columbus (to be launched in 2006) is the quasi-continuous measurement of the solar irradiance variability with highest possible accuracy. For this reason the total spectral range will be recorded simultaneously from 3000 to 17 nm by three sets of instruments: SOVIM is combining two types of absolute radiometers and three-channel filter radiometers. SOLSPEC is composed of three double monochromators using concave gratings, covering the wavelength range from 3000 to 180 nm. SOL-ACES has four grazing incidence planar grating spectrometers plus two three-signal ionization chambers (two signals from a two stage chamber plus a third signal from a silicon diode at the end of the chamber) with exchangeable band pass filters to determine the absolute fluxes from 220 to 17 nm repeatedly during the mission. For the TSI the relative standard uncertainty (RSU) to be achieved is of the order of 0.15% and for the SSI from 1% in the IR/Vis, 2% in the UV, 5% in the FUV up to 10% in the XUV spectral regions. The general requirements for the TSI and SSI measurements and their conceptual realization within this payload will be discussed with emphasis on instrumental realization and calibration aspects. Ó 2006 Published by Elsevier Ltd on behalf of COSPAR. Keywords: Total solar irradiance; Spectral solar irradiance; Variability of solar irradiance; Calibration methods; Instrumentation of the ISS-SOLAR payload and its calibration in the NIR–Vis–VUV–EUV–XUV spectral regions

1. Introduction In December 1997 three instruments, SOVIM (SOlar Variabilty Irradiance Monitor), SOLSPEC (SOLar SPECtrum) and SOL-ACES (SOLar Auto-Calibrating EUV/UV Spectrometers), have been selected by the ESA Microgravity and Space Station Utilisation Department following the evaluation of the ESA Announcement of Opportunity for Externally Mounted Payloads (SP-1201). The three complementary instruments shall be integrated into the payload SOLAR in early 2005 for testing and to be launched with the

*

Corresponding author. Tel.: +49 761 8857 176; fax: +49 761 8857 224. E-mail address: [email protected] (G. Schmidtke). 0273-1177/$30 Ó 2006 Published by Elsevier Ltd on behalf of COSPAR. doi:10.1016/j.asr.2005.01.009

Columbus Payload in October 2006 for integration on the International Space Station.

2. Scientific background Taking into account the broad wavelength range covered from 3000 to 17 nm by the three instruments, there are several aspects of scientific interest. 2.1. Terrestrial climatology A primary aspect is related to climatology of the terrestrial atmosphere: the quasi-continuous measurements of the solar irradiance such as those presented in Fig. 1, with highest possible accuracy will provide data to investigate the impact of the solar irradiance variability

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Fig. 1. Extraterrestrial solar spectrum (Thuillier et al., 2004) from SOLSTICE, SUSIM on board UARS; SUSIM*, SSBUV and SOLSPEC on board ATLAS; SOSP (identical to SOLSPEC) on board EURECA; and a rocket data from Woods et al. (1998).

on the EarthÕs climate to separate this natural contribution from the man-made sources influence. For this topic, spectral and total solar irradiance are needed. 2.2. Atmospheric physics In the wavelength range 300–450 nm solar photons are partially absorbed by ozone. At wavelengths 300– 100 nm most of the photons are absorbed in the terrestrial atmospheric altitude regime from ground up to the mesopause at about 80 km leading to photodissociation, fluorescence, photoionisation and other primary reactions. These reactions initiate a large number of secondary, tertiary, etc. reactions including photochemical and catalytic interactions with ozone, nitric oxides and many other gaseous and aerosol components. Again, the accurate knowledge of the spectral solar irradiance is one of the cornerstones in atmospheric physics for modeling the large number of these complex processes involved. The physics of the thermosphere and ionosphere (T/I) from 80 to 600 km altitude is primarily controlled by the solar EUV radiation in the spectral range from about 100 to 16 nm. The primary interactions of the EUV photons with the T/I species are photoionisation, dissociation, fluorescence and resonant scattering. Secondary reactions thermalise the high-energetic photoelectrons heating up the neutral and ionised particles and excite the ions and neutral particles generating airglow emissions. In the IR and Vis spectral regions the solar variability is estimated to 0.1% during a solar cycle. Therefore, the required accuracy of the measurements should be at least <0.05%, which is difficult to achieve with respect to long-term (minimum 1 year) stability of the calibra-

tion parameters. This is why internal calibration means have to be incorporated in the instrument. In the spectral region from 450 to 200 nm the longterm solar variability increases with decreasing wavelength from about 0.1% to 2%. Due to the highest solar variability from about 2% up to more than one order of magnitude and due to the highest uncertainties in the determination of the absolute solar fluxes in the spectral range from 200 to 17 nm, very specific calibration tools and procedures have been developed. Thus the determination of the time dependent individual instrumental calibration parameters is playing a key role in TSI and SSI measurements. For this reason instruments of the SOLAR payload and the applied calibration procedures are described in more detail. 2.3. Solar and solar-stellar physics The TSI variability is well established for the last three cycles with a long-term uncertainty of about 10 ppm/year, thanks to overlapping space missions with accurate radiometers (Fig. 2, see figure caption). However, a full understanding of the underlying physical mechanisms is still missing, and the continuation of the record is very important, especially due the fact that the most recent of the three solar cycles (23) looks quite different. The spectral irradiance variability is highest in EUV becoming less in the UV, Vis and IR (e.g., Fontenla et al., 2004) and the wavelength dependent variability is quite well established for short to medium-term time scales, but the long-term (solar cycle) changes need to be determined with higher accuracy.

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Fig. 2. Total (TSI) and Spectral Irradiance (SSI) traces from 1981 to 2002 The variability is increasing strongly from top to bottom traces (see ordinate scales) that are representing irradiances as generated primarily in the solar photosphere (first trace), in the lower chromosphere (second trace) up to the solar corona (fourth trace). Upper panel: Daily averaged values of the SunÕs total irradiance TSI from radiometers on different space platforms since November 1978: HF on Nimbus7, ACRIM I on SMM, ERBE on ERBS, ACRIM II on UARS, VIRGO on SOHO, and ACRIM III on ACRIM-Sat. Next panels show modelled spectral irradiances (daily average values) from the declining solar cycle 21 to the cycle 23. Variability is shown on a relative scale normalised to the solar minimum emission (Tobiska, 2004).

The detailed understanding of the spectral distribution of the radiation during changes of TSI is badly needed and it can only be achieved by more and better spectral and total solar irradiance measurements. Other topics such as the varying amplitude and length of the 11-year cycle remain to be explained. The Sun is a variable star that is also representing stars of similar size, age and composition. The investigation of the Sun leads to conclusions applicable to other stars, too.

3. SOLAR instruments The ISS SOLAR payload consists of three instruments to be placed on the External Payload Facility of the Columbus Laboratory (Fig. 3). The instruments complement each other with the following objectives:  SOVIM for absolute total irradiance measurements and for spectral irradiance measurements at three wavelengths;  SOLSPEC for absolute spectral irradiance measurements from 3000 to 180 nm;  SOL-ACES for absolute spectral irradiance measurements from 220 to 17 nm.

By these three instruments a wide spectral range from IR to EUV spectral regions is covered representing 99% of the total solar irradiance to be accurately measured simultaneously. The SOLAR mission is described in some more detail by Thuillier et al. (1999). The experiments are mounted on the two-axis Course Pointing Device (CPD). Taking into account the ISS orbit and the constraints given by the angular range accessible by the CPD, up to 20 min measuring time per orbit will be available. Due to seasonal effects of the low-inclination ISS orbit and due to operational constraints only about 2350 useful orbits per year are anticipated (about 40% of all possible). An important amount may be lost due to ISS maintenance, docking and orbit raises. Thus, a total Sun observational time between a minimum of 450 and a maximum of 790 h per year can be used for SOLAR observations. This is only about half of what was promised at the time of the announcement of opportunity and it may well jeopardise the achievement of some of the science objectives announced in the proposals. SOVIM, SOLSPEC and SOL-ACES are under the responsibility of Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center (Switzerland), Service dÕAe´ronomie du CNRS (France) and

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Fig. 3. The External Payload Facility with the three SOLAR instruments (upper part) and the Columbus laboratory (below).

Fraunhofer Institut fu¨r Physikalische Messtechnik (Germany), respectively. 3.1. SOVIM An overall view of the SOVIM instrument is shown in Fig. 4(a). Two types of radiometers, PMO6 and DIARAD, will enable a consistent determination of possible changes (e.g., degradation) during the mission similar to the philosophy adopted for VIRGO on SOHO (Fro¨hlich et al., 1997). Moreover, a third PMO6 radiometer is added which had been flown on EUropean REtrieval CArrier platform (EURECA) and will be used for comparison of possible long-term changes since then. The PMO6 are improved versions of the VIRGO type. Two of them are included in the payload for the determination of exposure dependent changes by comparison of the operational and the less exposed (backup) radi-

ometers. The same applies for DIARAD which has two channels. They can be operated sequentially for solar measurements. Furthermore, SOVIM includes sunphotometers (SPM) and a solar attitude sensor TASS with a resolution of <100 in x and y directions. The latter is to monitor the solar pointing which is specified to be only within ±1°. This information is needed to correct the instrument signals. The SPM are filter radiometers with three channels of 5 nm bandwidth centered at 402, 500 and 862 nm using interference filters and silicon detectors. The filters and detectors are heated to a few degrees above ambient temperature to reduce condensation of gaseous contaminants on the optical surfaces. An additional radiation-hard glass window protects the filters from UV radiation below 380 nm. The optical compartment, sealed by o-rings and a cover, are permanently flushed with pure nitrogen during

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Fig. 4. (a) The SOVIM instrument. (b) Arrangement of the instruments and subunits within SOVIM.

integration and testing until the latest moment before launch. Moreover, all parts were vacuum baked before assembling in order to clean them to the highest possible degree. Again, two SPM are included one of which will be used operationally and the other one only from time to time. Although, this strategy did not really work for degradation control on VIRGO it will be used again on ISS where the environment is much harsher. Some more information can possibly be

gained. The arrangement of the instruments and subunits is shown in Fig. 4(b). The SOVIM data products are of level-1 and level-2. The former includes all a priori known corrections (Sun–Satellite distance to 1 AU, temperature and pointing corrections) and will be presented as individual measurements as taken. The data will comprise the time series of the PMO6, of DIARAD, of the SPM and of the TASS. Level-2 can only be evaluated after at least

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a half year of data recording as it needs careful determination of the exposure dependent degradation. These data will be presented as averages over the orbit measuring period. The following products will be available at level 2:  SOVIM TSI, a composite of the PMO6 and DIARAD time series, together with the two radiometer time-series each corrected for exposure dependent changes;  Time series of the three channels of the SPM. Note that the corrections to level 2 may be less reliable than in the case of TSI. All these data will be published as soon as they are available (level-1 within a week and level-2 after 6–12 months at the beginning and then about monthly). 3.2. SOLSPEC This instrument is designed to measure the solar spectral irradiance from 3000 to 180 nm (Fig. 5). The expected accuracy (relative standard uncertainty, RSU) will range from about 1% in the IR/Vis spectral regions up to 3% at 180 nm. These numbers are based on previous results obtained with the SOLSPEC instrument run

with the ATmospheric Laboratory for Applications and Science (ATLAS) and EURECA missions. The instrument for the ISS has a significant heritage from the one which flew with the previous missions. It incorparates three double monochromators using holographic gratings, dedicated to the three spectral domains, 3000–900, 950–350 and 370–180 nm, respectively. The original design has been kept but new optics and electronics are used. Optics contamination on ground and in orbit is a source of instrument ageing. First, the instrument will be outgased in vaccum. Afterward, to prevent material condensation on entrance optics, pure nitrogen flushing will be performed as long as possible on ground. In addition, the entrance optics will be kept a few degrees above the ambient temperature in orbit. Furthermore, the instrument uses several quartz plates to absorb the solar radiation below 170 nm. Being placed on a wheel, they will be used in flight at different rates, intercompared, and periodically calibrated by using the direct view of the Sun. Data processing includes several corrections due to pointing, instrument ageing, etc., which will enable a level-2 data delivery in a time frame of about a year. Data obtained during the ATLAS and EURECA missions have produced several spectra (Thuillier et al., 1997, 1998a,b, 2003, 2004) in UV, Vis and IR.

Fig. 5. Functional schematic of the SOLSPEC subsystems.

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They were assembled in a single spectrum. Adding EUV data from Woods et al. (1998) and from the UARS spectrometers, a spectrum from 0.5 to 2400 nm has been composed as it is shown in Fig. 1. The present payload has the capability to generate similar results extended up to 3000 nm and with a better accuracy given the improvements made on the instruments. 3.3. SOL-ACES Fig. 6(a) is a view on the SOL-ACES instrument with the optical apertures marked. For a better understanding of the system the schematics of the SOL-ACES subcomponents with four spectrometers and two ionisation chambers is shown in Fig. 6(b). These subsystems are being ÔconnectedÕ by a filter wheel with 48 filter locations. Gas reservoirs (also marked in Fig. 6(a)), supply systems and electronics add to the complexity of the instrument. The spectrometers are of the Bedo-Hinteregger type (Wienhold et al., 2000): The Sun is acting as the entrance slit with the nearly parallel incoming radiation diffracted by a planar grating. Depending on the angle of diffraction wavelengths are selected by a parabolic mirror with a channeltron detector for the photon-electrical pulse conversion. In the EUV spectral region these grazing incidence spectrometers provide an excellent data statis-

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tics between 105 and 108 cps depending on wavelength. However, in order not to exceed the limits of accumulated counts for the channeltrons when degradation is becoming a severe problem, optical attenuators have to be inserted. The ionisation chambers are of the double-ionisation chamber type with two ion-collectors of 250 mm length, each (providing two signals). At the end of the cell there is a large AXUV-576G silicon diode of IRD (International Radiation Detectors, Inc.) company in order to record the photons after passing the cell from the entrance as defined by one of the filters to the diode (third signal). Inter-comparison of the two signals from the ion-collectors with the diode current allows the separation of the current contribution by primary absorption processes from secondary effects, such as secondary generation of ion-electron pairs by photo-electron impact and/or the amount of absorption due to vibration–rotational excitation. At normal operation the four spectrometers will record solar EUV/UV emission spectra simultaneously. At the same time the two ionisation chambers are covered with filters, the first one with an aluminium filter and the second one with a magnesium filter to record wavelength ranges from 17 to 70 nm and from 100 to 127 nm with the silicon diodes at high-temporal resolution of 0.683 s. During these measurements the ionisation chambers will not be filled with absorbing gas.

Fig. 6. (a) The SOL-AECS instrument: S1–S4, apertures of the spectrometers S1–S4; IC1/2, ionisation chamber to be operated with S1 and S2; IC3/4, ionisation chamber to be operated with S3 and S4; GR, gas reservoirs (to be designed as structural elements, too). (b) Functional schematics of the SOL-AECS subsystems.

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Within the project SOLAR level-1 and level-2 data products will be generated in context with the data evaluation of the other two instruments.

4. Calibration 4.1. SOVIM The radiometers within SOVIM are fully characterised and thus individual radiometric references. The RSU of the SI realization is estimated to ±0.15%. Moreover, the PMO6 radiometers are compared to the World Radiometric Reference maintained at PMOD/WRC and are directly compared to a cryogenic radiometer at NPL (National Physical Laboratory) which is supposed to realise the SI unit to ±0.01%. The SPM are calibrated against lamps, which are traceable to NIST (National Institute of Standards and Technology). Furthermore they have spectral calibrations against trap detectors, which are calibrated against cryogenic radiometers. The RSU is estimated to ±0.2–1% depending on wavelength. The long-term behaviour as shown by the SPM on VIRGO/SOHO, however, does not allow maintaining this level of uncertainty over the mission. 4.2. SOLSPEC The instrument incorporates four tungsten ribbon lamps, two deuterium lamps for in flight photometric control and one hollow cathode lamp for in flight wavelength scale control. The instrument is calibrated with the use of the Heidelberg (G) blackbody in the RSU scale of the Physikalisch-Technische Bundesanstalt of Berlin (Germany) (Mandel et al., 1998). The temperature of the blackbody is adjusted as a function of the spectrometer to be calibrated. Temperature as high as 3050 K is used for UV while in the IR range, 2700 K are used. The use of high temperature generates significant difficulties in term of emission stability and duration of the blackbody cavity. Below 200 nm, the instrument uses calibrated D2 lamps. SOLSPEC will overlap with SOLACES in providing solar spectral irradiance below 220 nm (see Table 1).

Table 1 SOLSPEC spectral domains and their in-flight calibration means UV (nm)

Visible (nm)

IR (nm)

Spectral range Photometric control

180–370 2D2 sources

350–950 2 Tungsten ribbon lamps

900–3000 2 Tungsten ribbon lamps

Wavelength scale

1 Hollow cathode lamp

4.3. SOL-ACES Accurate measurements of the solar spectral irradiance require spectrophotometers that are radiometrically calibrated spectrometers. Since the efficiency g of spectrometers (g, number of counts per incoming EUV/UV photon) is a complex function of wavelength, temperature, time, the local area of the interaction of photons with optical surfaces, accumulated counts by the channeltrons and other parameters, g is changing in a non-predictable way during the mission in space. Therefore, in-flight calibration is an indispensable requirement to achieve the SOL-ACES scientific goals. The applied method is described in the following part. Ionisation chambers are primary detector standards (Samson and Haddard, 1974). By using thin film metal and interference filters as entrance aperture of the SOL-ACES ionisation chambers intervals of the order of 10 nm of the solar emission spectrum are selected. By increasing the absorbing gas pressure inside the chambers from zero to the order of mbars the incoming EUV/UV photons will be absorbed generating ion-electron pairs. Collecting the ions the measured current can directly be converted into photon fluxes. Since the transmission of the filters is also changing with time, it has to be measured for each calibration cycle. For this purpose solar emission spectra are to be recorded by four spectrometers in overlapping wavelength ranges (16–65, 25–99, 39–151 and 115–226 nm) at the same time (see Figs. 6(a) and (b)). After turning the filter wheel the transmissions of four filters will be determined simultaneously. Thereafter two of these filters will be placed at the entrance of both ionisation chambers selecting corresponding band passes of the solar radiation to be absorbed in the chambers. Thus, absolute calibration of the spectrometers will be performed repeatedly by a primary detector standard during the mission. In addition, for the total spectral range of 17– 220 nm AXUV silicon diodes are applied as a secondary detector standard. The diodes are calibrated at the BESSY electron synchrotron in Berlin. They will be recalibrated in-flight by the auto-calibration capability in the spectral region from 16 to 140 nm. A cross-calibration with the SOLSPEC instrument will also be carried out. The (auto-) calibration mode will be performed twice per week during the first four weeks in orbit. This sequence will be decreased to once per week thereafter and finally twice per month at the end of the mission. Modes for stray-light and background measurements, gas pressure and solar pointing accuracy determination as well as operational parameter tests will be performed on demand. More details are described in Schmidtke et al. (2005).

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4.4. Calibration strategy of the SOLAR mission The precision of radiometric measurements are about a factor of 10 greater than their accuracy. Consequently, only overlapping measurements can ensure accurate long-term monitoring. This strategy has been successfully applied since the beginning of accurate TSI observations in late 1978 (Fro¨hlich, 2003). Furthermore, two or more instruments observing at the same time allow to discriminate which one presents a particular behaviour due to a specific problem or ageing. But, this depends obviously on the availability of more than one simultaneous missions. In this context, SOLAR is a very important contribution to the long-term record of TSI and SSI. The UV measurements showed discrepancies of the order of 10% or more (wavelength dependent) in the 1970s. A similar strategy as for the radiometric TSI measurements was applied by the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM, Brueckner et al., 1993) and SOLar STellar Irradiance Comparison Experiment (SOLSTICE, Rottman et al., 1993), by the spectrometers operated on board the Upper Atmosphere Research Satellite (UARS), and by the three ATLAS missions which took place during the UARS period with the Shuttle Solar Backscatter UltraViolet (SSBUV, Cebula et al., 1996)), SUSIM* and SOLSPEC. The accuracy reached by the measurements made during these missions (Cebula et al., 1996; Woods et al., 1996) demonstrated the efficiency of this strategy which is illustrated by the resulting spectrum shown in Fig. 1. This strategy will be still applied in relation with the SORCE investigation (Woods et al., 2000) which carries out similar measurements employing instruments using different technology as compared to SOLAR. Because of the complex technology in EUV spectroscopy and due to the strongly changing efficiencies of the optical components discrepancies up to a factor of four have been observed just a few years ago (Solomon et al., 2001). In order to overcome the shortcomings of the past true in-flight calibration will be applied for the first time with the EUV/UV measurements by SOL-ACES.

5. Operations The launch of the SOLAR mission is planned in late 2006 with a nominal duration of 1.5 years. Scientific measurements will be performed during 16–20 min depending on the orbit conditions compatible with CPD (see top of Fig. 3). However, not all of the orbits can be used for SOLAR science operations because of special ISS activities and other limitations. SOVIM will monitor TSI and the three spectral channels whenever measurement orbits are available. About

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once per week at the beginning and once per month later-on the back-up instruments will be operated during one orbit for the internal determination of the exposure dependent degradation. SOLSPEC will record the solar irradiance spectrum and determination of the Mg II index. When the instrument is not Sun-oriented, it will run in calibration mode using internal lamps once a day. SOL-ACES will be able to record solar EUV/UV emission spectra minimum once per orbit. The (auto-) calibration mode will be performed twice per week during the first four weeks in orbit, and be repeated less according to the evolution of the instrument responsivity. Modes for straylight, background measurements, gas pressure and solar pointing accuracy determination, will be performed on request.

6. Summary The payload ISS-SOLAR is formed by a set of three instruments which will measure the total and spectral solar irradiance variability in the wavelength range from 3000 to 17 nm with highest possible accuracy. It will be an important contribution to the data sets and time series of the solar irradiance variation. These data will improve the understanding of the Sun-climate aspects as well as the thermospheric–ionospheric modeling and special T/I applications.

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