SOL–ACES: Auto-calibrating EUV/UV spectrometersfor measurements onboard the International Space Station

G. Schmidtke *, R. Brunner, D. Eberhard, B. Halford, U. Klocke, M. Knothe,W. Konz, W.-J. Riedel, H. Wolf

Fraunhofer-Institut fur Physikalische Messtechnik (IPM), Heidenhofstrasse 8, D-79110 Freiburg, Germany

Received 1 September 2004; received in revised form 11 January 2005; accepted 21 January 2005

Abstract

The SOL–ACES experiment is prepared to be flown with the ESA SOLAR payload to the International Space Station as plannedfor the Shuttle mission E1 (Columbus) in August 2006. Four grazing incidence spectrometers of planar geometry cover the wave-length range from 16 to 220 nm with a spectral resolution from 0.5 to 2.3 nm. These high-e!ciency spectrometers will be re-cali-brated by two three-signal ionization chambers to be operated with 42 band pass filters on routine during the mission.Re-measuring the filter transmissions with the spectrometers also allows a very accurate determination of the changing second order(optical) e!ciencies of the spectrometers as well as the stray light contributions to the spectral recording in di"erent wavelengthranges. In this context the primary requirements for measurements of low radiometric uncertainty are discussed in detail. Theabsorbing gases in the ionization chambers are neon, xenon and a mixture of 10% nitric oxide and 90% xenon. The laboratory mea-surements confirm very high count rates such that optical attenuators have to be applied. In addition, possible interfering contri-butions to the recorded data as generated by secondary e"ects can be determined to a high degree of accuracy by this method.Hence, very accurate irradiance measurements are expected in terms of relative standard uncertainties (RSU) ranging from 5%to 3% depending on the wavelength range.! 2006 Published by Elsevier Ltd on behalf of COSPAR.

Keywords: In-flight calibration of EUV spectrometers; Three-signal ionisation chamber

1. Introduction

The accuracy of space measurements determining so-lar, terrestrial, planetary or interplanetary EUV spectralirradiance variations is strongly impeded by inherentand inevitable e!ciency (counts per incoming photon)changes of the spectrometric devices used. The processeschanging the e!ciency of EUV instrumentation in spacedepend on wavelength, on time, on temperature, on theinteraction with EUV and particle radiations. In addi-tion di"usion processes in the upper layers of the optical

surfaces involved, contamination from the environmentand other specific parameters of components such asaccumulated counts of channeltrons are causing degra-dation of these components. The processes involvedare only understood qualitatively in some cases, there-fore there is no method available, yet to calculate the de-gree of e!ciency changes occurring in space.

In the laboratory, a method has been developed forre-calibrating EUV spectrometers in space repeatedlyon a time base su!cient to separate the relativelyshort-time irradiance variations from the longer-terme!ciency changes of the instruments. It will be appliedin space by the SOL–ACES (SOLAR Auto-CalibratingEUV/UV Spectrometers) experiment for the ISS ESASOLAR mission (Schmidtke et al., 2005; Thuillier

0273-1177/$30 ! 2006 Published by Elsevier Ltd on behalf of COSPAR.doi:10.1016/j.asr.2005.01.112

* Corresponding author. Tel.: +49 761 8857 176; fax: +49 761 8857224.

E-mail address: schmidtke@ipm.fraunhofer.de (G. Schmidtke).

www.elsevier.com/locate/asr

Advances in Space Research 37 (2006) 273–282

et al., 1999). The method and the SOL–ACES instru-mentation are described in detail.

The EUV/UV irradiance aspects including the model-ling and the thermospheric and ionospheric topics areintegrated into the TIGER (Thermospheric/IonosphericGEospheric Research) program. The latter was startedin 1998 at the 1st TIGER Symposium held in Frei-burg/Germany.

2. Scientific goals of SOL–ACES

In order to work on the scientific topics theSOL–ACES team is supported by the SOVIM (SOlarVariabilty Irradiance Monitor) and SOLSPEC (SOLarSPECtrum) teams. SOL–ACES Co-Investigators are alsoR. Hammer and R. Schleicher, KIS Freiburg/Germany,J. Hildebrandt, AIP Potsdam/Germany, Ch. Jacobi,University of Leipzig/Germany, N. Jakowski, DLR/DFD Neustrelitz/Germany and K. Tobiska, SET PacificPalisades CA/USA.

SOL–ACES is one of the three experiments of theSOLAR payload. The other ones are SOVIM andSOLSPEC. This set of experiments is adding to eachother with respect to the primary objectives:SOVIM – measurement of the absolute totalirradiance.SOLSPEC – measurement of absolute spectral irradi-ance from 3000 to 180 nm.SOL–ACES – measurement of the absolute spectralirradiance from 220 to 16 nm.

By the SOLAR payload, a wide spectral range fromIR to EUV spectral regions will be covered representing99% of the total solar irradiance to be accurately mea-sured at the same time. More details are described byThuillier et al. (1999, 2005) and Schmidtke et al. (2005).

In the field of terrestrial climatology the most impor-tant scientific goal of the SOLAR mission is the quasi-continuous measurement of the solar irradiance withhighest possible accuracy. The irradiance data will beprovided for the investigation of the impact of the solarirradiance variability on the Earth!s climate changes.

In atmospheric physics more accurate solar spectralirradiance data will support the further modeling ofthe altitude regions from ground to the exosphere withthe numerous physical and chemical processes involved.

In addition to the climatic aspects SOL–ACES datawill contribute to the:

determination and modelling of the solar EUV/UVspectral irradiance;derivation of EUV/UV indices;semi-empirical modelling of active regions of the sun;modelling of dark spot/enhanced plage emissions;

investigation of solar-terrestrial relations (in the earthaltitude region from 80 to 1000 km);investigation of stellar-stellar connections;aspects of space weather (impacts on satellite commu-nication & navigation);improvement of EUV/UV space instrumentation andits calibration.

3. Requirements to design optical EUV/UV systems forsolar flux measurements

Summarising the today!s state of the development ofT/I (thermosphere/ionosphere) models and of the appli-cations of EUV/UV solar fluxes in navigation, commu-nication and space-related radar measurements thefollowing requirements are derived:

3.1. The most important solar emission lines shall beresolved

There are several groups of strong and medium-strong emission lines exceeding fluxes of 108 pho-tons cm!2 s!1. To separate these lines from each othera spectral resolution of <1 nm is required.

3.2. The relative standard uncertainty (RSU) of themeasurements shall be below the 10% level

The statistical uncertainty of the data shall be of theorder of some per cent only. Since in most cases photondetectors are used, they shall provide count rates of theorder of 104 cps for medium intensity solar emissions.

3.3. The solar variability shall be measured on a daily base

The primary interest in T/I aeronomy is to improvethe existing thermospheric and ionospheric models min-imum on a daily base. If the solar fluxes and their vari-ability would be accurately known at a RSU level of10% every day, it would mean real progress in this fieldof research.

Three more requirements have to be added from atechnical point of view: The optical surfaces do notshow uniform e!ciencies in the EUV/UV spectral re-gion on one side and their e!ciencies may changestrongly during the mission on the other side.

3.4. The radiation shall interact with the same surfacearea at all wavelengths

Optical surfaces seem to change their optical proper-ties in a more similar way with increasing size. Conse-quently it is more di!cult to trace smaller surfacessuch as pixels in channel plates, because each of the pix-

274 G. Schmidtke et al. / Advances in Space Research 37 (2006) 273–282

els undergoes individual e!ciency changes in space thatcannot be determined in detail. For example, parameterslike accumulated counts and "burned! points cannot betraced for each pixel. However, for accurate measure-ments the actual e!ciency has to be known at each indi-vidual wavelength (pixel) of the spectrum for eachperiod of the measurement. On larger surfaces strong lo-cal variations can be averaged.

3.5. For broad wavelength ranges below 50 nm grazingincidence optics has to be applied

Optical components in grazing incidence (reflectancegratings, mirrors, channeltrons) show best performanceswith respect to their e!ciency. In the spectral range<50 nm there is no alternative for using these devicesin grazing incidence for broad wavelength ranges.

3.6. Second order and stray light contributions to thespectra shall be determined in space

Second order and stray light contributions increasethe uncertainty of the derived solar fluxes. These contri-butions do change with solar activity as well as withwavelength-dependent e!ciency changes of the instru-ment. The repeated measurement and determination ofthese contributions shall be part of the operation inspace and of the data evaluation on ground.

The requirements Sections 3.1–3.6 are met by SOL–ACES as is shown in the following sections.

4. SOL–ACES instrument description

Four spectrometers (S1, S2, S3 and S4) and two ioni-sation chambers (IC1/2 and IC3/4) are the active instru-ments of SOL–ACES.

The spectrometer type is described in more detail byHinteregger et al. (1973), Schmidtke et al. (1974) andWienhold et al. (2000): The sun is acting as entrance slitinto a planar grating spectrometer (Fig. 1). Taking intoaccount the length of the ruled lines and the uncertaintyof the solar pointing as well as the angle of incidence(83"; see requirement in paragraph Section 3.5) on theplanar grating an aperture of 3.6 mm width times8 mm length is chosen. The solar radiation enteringthe spectrometer via this aperture is di"racted by thegrating and collected by a parabolic mirror. In its focalarea as defined by the exit slit radiation is selected onlyfrom one angle of di"raction. By rotating the subassem-bly consisting of a parabolic mirror and a channeltronspectra are scanned. The wavelength scale will be pro-vided by the known position of spectral lines.

Double ionisation chambers providing two currentsignals are investigated by Samson (1967) and Samsonand Haddad (1974). To this arrangement an AXUV-

576G silicon detector of IRD (International RadiationDetectors Inc.) is added at the end of the chamber (seeFig. 2) generating the third current tracing during thecalibration measurement. The data evaluation of thesemeasurements is described in Section 5.

The geometrical arrangement of SOL–ACES isshown in Fig. 3. The filter wheel is providing a total of48 apertures on two circles with 24 locations, each ontop of the instrument. All apertures are of identical geo-metrical size of 3.6 · 8 mm. Four of the apertures areopen (for the spectrometers), two of them are closed(for back ground recording) and 42 of them are formedby EUV/UV filters of 10–20 nm band pass covering thespectral range from 16 to 220 nm.

The arrangement of the optical subsystems is suchthat the longer wavelength ranges are with the outer cir-cle of the filter wheel at the filter positions A1, . . . ,A24(S3, S4 and IC3/4: see Figs. 4 and 5). The shorter wave-length ranges are at the inner circle with I1, . . . ,I24 (S1,S2 and IC1/2).

Using the four open apertures for the spectrometers(see Fig. 4) solar emission spectra are recorded simulta-neously in the overlapping wavelength ranges of 16–

Entrance Aperture Pointing Deviation

Solar Disk ExtensionDiffraction

Grating(fixed)

Parabolic Mirror

Exit Slit

ChannelElectron

Multiplier

Center ofRotation

RotatingSubassemly

Solar Radiation

Fig. 1. Scheme of the SOL–ACES spectrometer (the parabolic mirrorand the channeltron are fixed on a platform with the rotating point atthe center of the ruled area of the planar grating).

G. Schmidtke et al. / Advances in Space Research 37 (2006) 273–282 275

65 nm (S1), 25–99 nm (S2), 39–151 nm (S3) and 115–226 nm (S4) in 500 steps of 0.683 s, each (Fig. 5). Atthe same time the two ionisation chambers are coveredby an aluminium (IC1/2) and by a magnesium filter(IC3/4), respectively (see also Fig. 6). Important: In thisgeometry the solar radiation is hitting the same opticalsurface areas in the spectrometers for all of the scannedwavelengths (see requirement in Section 3.4).

Since the ionisation chambers are not filled with gasduring spectrometer recordings, the silicon diode will de-tect solar EUV radiation in the band passes of alumin-ium and magnesium, respectively, at the same time. Bythese broad band measurements data are recorded at0.683 s temporal resolution.

The ionisation chambers are supplied by gas controlsystems with the gas reservoirs also designed as struc-tural elements. This design is adding to the complexityof the SOL–ACES system that includes the control elec-tronics and data handling devices.

Fig. 2. Scheme of the three-signal ionisation chamber: "1st chamber! isproviding the "front! signal, "2nd chamber! is providing the "rear! signal,"Photodetector! (AXUV diode) is providing "photo! signal.

Fig. 3. SOL–ACES: Geometrical arrangement of the subsystems (see also Fig. 4): The primary optical subsystems are four spectrometers S1, S2, S3and S4 and two ionisation chambers 1/2 and 3/4.

Spectrometers S3 / S1 Ionisation Chamber 3/4

Spectrometers S2 / S4

48 Filter Positions

Ionisation Chamber 1/2

Fig. 4. Top view on the filter wheel: Filters are located on two circles(the inner and the outer one). There are 24 inner filter positions(marked I1–I24) and 24 outer positions (A1–A24). Below the filterwheel spectrometers S1 and S2 and the ionisation chamber 1/2 areallocated to the inner circle while S3 and S4 and the ionisationchamber 3/4 are operated with outer filter positions (see also Fig. 3).

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Fig. 5. Optical parameters of the SOL–ACES spectrometers and the wavelength coverage (grating constants in lines/mm, spectral resolution Dk innm; spectrometer S1 is equipped with the grating 3500 lines/mm, S2 with 2300 lines/mm, S4 with 1000 lines/mm, etc.).

Fig. 6. (a) Scheme of the spectrometer – filter wheel – ionisation chamber arrangement. (b)–(c) Schematic sequence of the Calibration Mode.

G. Schmidtke et al. / Advances in Space Research 37 (2006) 273–282 277

In Fig. 5, the spectral resolution is also quantifiedmeeting the requirement in Section 3.1.

5. Calibration method and laboratory results

Self-calibration is applied in the EUV spectral regionfrom 16 to 145 nm by using neon, xenon and nitric oxideas absorbers in the ionisation chamber, which is a pri-mary detection standard.

First step (solar spectrum recording): The filter wheelis turned into the standard position with open aperturesat the spectrometers (Fig. 6(a)). Then the wavelengthrange from 16 to 220 nm will be recorded by the fourspectrometers. In order to exclude measurements duringsolar flare activity short-term variability will be tracedby the AXUV diodes of the ionisation chambers witha temporal resolution of 0.683 s. The latter measure-ments will be taken into account for the data evaluation.

Second step (solar spectrum recording with filter):After another turning of the filter wheel four filters areplaced at the spectrometer entrances. Recording the so-lar spectrum with the given spectral resolution the filtertransmission will be determined. At the same time thesolar radiation in the band passes of two other filtersis recorded by the AXUV diodes (Fig. 6(b)). Second or-der and stray light contributions are determined at thesame spectral scan (meeting requirement 3.6). Evalua-tion of the measurements in Step 1 and Step 2 will pro-vide the actual filter transmission of four filters for eachrecording. One scan will take about 5 min.

Third step (ionisation chamber measurement): Afterturning the filter wheel once more two filters are coveringand sealing the ionisation chambers at the same time(Fig. 6(c)). Gas will be released from the gas reservoir intwo steps within 180 s, using a low flow valve at low pres-sures (0–10!2 mbar) and a high flow valve at higher pres-sures (10!2–1 mbar or more). During this time threecurrents are recorded for each of the two ionisation cham-bers of 500 mm optical path (spectrometers are o").

Fourth step (gas release): The filter wheel will belifted to release the absorbing gas into space (Fig. 6(d)).

Whenever calibration will be performed, this cycle(Steps 1–4) will be operated at the end of the measuringperiod of an ISS-SOLAR orbit providing in flight cali-bration for a spectral interval of 10–20 nm at longer(outer circle) as well as at shorter wavelengths (inner cir-cle). Repeating these cycles up to twelve times with dif-ferent filters (see Table 2) the total wavelength range ofSOL–ACES will be covered.

A laboratory EUV emission spectrum of helium isshown in Fig. 7 with the dominant HeI and HeII reso-nance lines in di"erent optical orders (the blaze of the1000 groves/mm grating is acting at the center of thewavelength coverage). The laboratory spectrometer isof identical geometry as the SOL–ACES spectrometers.

This helium emission spectrum as selected by an alu-minium filter is absorbed by xenon in the ionisationchamber with the current traces given in Fig. 8.

The EUV hollow cathode radiation source (OmicronHIS 13) can also be operated in nitrogen generating amultiple of atomic line and molecular band emissions.Using an indium filter the spectral range from "90 to100 nm is selected. The corresponding current tracesare given in Fig. 9.

In these cases, one EUV photon is producing one ion-electron pair by absorption.

The data evaluation of these measurements is startingwith the equation representing the current as generatedby photon absorption in the 1st stage of the ionisationchamber (see Fig. 2 and trace "front! in Figs. 8 and 9)

i1 # ge $ e $ Io $ %1! exp%!n $ r $ L1&&; %1&

where i1,2, current generated in the 1st and 2nd stage ofthe ionisation chamber, respectively; ge, number of elec-trons generated by one photon absorbed in a gas; e, elec-tron charge; Io, solar photon flux entering the ionisationchamber; n, number of absorbing gas particles per cm3;

0 20 40 60 80 100 120 140 160 180 200 2200.0

2.0

4.0

6.0

8.0

1.0

1.2

1.4

1.6

1.8

2.0

Spectrum He

coun

trat

e / c

psx

106

wavelength / nm

Fig. 7. Laboratory EUV emission spectrum of helium: The dominantlines are HeI 58.4 nm first, second and third orders. HeII 30.4 nm iseven found in four orders.

He-Xe-Al-040319-01.dat

0

500

1000

1500

2000

2500

3000

0 1 2 3pressure / mbar

i / p

A rear frontPhoto

Fig. 8. Current traces in the ionisation chamber of a helium emissionspectrum as selected by an aluminium filter and absorbed by xenon.

278 G. Schmidtke et al. / Advances in Space Research 37 (2006) 273–282

r, photon absorption absorption cross section of thegas; L1,2, length of the optical absorption path (L1 =L2 = 250 mm.

For total absorption of the photons in the 1st ionisa-tion chamber with n * r * L ' 1 a first approximation is

i1 ( ge $ e $ Io. %2&

In inert gases such as neon and xenon ge = 1 for most ofthe energy range of the solar photons for the givengeometry and electrical field distribution in the specialionisation chamber. For nitric oxide ge ranges fromabout 0.6 to nearly 1 in the wavelength range of interest.

As given in Figs. 8 and 9, the maximum readings ofthe current (see trace "front!) can be used to determinethe first approximation of the incoming photon fluxes.With this value Io equation (1) can be computed and

compared with the measurement for the relevant pres-sure interval. In addition the current reading for the2nd ionisation chamber (see trace "rear! in Figs. 8 and 9)

i2 # i1 $ exp%!n $ r $ L&; %3&

as well as the current generated in the silicon diode atthe end of the absorbing path of the ionisation chamber(trace "Photo! in Figs. 8 and 9)

i3 # gSi $ e $ Io $ exp%!n $ r $ 2 $ L&; %4&

where i3, current generated in the silicon diode; gSi, num-ber of electrons generated by one photon in the silicondiode will be compared with computed data sets.

Simulating these traces by computation is a very sen-sitive method to determine the incoming photon fluxeswith an uncertainty in the low percentage range. Thismethod has been applied to measurements at theBESSY (Berliner Elektronen-Synchrotron) electron syn-chrotron providing agreements of the measured andcomputed traces to <1% accuracy.

After collecting and evaluating the measurements forthe wavelength sub-ranges (see also Tables 1–4) the e!-ciency is derived for the four spectrometers. With thesedata sets a final check on the derived solar photon fluxeswill also provide a measure of the inherent datauncertainty.

This way absolute calibration of the spectrometerswill be performed repeatedly during the mission. Theionisation chambers will be filled from zero pressure tototal EUV absorption pressure during each of the cali-bration measurements.

N2-NO-In-040325-01.dat

-100

0

100

200

300

400

500

600

700

0 0.1 0.2 0.3 0.4 0.5

pressure / mbar

i / p

A

rearfrontPhoto

Fig. 9. Current traces in the ionisation chamber of a nitrogen emissionspectrum as selected by an indium filter and absorbed by xenon.

Table 1Expected count rates (CR) of the spectrometers (without optical attenuation)

k (nm) Solar flux (cm!2 s!1) eGi eSpi eCh eCsI Open channeltron CR (s!1) CsI channeltron CR (s!1)

220 5 · 1012 0.1 0.7 0.0006 6 · 107

200 8 · 1011 0.1 0.7 0.005 8 · 107

180 2 · 1011 0.1 0.7 0.02 8 · 107

160 3 · 1010 0.1 0.7 0.05 4 · 107

140 2 · 1010 0.08 0.7 0.003 0.08 1 · 107 3 · 107

121 3 · 1011 0.07 0.6 0.03 0.07 1 · 108 2 · 108

110 2 · 1009 0.07 0.6 0.08 3 · 106

100 2 · 1009 0.06 0.6 0.1 3 · 106

98 7 · 1009 0.05 0.5 0.1 6 · 106

90 2 · 1009 0.05 0.5 0.1 1 · 106

80 1 · 1009 0.05 0.5 0.1 8 · 105

70 4 · 1008 0.05 0.5 0.1 4 · 105

60 1 · 1009 0.05 0.5 0.1 8 · 105

50 5 · 1008 0.04 0.5 0.1 4 · 105

40 2 · 1008 0.04 0.5 0.09 1 · 105

30 6 · 1009 0.03 0.5 0.08 3 · 106

20 8 · 1008 0.03 0.5 0.06 3 · 105

CR = Uk * FA * eGi * eSpi * eDet.CR – pulses/s.U – solar photon flux/cm2 s1.FA = 0.31 cm2 – aperture.eDet – detector e!ciency.eGi – grating e!ciency.eSpi – reflectivity of the parabolic mirror.

G. Schmidtke et al. / Advances in Space Research 37 (2006) 273–282 279

Table 2Expected currents iIC of the first ionisation chamber (IC) (see also Fig. 2)

No. Filter material Thickness (nm) k. . .k (nm) TF Solar photon flux (cm!2 s!1) IC current (A)

6 135-N-.5D 125–145 0.15 5 · 1010 3 · 10!10

7 122-XN-.5D 117–127 0.05 3 · 1011 6 · 10!10

8 In 160 73–103 0.18 5 · 1010 4 · 10!10

9 In 250 76–93 0.13 3 · 1010 2 · 10!10

10 Ga 160 64–83 0.16 5 · 1009 3 · 10!11

11 Gd 150 62–77 0.11 5 · 1009 2 · 10!11

12 In/Ge 160/5 53–75 0.18 1 · 1010 7 · 10!11

13 In/Ge 200/30 54–64 0.12 7 · 1009 3 · 10!11

14 Ge 150 41–60 0.20 9 · 1009 7 · 10!11

15 Ge 300 43–52 0.14 4 · 1009 2 · 10!11

16 Sb 300 38–52 0.16 5 · 1009 3 · 10!11

17 Te 300 29–43 0.21 2 · 1010 2 · 10!10

18 Se 150 13–42 0.30 3 · 1010 4 · 10!10

19 Se 600 22–29 0.15 9 · 1009 6 · 10!11

20 Al 600 16–70 0.22 4 · 1010 4 · 10!10

21 Al/C 300/15 16–52 0.21 3 · 1010 3 · 10!10

22 Al/C 300/30 16–36 0.20 2 · 1010 2 · 10!10

23 Al/C 300/60 16–27 0.17 6 · 1009 4 · 10!11

24 Al/C 300/99 16–23 0.14 4 · 1009 2 · 10!11

iIC = U * TT * TF * e = U * TF * 0.42 * 10!19 (A).

iIC – IC current.U – Photonenfluss*/cm

!2 s!1 Dk.TT = 0.26 – transmission of the filter support.TF – transmission of the filter.e – 1.6 · 10!19 As.

Table 3Expected current of the AXUV diodes

No. Filter material Thickness (nm) k. . .k (nm) gSi TF Solar flux (cm!2 s!1) Diode current (A)

1 220-B-.5D 203–237 0.6 0.30 2 · 1014 2 · 10!06

2 200-N-.5D 190–210 0.7 0.15 2 · 1013 8 · 10!08

3 180-B-.5D 160–200 0.9 0.30 8 · 1012 9 · 10!08

4 172-N-.5D 162–182 1 0.15 2 · 1012 1 · 10!08

5 157-N-CF.5D 142–172 1.2 0.12 8 · 1011 8 · 10!09

6 135-N-.5D 125–145 1.2 0.15 5 · 1010 3 · 10!10

7 122-XN-.5D 117–127 1.5 0.05 3 · 1011 9 · 10!10

8 In 160 73–103 2 0.18 5 · 1010 8 · 10!10

9 In 250 76–93 2 0.13 3 · 1010 3 · 10!10

10 Ga 160 64–83 3 0.16 5 · 1009 9 · 10!11

11 Gd 150 62–77 3 0.11 5 · 1009 7 · 10!11

12 In/Ge 160/5 53–75 4 0.18 1 · 1010 4 · 10!10

13 In/Ge 200/30 54–64 5 0.12 7 · 1009 2 · 10!10

14 Ge 150 41–60 6 0.20 9 · 1009 4 · 10!10

15 Ge 300 43–52 6 0.14 4 · 1009 1 · 10!10

16 Sb 300 38–52 7 0.16 5 · 1009 2 · 10!10

17 Te 300 29–43 8 0.21 2 · 1010 1 · 10!09

18 Se 150 13–42 10 0.30 3 · 1010 3 · 10!09

19 Se 600 22–29 10 0.15 9 · 1009 6 · 10!10

20 Al 600 16–70 10 0.22 4 · 1010 4 · 10!09

21 Al/C 300/15 16–52 10 0.21 3 · 1010 3 · 10!09

22 Al/C 300/30 16–36 10 0.20 2 · 1010 2 · 10!09

23 Al/C 300/60 16–27 10 0.17 6 · 1009 4 · 10!10

24 Al/C 300/99 16–23 10 0.14 4 · 1009 2 · 10!10

iSi = gSi * U * TT * TF * e = U * eSi * TF * 0.42 · 10!19 (A).iSi – diode current.gSi – e!ciency of the AXUV diode (electrons per photon).U – solar photon flux/(cm2 s1 Dk).

280 G. Schmidtke et al. / Advances in Space Research 37 (2006) 273–282

In addition, for the total spectral range of 16–220 nmAXUV silicon diodes are applied as a secondary detec-tor standard. The diodes are calibrated at the BESSYelectron synchrotron in Berlin. They will also be recali-brated in-flight by the auto-calibration capability inthe spectral region from 16 to 140 nm. A cross-calibra-tion with the SOLSPEC instrument will also be appliedin the spectral region of 180–220 nm.

The application of the silicon diodes in the three-sig-nal ionisation chamber is providing another advantage:Possible secondary e"ects such as the eventual genera-tion of additional ion-electron pairs in the absorbinggas by higher energetic photoelectrons or the vibrationalexcitation of nitric oxide molecules can be quantified byinter-comparison of the trace "Photo! (Figs. 8 and 9)with the theoretical results from

i3=%gSi $ e $ Io& # i3=Io # exp%!n $ r $ 2 $ L&. %5&

For this special purpose the silicon detector must notbe calibrated. The primary requirements on the Si detec-tor are full linearity of the signal within the dynamic rangeof themeasurement (that is fulfilled for silicon diodes) andthe stability of the radiation source during the measure-ment. Within the expected accuracy (RSU (3–5%) thelatter requirement is met for most of the short term peri-ods of less than 3 min for a calibration cycle.

6. Operational modes

The Spectral Mode will provide full spectral coverageminimum once per orbit. Thus up to 16 spectral mea-surements can be performed per day meeting require-ment in Section 3.3.

The (Auto-) Calibration Mode will be performedtwice per week during the first four weeks in orbit. Thissequence will be decreased to once per week thereafterand finally twice per month at the end of the mission.The gas reservoirs will provide gas for a three years mis-sion taking into account the gas consumption and themeasured leakage with the expected storage periods in-cluded. The leakage rate is determined in the laboratory.

Modes for stray light and background measurementsas well as recalibration of the gas pressure sensors willbe performed on demand. The same holds for the cross-

calibration of the three solar pointing sensors within theSOLAR payload and for operational parameter tests.

The launch of the SOLAR mission is planned in late2006 with a nominal duration of 1.5 years, though spec-ifications allow for a three years mission. Scientific mea-surements will be performed during 16–20 mindepending on the orbit conditions compatible withCoarse Pointing Device (CPD, see top of Fig. 3). How-ever, not all of the orbits can be used for SOLAR scienceoperations because of special ISS activities and otherlimitations.

7. Laboratory measurements

The estimate of the signal strength to be expectedduring the mission is calculated for the spectrometercount rates (Table 1), for the ionisation chambers cur-rents (Table 2) and for silicon diodes currents (Table3) is demonstrating the very high-e!ciency of theinstrumentation.

One of the restricting parameters is the accumulatedcount rate of 1011 of the channeltrons. Taking into ac-count the planned mission period of 1.5 years with anaverage of 15 min measuring period per orbit and upto 16 orbits per day average count rates of "104 s!1

are adequate. This number does meet the requirement3.2. However, the e!ciency of the spectrometers en-forces an optical attenuation of the incoming solar pho-ton flux – a very unusual case in EUV spectroscopy. Theaperture size is not decreased in order to optimise thesensitivity of the ionisation chambers.

The expected currents of the ionisation chambers andof the silicon diodes are in a range that exceeds the sen-sitivity limits by up to four orders of magnitude – againa comfortable signal-to-noise ratio, since dark current isbelow 0.1 pA.

In Table 4, examples are demonstrating the resultsfrom laboratory measurements with flight-like opticalcomponents. The measurements are supporting the or-der of magnitude of the results as given in the Tables1–3 for longer wavelengths. However, for wavelengthsbelow 40 nm the consequent application of grazing inci-dence geometry is increasing the spectrometric e!ciencyby a factor of five.

8. Summary

The results of laboratory measurements prove theanticipated capability for true in-flight calibration ofthe SOL–ACES space experiment at RSU levels of 3–5%.

In the past RSU levels of 10–15% are quoted in mostof the relevant publications. However, some years agoa disagreement by a factor of four has been noted

Table 4Expected and measured count rates at solar flux level

k (nm) Solar photon flux(cm !2 s!1)

Count rates!1 (expected)

Count rates!1 (measured)

121 3 · 1011 1 · 108 1.8 · 108

90 2 · 1009 1 · 106 1.2 · 106

60 1 · 1009 8 · 105 9 · 105

30 6 · 1009 3 · 106 1.5 · 107

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(Solomon et al., 2001) for the strong solar emissions be-low 25 nm. More accurate measurements shall improvethe situation.

The development in thermospheric–ionospheric mod-elling has reached a level that the knowledge of theincoming solar EUV radiation as the most importantenergy contribution should be accurate to a degree ofsignificantly better than 10% RSU.

Acknowledgements

This project is sponsored by DLR Bonn and by ESA/ESTEC Noordwijk. We are very grateful for the strongsupport by W. Pfe"er, EADS Friedrichshafen. Also theIPM team deserves a strong acknowledgement for theremarkable work during the development, fabricationand testing of SOL–ACES. In particular, S. Adolph, J.Anders, D. Binder, R. Ho"mann, A. Hofmann, R. Sin-gler and U. Ulmer have shown a very strong engage-ment in this project.

References

Hinteregger, H.E., Bedo, D.E., Manson, J.E. The EUV spectropho-tometer on atmospheric explorer. Radio Sci. 8, 349–359, 1973.

Samson, J.A.R. Techniques of Vacuum Ultraviolet Spectroscopy.Wiley, New York, 1967.

Samson, J.A.R., Haddad, G.N. Absolute photon flux measurements inthe vacuum ultraviolet. J. Opt. Soc. Am. 64, 47–54, 1974.

Schmidtke, G., Schweizer, W., Knothe, M. The AEROS-EUVspectrometer. J. Geophys. 40, 577–584, 1974.

Schmidtke, G., Thuillier, G., Frohlich, C. Total (TSI) and spectral(SSI) irradiance measurements, JASR-D-04-00350, 2005.

Solomon, S.C., Bailey, S.M.,Woods, T.N. E"ect of solar soft X-rays onthe lower ionosphere. Geophys. Res. Lett. 28 (11), 2149–2152, 2001.

Thuillier, G., Frohlich, C., Schmidtke, G. Spectral and total solarirradiance measurements on board the international Space Station,in: Utilisation of the International Space Station, Second EuropeanSymposium, ESA-SP433, 605–611, 1999.

Thuillier, G., Frohlich, C., Schmidtke, G. Measurement of the totaland spectral solar irradiance: What do we have and will get infuture? COSPAR04-D2.1/C2.2/E3.1-0047-04, 2005.

Wienhold, F.G., Anders, J., Galuska, B., Klocke, U., Knothe, M.,Riedel, W.J., Schmidtke, G., Singler, R., Ulmer, U., Wolf, H. In:The Solar Package on ISS: SOL–ACES, TIGER Proceedings,Phys. Chem. Earth (C), 25(5–6), 473–476, 2000.

282 G. Schmidtke et al. / Advances in Space Research 37 (2006) 273–282