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Advances in Space Research 48 (2011) 899–903

Distinct EUV minimum of the solar irradiance (16–40 nm) observedby SolACES spectrometers onboard the International Space

Station (ISS) in August/September 2009

B. Nikutowski a,b,⇑, R. Brunner a, Ch. Erhardt a, St. Knecht a, G. Schmidtke a

a Fraunhofer-Institut fur Physikalische Messtechnik (IPM), Heidenhofstraße 8, 79110 Freiburg, Germanyb Institute for Meteorology, University of Leipzig, Stephanstr. 3, 04103 Leipzig, Germany

Received 30 November 2010; received in revised form 19 April 2011; accepted 2 May 2011Available online 10 May 2011

Abstract

In the field of terrestrial climatology the continuous monitoring of the solar irradiance with highest possible accuracy is an importantgoal. SolACES as a part of the ESA mission SOLAR on the ISS is measuring the short-wavelength solar EUV irradiance from 16–150 nm.This data will be made available to the scientific community to investigate the impact of the solar irradiance variability on the Earth’sclimate as well as the thermospheric/ionospheric interactions that are pursued in the TIGER program. Since the successful launch withthe shuttle mission STS-122 on February 7th, 2008, SolACES initially recorded the low EUV irradiance during the extended solar activityminimum. Thereafter it has been observing the EUV irradiance during the increasing solar activity with enhanced intensity and changingspectral composition. SolACES consists of three grazing incidence planar grating spectrometers. In addition there are two three-signalionisation chambers, each with exchangeable band-pass filters to determine the absolute EUV fluxes repeatedly during the mission.One important problem of space-borne instrumentation recording the solar EUV irradiance is the degradation of the spectrometer sen-sitivity. The two double ionisation chambers of SolACES, which could be re-filled with three different gases for each recording, allow therecalibration of the efficiencies of the three SolACES spectrometers from time to time.� 2011 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Solar EUV spectral irradiance; Measurements; Calibration

1. Introduction

The irradiance of the sun is the basis for life on earth. Itis accepted that changes in this radiation have influencedthe climate in the past. One periodic component containedin this variation is the solar cycle (Willson et al., 1981).Although the total solar irradiance has only a variationof 0.1% (Willson and Hudson, 1991), the short wavelengthrange, the extreme-ultraviolet (EUV) solar irradiance, canvary by some 100%. The determination of the EUV solarirradiance, which is absorbed by ionising and heating the

0273-1177/$36.00 � 2011 COSPAR. Published by Elsevier Ltd. All rights rese

doi:10.1016/j.asr.2011.05.002

⇑ Corresponding author at: Fraunhofer-Institut fur Physikalische Mess-technik (IPM), Heidenhofstraße 8, 79110 Freiburg, Germany.

E-mail address: bernd.nikutowski@ipm.fraunhofer.de (B. Nikutowski).

high atmosphere, is an important task for terrestrial clima-tology. Thus, continuous measurements of the EUV arenecessary to derive solar indices which describe the solarvariability and could be used to model the high atmosphereand ionosphere (Unglaub et al., this issue).

A major problem, associated with measuring EUV irra-diation, is the instability of the EUV photon collectingdevices, mainly due to degradation. Up to now these pro-cesses are understood only qualitatively. One way to over-come this problem is to launch a rocket every year withfreshly calibrated devices parallel to the continuous satelliteobservations (Woods et al., 2000, 2005, Woods, 2008).Another option is with a frequent in-situ recalibration ofthe instruments as chosen by SolACES: two double ionisa-tion chambers allow the calibration of the spectrometers at

rved.

a) b) Chamber

0 V

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I 2

I 3

I 1

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CathodeAnode

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Photo detector

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I 3

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0 V

-23 V

-23 V

Aperture

Ionisation Area

Fig. 1. Ionisation chamber measurements, (a) sketch of a double ionisation chamber, (b) three current output of an ionisation chamber measurement(dotted line – photodiode current with left scale, solid line – first chamber current, dashed line – second chamber current, both with right scale).

Fig. 2. Transmission curves of the filter I4 determined in the distance often months.

900 B. Nikutowski et al. / Advances in Space Research 48 (2011) 899–903

intervals directly in space (Schmidtke et al., 2006a). As anexample we will present integral measurements of one ion-isation chamber as an absolute measurement of the EUVradiation in the wavelength range from 16 to 40 nm.

2. Instrument characteristics

SolACES (Schmidtke et al., 2006b) is a part of the ESAmission SOLAR on the International Space Station (ISS)(Schmidtke et al., 2006c) measures the short-wavelengthsolar EUV irradiance from 16–150 nm. Since the successfullaunch with the shuttle mission STS-122 on February 7th2008, SolACES has been recording the low EUV irradianceduring the extended solar activity minimum between thesolar cycles 23 and 24 and the enhancing EUV irradianceduring the increasing solar activity of the new solar cycle24.

SolACES is working with three grazing incidence planargrating spectrometers and two three-signal double-ionisa-tion chambers with exchangeable band-pass filters to deter-mine the absolute EUV fluxes. The ionisation chambers(Fig. 1a) of SolACES, which can be refilled with three dif-ferent gases (Ne, Xe or Xe/NO), allow the recalibration ofthe efficiencies of the three SolACES spectrometers fromtime to time. The ionisation currents (Fig. 2) measured inboth chambers represent a quantitative measurement ofthe EUV flux transmitting through the filter. A detaileddescription of SolACES is given by Schmidtke et al.(2006b).

SolACES measurements were originally planned asspectrometer measurements with calibration measurementsfrom time to time. Evaluations of the first measurementsshowed a strong temporal variability of the spectrometerefficiencies. On the other hand, the leakage rates of the ninevalves involved are much lower than specified; hence thereis more gas available for the ionisation chamber measure-ments, allowing recent spectrometer observations to bemore frequently recalibrated. Since these ionisation cham-ber measurements are well modelled, irradiance spectra candirectly be derived, too. As a result, all spectrometer obser-vations can be calibrated directly.

Fig. 3. Ionisation chamber 1 current of the filter I4 at 1 mbar (black line)compared with the sunspot number (gray line).

Fig. 4. Part of Fig. 3 zoomed in around the minimum of the ionisationchamber 1 current (the sunspot number is shown by a dashed line).

B. Nikutowski et al. / Advances in Space Research 48 (2011) 899–903 901

Due to the ISS constraints each orbit is providing up to20 min of measuring time (out of about 90 min). Also thereare periodical gaps of two to three weeks when the CoarsePointing Device of the SOLAR platform can not bepointed to the sun. In addition, during special maneuversthe SOLAR platform is out of operation.

3. Results

One of the advantages of SolACES is that the current ofthe first ionisation chambers depends only on the intensity

Fig. 5. The solar “EUV Minimum” between solar cycles 20 and 21 in April 1971984).

of the photon flux transmitted through the filter. In orderto trace degradation effects of the filters their characteristictransmission features are repeatedly derived from directmeasurements showing that the aluminium/carbon lowband-pass filter I4 (Fig. 2a) is absolutely stable, thus verify-ing it does not show any degradation. Therefore, we canuse the ionisation current in the first chamber filled withneon at a pressure of 1 mbar as an absolute integral mea-sure of the 16–40 nm EUV irradiance. Fig. 3 presents thiscurrent during the complete SolACES observation periodtogether with the sunspot number. The current, starting

5 observed at 58.4 nm (He I) on satellites AE-C and AEROS-B (Schmidtke

15 20 25 30 35 40 45 50 55 6010−6

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10−3

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flux (

W m

−2 n

m−1

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SolACES: calibrated photon fluxes

06.08.200820.08.200915.02.201012.07.2010

Fig. 6. EUV irradiance in bins of 1 nm from 16 to 58 nm on four different days in 2008–2010.

15 20 25 30 35 40 45 50 55 60 6510−6

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Calibrated Photon Fluxes

Fe IX

Fe

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20.08.2009 (1)14.02.2010 (2)

Fig. 7. EUV irradiance in original resolution from 16 to 58 nm at the minimum and local maximum of Fig. 3.

902 B. Nikutowski et al. / Advances in Space Research 48 (2011) 899–903

with a value of 199 pA on April 10th, 2008, decreased con-tinuously down to 160 pA on September 16th, 2009. Itreached the distinct EUV minimum of 156 pA on August21st, 2009. At the end of August 2009 the second longestperiod free of sunspots (51 days) appeared after the solarcycle 23 finished, and indeed the solar EUV irradiance ofthe wave length range 16–40 nm reached its minimum inthe middle of September 2009 (Fig. 4). A similar observa-tion of a distinct solar minimum was published for thesolar EUV minimum in 1975 (Schmidtke, 1984) by inde-pendent measurements onboard the AE-C and AEROS-Bsatellites (Fig. 5). However, the total lengths and the start-ing solar activity of both solar minima show characteristicdifferences. The minimum period between cycles 23 and 24

lasted longer than one year compared to the minimumbetween cycles 20 and 21, and the onsets of the increasingsolar activity look different, too. As to the SolACES obser-vations, is there an overlap of the eleven-year solar cycleswith a starting Gleissberg cycle of about 87 years duration(Sonett, 1990)?

Fig. 6 presents binned spectra from 16 to 58 nm for fourdifferent days in the period 2008 to 2010. In Fig. 7 we showtwo non-binned spectra at the minimum on August 21st,2009 and near the local maximum on February 14th and15th, 2010 (see Fig. 3). The spectrum at the solar EUV min-imum has always the lowest intensity, the spectra of thehighest solar activity during the period of the solar mini-mum exhibit always the highest intensity, however the dif-

B. Nikutowski et al. / Advances in Space Research 48 (2011) 899–903 903

ference varies with wavelength. The largest enhancementby more then 100% occurs between 20 and 28 nm, whilethe smallest enhancements of only a few percent are foundfor wavelengths larger than 40 nm. Looking for concretespectral lines (Fig. 7), the strongest enhancement is causedby the Fe spectral lines which are dominating for wave-lengths smaller then 30 nm.

4. Summary

The first ionisation current of the aluminium/carbon fil-ter I4 as an absolute integral measure of the 16–40 nmEUV irradiance exhibits its minimum in the middle of Sep-tember 2009. The comparison of spectra at different activ-ity levels shows that at lowest activity the EUV irradianceis always the lowest, too. In general the differences in theintensity are much higher for wavelengths shorter than40 nm than for wavelengths longer than 40 nm. The stron-gest enhancements are observed by the Fe spectral lines.

Acknowledgement

We would like to thank the Solar Influences Data Anal-ysis Center of the Royal Observatory of Belgium for pro-viding the sunspot data through http://sidc.oma.be/sunspot-data.

This project is sponsored by DLR Bonn and by ESA/ESTEC Noordwijk.

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