Pergamon Adv. Space Rex. Vol. 16, No. 8, pp. (8)2!%(8)32, 1995

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SOLAR TOTAL IRRADIANCE VARIABILITY MEASURED BY SOVA-2 ON BOARD EURECA

J. Romero, C. Frijhlich and Ch. Wehrli

Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Dorftrasse 33. CH-7260 Davos Do& Switzerland

ABSTRACT

During its 9 months of operation, the experiment SOVA-2 (Solar Constant and Variability) on board the European Retrievable Carrier (EURECA) bas measuted the total solar irradiance with two radiometers of the type PMO6-R The solar variability has been investigated by bivariate spectml analysis of the irra- diance and the Photometric Sunspot Index (PSI) to account for the sunspot blocking and after removal of tlte spot influence with the MgII-Index as proxy for the conttibution of faculae and other bright magnetic elements.

INTRODUCTION

The SOVA (Solar VAtiabiity) experiment on board EURECA (EUropean FBrievable Chrier) is a collaboration between the Institut Royal M&!otologique de Belgique (IRMB), the PhyssMeteotolo- gisches Observatorium Davos, World Radiation Center (PMOD/WRc) and the Space Science Depattment (SSD) of the Eutopean Space Agency (ESA) with D. Crommelynck from the IRMFi as principle investigator. The experiment is composed of SOVA-1, SOVA-2 and SOVA-3 designed and built by tlte IRMB, the PMOD/wRC and tbe SSD mspectively and is descrii in /l/. EURECA was launched by the shuttle Atlantis on 31 July 1992 and mcovered by Endeavor on 24 June 1993. SOVA was operated from 11 August 1992 until 16 May 1993.

Here we report on results of the total solar irradiance from the absolute radiometers PMO6-R of SOVA-2. Their scientific objectives am to observe the solar irradiance with an accuracy co.196 and short term variations with a resolution of <le. In order to achieve these objectives two PMO6-R are used: the ‘active’ is continuously measuring and the ‘back-up’ monitors possible degradation of the ‘active’ by taking simultaneous measumments from time to time. The operation and evaluation of the radiometers during the EURECA mission is described in f2/ and details of their performance and chamcterixation can be found in /3/. The solar irradiance variability is investigated for the sunspot blocking by tbe Photometric Sunspot Index (PSI) and for the faculae and other bright magnetic elements by the Mg II-Index.

BXRADIANCE VARIABLlTY

Figure 1 shows the data (daily means) used in the analysis: l total solar irradiance measumd by the active and back-up radiometers. l photometric sunspot index (PSI) from /4/, l Magnesium core-to-wing ratio (MgII-Index) from the SBUV/2 instrument aboard the NOAA-9

satellite IS/. The large dips in the itradiance anumd days 232,300 and 405 am due to the passage of large sunspots groups and peaks around days 270,360 and 470 coincide with increases of the MgII-Index illustrating the influence of bright faculae.

A comparison between the active and back-up radiometers during the mission suggests, although not unambiguously, a loss of sensitivity of the active instrument of the order of 1 ppm/day which is corrected for in the Figure 1. Similar degradation has been found during the first year of ACRIM-I /6/ on tbe Solar Maximum Mission. After reuieval of EURECA the instruments could be investigated in detail again. Although the post-flight characteriz.ation is not yet completed and there are still some unexplained discrepancies between the pxe- and post-flight comparisons, the preliminary results indicate that the ori- ginally specular paint in the cavity of the active instrument has incxeased its diffuse part substantially.

J. Romero et al.

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Figure 1. ihily muus of the solar total sola irradiance during the EURECA mission mcasurd b the active (solid line) and &k-up (dots) rwlicnn~r (middle), of the photometric sunspot index (PSI, bottom) ad of the MgI[-Jnd= (top).

The back-up instrument shows no such effect, at least them is no measurable change observed. The change in the paint may explain the degradation by increasing the reflectivity of the cavity of the active instrument and may be mainly due to the h-radiance by the strong W-radiation from the Sun in space. PSI has been devebped (e.g. /N) for the purpose of the modelllng of solar total itradisnce variations by *the effect of the dark sunqx~s and describes the change of solar itradiance during to the passage of sunspot groups over the visible disk The values am based on obsen&ons of sunspots as published in the Solar Geophysical Data catalogue by NOAA. ‘lhe PSI used here is calculated by an improved algorithm 141 which takes into account a size dependent contrast of the sunspots and cslculates ‘true’ daily means for each observation using the latitude dependent surface rotation of the spots. Bivariate speetml analysis is used to investigate the influence of PSI on the it-radiance. This analysis calculates the association between two time series by determining a linear filter (characterized by gain and phase) which transforms one time series into the other. The degree of association is given by the coherence, the squam of which multiplied by 100 is the percentage of the signal in one time series explained by the other transformed with the filter. Pigum 2 shows the result of this analysis as spectml power densities. It is interesting to note that the highest coherence (-90%) and thus the lowest uncertahny of the gain is in the range from 0.5 to 0.9 pHx (13-23 days period) and the period of the solar rotation is in a reglon of low col~tence indicating that the rotationsl time scale is less well determined than the evolutionary one. The mean value of the gain is very close to one which is not astonlshing as the PSI values used here have been ‘calibrated’ by irradiance data /4/. Another source of solar variation is the enhanced emission from bright faculae 181. Furthermore. relative to the emission of the quiet Sun, facuhr emission is greater at W wavelengths than at other wavelength. It has been shown/Y/that the Mg II-Index can be used to reproduce variations in the solar W irradiance and thus may be also used as a proxy for total itradiance enhancement This association is also investigated by bivarlate spectral anslysis, but with the total solar h-radiance reduced by PSI, that is with thevariancedueto~~lemovcd.Figun3showsthetwotimestricsandpigun4theresultofthe bivariate analysis in the same fotmat as Pigure 2. The cohemnce no longer reach& values as high as for

Solar Total Imdiance Vtiability

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F&.2 Spectral power density of the total solar indance (uppa did curve) for periods of 80-5 days. The hwr did line hdiatfs the hit of that part of the i~xdixnce explained by PSI with iu 90 % confidence inlavds (dotted lines). Thus. at 18 days peaiod about 90% of the irmdiance vxxinnce is exphind by PSI.

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Fit3.Theuppaprmelshowsakain~M~-lndwr(MinPigure2)mdbrtlowaonethe~~~imdiance witi the influence of amspots ranoved. For this the PSI multiplied by the gain obtained from ttU: bivmiate an@iii6ddedtothetotalsolarirmdiwe.

the PSI correlation. It is interesting to note that in contrast to the PSI the MgII-Index does explain some vatiance (-70%) at and around tk solar rotation period. This can be undersmod because the W irradiance shows also a strong modtdation at this frequency.

CONCLUSIONS

Good agwuuent exists between the measurements of the solar mtal inadiance during the EURECA mission done by the two PMO6-R radiometers of SOVA-2. The solar variability during these nine months has been investigated using the PSI to account for the sunspot deficit and the Mg II-Index as a proxy for

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Fig.4. Spectral powa density of the solar total inadhcc augma~ted by PSI, that is with the intlueme of the sunspots removed (upper Aid curve). The lower solid line indicstes the limit of that part of the irradiance explained by MgII-Index with its 90 % confikce intfmxls (dotted lines).

the W variations associated with the excess of irradiance originating fmm faculae and the bright magnetic network.

ACKNOWLEDGEMENTS

The support of all the SOVA team (IRMB, PMOD/WRC and SSD/ESIEC) is gratefully acknowledged as well as the continuous financial support of Swiss National Science Foundation. Thanks am extended to J. Pap from JPL for supplying to us the data of the Mg II-Index.

REFERENCES

1. D. Crommelynck, V. Dan-o, A. Fichot, C. Ftihlich, B. Penelle. J. Romero, Ch. Wehrli, Preliminary results from the SOVA experiment on board the EUropean REtrievable CArrier (EURECA), Metrologin, 30, 375-379 (1993).

2. J. Romero, Ch. Wehrli and C. Ftihlich, Solar total irradiance variability from SOVA 2 on board EURECA, Sol. Phys, 152,23-30 (1994). 3. R.W Brusa and C. Ftihlich, Absolute radiometers (PMO6) and their characterization, Appl. Optics, 25 4173-80 (1986).

4. C. Ftihlich, J. Pap and H.S. Hudson, lmpmvement of the photometric sunspot index and changes of the disk-integrated sunspot contrast with time, Sol. Phys., in press. 5. R.F. Donnelly, Solar W Spectral Irradiance Variations, ~.Geom.Geuef., 43, 835842 (1991) 6. R.C. Wlllson and H.S. Hudson, The Sun’s luminosity over a complete solar cycle, IVUZJU~?, 351.4244 (1991). 7. H.S.Hudson and R.C. Willson), Sunspots and Solar Variability, in ‘Physics of Sumpots’, eds. CramL. and Thomas,J., Sacramento Peak Obs: Sunspot, 434445 (1982) 8. J. Lean, Variations in the Sun’s radiative output, Rev. Geophys., 29.505-535 (1991). 9. D.F. Heath and B.M. Schlesinger, The Mg 280-nm doublet as a monitor of changes in solar UltraviOlet itradiance, J. Geophys. Res., 91 @8), 8672-82 (1986).