total solar irradiance variations

\PERGAMON Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 04Ð13

S0253Ð5715:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reservedPII] S 0 2 5 3 Ð 5 7 1 5 " 8 7 # 9 9 0 0 1 Ð 5

Total solar irradiance variationsJ[M[ Papa\�\ C[ Fro�hlichb

a Department of Physics and Astronomy\ University of California\ Los Anèles\ 7860 Math Science Bld[\ 394 Hilàrd Ave[\CA 89984!0451\ U[S[A[

b Physikalisch!Meteoroloìsches Observatorium Davos World Radiation Center\ CH!6159 Davos\ Switzerland

Received 06 February 0887^ received in revised form 2 August 0887^ accepted 08 October 0887

Abstract

Total solar irradiance has been monitored from space for nearly two decades[ These space!borne observations haveestablished conclusively that total solar irradiance changes over a wide range of periodicities*from minutes to the 00!year solar cycle[ Since the total energy ~ux of the Sun is the principal driver for all Earth|s atmospheric phenomena\ theaccurate knowledge of the solar radiation received by the Earth and its variations is an extremely important issue[ Inthis paper we review the long!term variations of total solar irradiance during solar cycles 10 and 11[ We conclude that\within the current accuracy and precision of the measurements\ the minimum level of total solar irradiance is about thesame for both solar cycles 10 and 11[ Þ 0888 Elsevier Science Ltd[ All rights reserved[

0[ Introduction

The Sun\ a fairly typical star\ dominates the physicalconditions throughout the solar system due to its in~u!ence on planetary atmospheres and the interplanetarymedium[ Speculation on the role of changes in the Sunand their in~uence on changes in the Earth|s climate hasbeen ongoing for centuries[ Since the sunlight suppliesmost of the energy that drives the dynamics of the ter!restrial climate\ small but persistent variations in the solarenergy ~ux may explain a wide range of climate changes"Eddy\ 0866#[ To establish the possible link between solarvariability and climate changes\ it is necessary to main!tain high!precision and long!term measurements of totalsolar irradiance*an important quantity which rep!resents the value of the integrated solar energy ~ux overthe entire solar spectrum arriving at the top of the Earth|satmosphere at 0 AU distance[

Total solar irradiance has been monitored from spacefor nearly two decades[ These space!borne observationsconvinced the sceptics that total irradiance varies over awide range of periodicities] from minutes to the 00!year

� Corresponding author[ Fax] ¦0 209 195 1985[E!mail address] papÝastro[ucla[edu[ "J[M[ Pap#

solar activity cycle "Willson and Hudson\ 0880^ Fro�hlich\0883^ Kuhn\ 0885#[ The e}ect of granulation\ meso!\ andsupergranulation determines the power spectrum of totalirradiance on time scales of minutes to hours "Fro�hlichet al[\ 0886a#[ The variations on the 4!min time scale aredue to the p!mode oscillations with amplitudes of a fewparts per million[ On time scales of days to months\ theevolution of active regions plays the most important rolein irradiance changes via the combined e}ect of sunspotsand faculae "Chapman\ 0876#[

The most important discovery of the space!borneirradiance observations is that total irradiance varies byabout 9[0) over the solar cycle\ being higher duringmaximum activity conditions "Willson and Hudson\0880#[ It has been shown that the long!term irradiancevariations are associated with the changing emission ofbright faculae and the magnetic network "Foukal andLean\ 0877#[ The limb!brightness measurements "Kuhnet al[\ 0877# indicate that the long!term change in totalirradiance may also be related to variations in the photo!spheric temperature[ Additional global e}ects\ such asradius changes "Delache et al[\ 0875^ Ulrich and Bertello\0884#\ large scale convective cells "Ribes et al[\ 0874^ Foxand So_a\ 0883#\ the di}erential rotation of the Sun|sinterior and the solar dynamo magnetic _eld near the baseof the convective zone "Kuhn\ 0885# may also producevariations in total irradiance[

J[M[ Pap\ C[ Fro�hlich:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 04Ð1305

1[ Measurements of total solar irradiance

The _rst and longest high precision total irradiancemonitoring program was carried out by the Earth Radi!ation Budget "ERB# experiment on the Nimbus!6 satellitebetween November 0867 and January 0882 "Kyle et al[\0883#[ The ACRIM I experiment on the Solar MaximumMission "SMM# satellite was launched in February 0879and it was operated until July 0878 "Willson and Hudson\0880#[ The ACRIM II experiment on the Upper Atmo!sphere Research Satellite "UARS# has continued theACRIM I irradiance observations from October 0880and it is still operating as of this time "Willson\ 0886#[The Variability of IRradiance and Gravity Oscillation"VIRGO# experiment on SOHO has been monitoringtotal irradiance since January 0885 from the Sun!EarthL0 point "Fro�hlich et al[\ 0886a#[ An additional long!term experiment\ the Earth Radiation Budget Experi!ment "ERBE# on the ERBS satellite\ has measured thesolar radiation on a biweekly cadence since October 0873"Lee et al[\ 0884#[ The SOlar VAriability "SOVA# experi!ment on the EUropean REtrievable CArrier "EURECA#provided total irradiance measurements between July0881 and June 0882[ Both the SOVA and VIRGO experi!ments have carried two di}erent radiometers\ i[e[\ the{Di}erential Dual Absolute Radiometer| "DIARAD# andthe PMO!5 type absolute radiometer "Crommelynck etal[\ 0883^ Romero et al[\ 0883^ Fro�hlich et al[\ 0886b^Anklin et al[\ 0887#[

The various observations of total irradiance are sum!marized in Fig[ 0 "updated from Fro�hlich and Lean\0887#[ As can be seen\ the scale of total irradiancemeasurements varies from one experiment to the other[This scale di}erence is related to the limited absoluteaccuracy "29[1)# of the calibration of the individualmeasurements[ However\ the precision and stability ofthe measurements is much better\ which makes it possibleto study the relative variations in total irradiance[ Themeasurements of various experiments plotted in Fig[ 0demonstrate that the total irradiance varies with the solarcycle\ being higher during maximum activity conditions[In order to study the climate impact of total irradiancevariability\ the most important task is to establish theamplitude of the change in total irradiance between solarmaximum and minimum\ and from one cycle to another[

The absolute radiometric accuracy of the various sat!ellite experiments is only about 29[1)\ which causes theo}sets between data sets in Fig[ 0 and is larger than the29[0) solar cycle variations which we are measuring[To detect and study the small relative changes in solartotal irradiance\ continuous and overlapping measure!ments are necessary[ However\ one of the largestobstacles of studying long!term changes in totalirradiance is the existence of the nearly two!year gapbetween the ACRIM I and ACRIM II data sets[ Becauseof this\ the ACRIM II data have to be scaled to the

ACRIM I level via the intercomparison of the ACRIMdata with measurements of {third party| instruments[ Thesolid line in Fig[ 1 shows the SMM:ACRIM I and UAR!S:ACRIM II total solar irradiances on the same scale"i[e[\ ACRIM I scale#\ whereas the dashed line showsthe measured ACRIM II irradiance without scaling[ TheACRIM II data presented in Fig[ 1 have been scaled tothe ACRIM I level via the mutual intercomparison ofthe overlapping ACRIM I and Nimbus!6:ERB andACRIM II and ERB measurements\ yielding a scalingfactor of 0[990510 "Willson\ 0886#[ The result of thisscaling implies that the minimum level of total solarirradiance is higher by about 9[7 Wm−1 during the mini!mum at the end of solar cycle 11 than during the mini!mum at the end of cycle 10[ Based on these results\ Will!son "0886# has concluded that the Sun was brighter byabout 9[92) during the minimum of cycle 11 than duringthe minimum of cycle 10 and has pointed to its potentialsigni_cance for climate change[ The question then is howaccurate is the adjustment of the ACRIM II data to theACRIM I scale[

Lee et al[ "0884# showed that a 9[3 Wm−1 upward shiftoccurred in the ERB data in September 0878\ which wasfollowed by an addition 9[3 Wm−1 shift between May0889 and January 0882\ indicating that the scale of theNimbus!6:ERB data increased with about 9[7 Wm−1

between 0878 and 0882[ Using irradiance modelsdeveloped from ground!based observations of sunspotsand faculae\ Chapman et al[ "0885# derived a somewhatlower increase of 9[56 Wm−1 in the ERB scale[ This shiftin the Nimbus!6:ERB irradiance measurements has notbeen taken into account in the scaling by Willson "0886#\as presented in Fig[ 1[ The scaling factor to adjust theACRIM II data to the ACRIM I level\ as shown in Fig[0\ has been derived by Fro�hlich and Lean "0887# takinginto account the drifts in the Nimbus!6:ERB totalirradiance data[ In addition\ Fro�hlich and Lean "0887#used both the Nimbus!6:ERB and the ERBS data toderive the scaling factor[ As can be seen from Fig[ 0\ thisscaling yields a factor of 0[99007929[999042\ lower thanthe 0[990510 derived by Willson "0886#\ and no signi_cantdi}erence is seen between the minimum level of totalirradiance during solar cycles 10 and 11[ This exercisedemonstrates the di.culty for adjusting data sets withgaps through the observations of a third party experi!ment[

In order to study the potential climate impact of solarirradiance variability\ it is necessary to establish a con!tinuous long!term data set from the existing measure!ments[ Fro�hlich and Lean "0887# have compiled a {com!posite total irradiance| using the Nimbus!6:ERB\SMM:ACRIM I\ UARS:ACRIM II\ and the SOHO:VIRGO measurements[ This composite is shown in Fig[2\ and its construction described in detail by Fro�hlich"0887# and Fro�hlich and Lean "0887#[ The so!called spin!mode ACRIM I data "December 0879ÐApril 0873# have

J[M[ Pap\ C[ Fro�hlich:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 04Ð13 06

Fig[ 0[ Time series of various space!borne irradiance experiments updated from Fro�hlich and Lean "0887#[

Fig[ 1[ The solid line shows the ACRIM I and ACRIM II data on the same scale "Willson\ 0886#^ the dashed line shows the measured\unscaled ACRIM II data[ Note that the solid line of ACRIM II data includes Willson|s "0886# scaling factor of 0[990510[


Fig[ 2[ Composite total solar irradiance times series updated from Fro�hlich and Lean "0887# which uses their scaling factor of 0[990079for the ACRIM II data[

been excluded from the composite time series because oftheir inherently large uncertainty "see also Pap et al[\0885#[ Note that during the normal operational mode ofSMM\ the measuring precision of ACRIM I was about29[991)\ and most of the observed events had a solar\rather than an instrumental origin "Willson\ 0873#[

As can be seen from Fig[ 2\ the most important featuresare nearly the same amplitudes of the peaks in 0868Ð0871 with respect to those in 0878Ð0881\ nearly equalamplitudes of the two minima and a slightly ~atter mini!mum phase in 0874Ð0876 than in 0884Ð0886\ and it isunlikely that the Sun was brighter during the minimumof cycle 11 than during cycle 10[ Obviously\ one can drawdi}erent conclusions depending on the way in which thescaling factor for the ACRIM II data is determined andwhat has been assumed for the two instrumentation shiftsin the Nimbus!6 data[ However\ it should be noted thatthe minima of solar cycles result as a combination of adecaying old cycle and the initial outbreak and early riseof an overlapping activity of the new cycle\ so one doesnot need to expect the minimum levels to be exactly thesame\ only nearly the same[

2[ Total irradiance variations during solar cycles 10 and 11

In the discussion to follow\ we shall compare the long!term variations of total irradiance during solar cycles 10and 11\ using the scaling factor derived by Willson "0886#for ACRIM II[ This will give an independent check ofthe assumptions used to construct the composite of Fig[2[

In addition to the ACRIM and Nimbus!6 totalirradiance measurements\ the ERBS data provide anindependent measure of long!term total irradiance vari!ations[ Although the ERBS solar monitoring experimentmeasures total solar irradiance only on a biweekly basis\the ERBS data still provide useful information about thelong!term changes over the solar cycle[ The ERBS totalirradiance data are plotted in Fig[ 3 "dots# together withtheir model estimates calculated from the PhotometricSunspot Index and the 09[6 cm radio ~ux "solid line#"updated from Pap et al[\ 0886#[ As can be seen\ themeasured ERBS total irradiance and its model estimatesdo not indicate a signi_cant di}erence between the mini!mum levels of solar cycles 10 and 11[


Fig[ 3[ The dots show the ERBS total solar irradiance[ The dashed line gives the standard deviation of the data^ the solid line representsthe regression model calculated from PSI and the 09[6 cm radio ~ux[ This _gure is provided by courtesy of Robert Lee\ III\ and it isupdated from Pap et al[ "0886#[

An additional two!parameter model of total solarirradiance has been calculated for the time interval of theERBS measurements "0873Ð0885# using the Mt WilsonMagnetic Indices[ In this case\ the Magnetic PlageStrength Index "MPSI# and the Mt Wilson Sunspot Index"MWSI# have been _tted to both the SMM:ACRIM Iand UARS:ACRIM II total irradiance measurementsusing multiple linear regression[ MPSI is de_ned as thesum of the absolute magnetic _elds of all pixels withmagnetic _eld strength between 209 and 099 Gauss div!ided by the total number of pixels in the images[ In asimilar manner\ MWSI is de_ned as the sum of the absol!ute magnetic _elds of all pixels with magnetic _eldstrength above 2099 Gauss[ Note that the MPSI andMWSI indices have been derived by careful comparisonof the Mt Wilson images with Ca II K observations ofplages and white!light observations of sunspots "Chap!man and Boyden\ 0875#[ The solid line in Fig[ 4 showsthe regression model\ derived from the ACRIM I obser!vations[ The dashed line shows the model based on theACRIM II data[ Note that for this latter model cal!culation we have used the ACRIM II data adjusted byWillson "0886#[ Similar to the ERBS model\ neitherregression model shows a signi_cant di}erence betweenthe minimum levels of the two solar cycles[ It is interestingto note\ however\ that the modeled total irradiancevalues\ which are estimated from the ACRIM II measure!

ments\ are higher than the ACRIM I irradiance estimates[Although the models of total irradiance\ based on mag!netic surrogates\ are less accurate than the measuredirradiance values "see summary by Pap et al[\ 0883#\ thesemodel calculations also suggest that the 0[990510 scaleof the ACRIM II total irradiance\ as derived by Willson"0886#\ may be too high[

To further study the irradiance changes during the twosolar minima\ the long!term trends have been separatedin both the ACRIM and ERBS total irradiance timeseries[ Since the long!term variations in both total andUV irradiances are associated with the changing emissionof plages and the magnetic network "Lean\ 0877#\ thelong!term trend of the combined Nimbus!6:SBUV0 andNOAA8:SBUV1 Mg c:w ratio "see review by Donnelly\0880# has been examined as well[ For this purpose wehave applied a relatively new and advanced statisticalmethod\ Singular Spectrum Analysis "SSA# "Vautard etal[\ 0881#[ SSA has been used extensively in climateresearch and has been applied in irradiance studies "Papand Varadi\ 0885^ Pap 0886a\ b#[ SSA is based on Prin!cipal Component Analysis in the time domain[ The giventime series is augmented into a number of lagged timeseries up to a _xed value[ The cornerstone of SSA is theeigenvalue!eigenvector decomposition of the lag!covari!ance matrix[ The eigenvectors provide moving average_lters which extract uncorrelated parts of the signal and


Fig[ 4[ The regression model of the ACRIM total solar irradiance\ calculated from the MPSI and MWSI plage and sunspot indices\ ispresented "updated from Pap et al[\ 0886#[

whose contributions to the complete signal are given bythe corresponding eigenvalues[ The eigenvalues them!selves\ arranged in a decreasing order\ form the SingularSpectrum\ which levels o} after a certain eigenvalue indexby forming the {noise ~oor|[ What is worth modeling inthe given time series is the variability associated witheigenvalues above the noise ~oor*trends and oscil!lations which appear to be statistically signi_cant[

The reconstructed solar!cycle!related trends in theACRIM I and ACRIM II data are plotted in Fig[ 5"heavy solid line#[ The thin lines show the reconstructedtotal irradiance data without the solar cycle trends[ Ascan be seen\ not only is the minimum level of the ACRIMII data is higher during cycle 11 than during cycle 10\ butthe detrended ACRIM II data and their mean value arealso above the ACRIM I level[ If the adjustment of theACRIM II data to the ACRIM I scale "Willson\ 0886# iscorrect\ the results imply that the Sun was also brighterduring the maximum and declining portion of solar cycle11 than during cycle 10\ not just at the minimum time ofcycle 11[ We note that neither the Nimbus!6:ERB totalirradiance data\ even on their higher scales due to instru!mental drifts "Lee et al[\ 0884^ Chapman et al[\ 0885#\ northe ERBS data\ support this result[

In a similar way\ the long!term trends have been sep!arated in the ERBS total irradiance and its correction forsunspot darkening "Sc#[ The _lled circles in Fig[ 6 showthe ERBS total irradiance^ the squares represent theERBS Sc[ The heavy lines are the SSA!reconstructed solar

cycle trends[ As can be seen\ in the case of the ERBStotal irradiance\ the trend values indicate a slightly higherlevel "about 9[2 Wm−1# of the minimum of cycle 11 thanthat of cycle 10[ However\ one has to keep in mind thatthe ERBS irradiance observations are performed onlyon a biweekly basis\ thus the long!term trend may bein~uenced by the temporary e}ect of solar active regions[As the ERBS Sc trend shows\ after removing the e}ect ofsunspots\ there is no di}erence between the minimumlevel of the two solar cycles[

To demonstrate the long!term UV variations\ thesolar!cycle!related trend of the Mg c:w ratio has beenreconstructed and plotted in the upper panel of Fig[ 7"heavy line#^ the thin line shows the detrended Mg c:wratio data[ As can be seen\ the level of the minima of thetwo cycles is the same\ and there is no evidence that thesun was brighter in the UV irradiance during cycle 11than in cycle 10[ To carry the comparison between theUV and total irradiance further\ an arti_cial gap hasbeen introduced into the long!term Mg c:w ratio\ whichmatches exactly the time interval of the gap in theACRIM data[ The heavy lines in the lower panel of Fig[7 show the reconstructed trends in the Mg c:w ratio forthe ACRIM I and ACRIM II time intervals\ respectively\while the thin lines show the corresponding detrendedMg c:w time series[ It can be seen that the reconstructedtrend components indicate the same level of the solarminimum during solar cycles 10 and 11[ In contrast\ thedetrended Mg c:w data for the ACRIM II time period


Fig[ 5[ The heavy line shows the reconstructed solar cycle trends in the ACRIM I and ACRIM II total solar irradiance[ The thin lineshows the detrended total irradiance[

Fig[ 6[ The _lled circles show the ERBS total irradiance and the squares represent the ERBS Sc values[ The heavy lines are thecorresponding solar cycle trends reconstructed by SSA[


Fig[ 7[ The heavy line in the upper panel shows the solar cycle trend in the Mg c:w ratio\ while the thin line represents the detrendeddata[ The heavy line in the lower panel shows the reconstructed trends for the ACRIM I and ACRIM II time intervals\ respectively[Again\ the thin lines show the corresponding detrended data[

"0880 to 0885# and their mean is now placed below thedetrended data for cycle 10[ Note that the lower valuesof the detrended data and their mean are not surprisingsince the high UV irradiance data during the maximumof cycle 11 have been excluded from the analysis\ leadingto a lower mean value[

3[ Conclusions

Results on the long!term variations of total solarirradiance from solar cycle 10 to cycle 11 have been dis!cussed in this paper[ Because of the almost two!year longgap between the SMM:ACRIM I and UARS:ACRIM


II total irradiance measurements\ the ACRIM II dataneed to be scaled to the ACRIM I level via comparisonwith third party measurements\ such as the Nimbus!6:ERB and ERBS:ERBE total irradiance[ Willson "0886#concluded from his correction that the minimum level oftotal solar irradiance for solar cycle 11 was higher byabout 9[92) than for cycle 10[ In contrast\ the compositederived by Fro�hlich and Lean "0887# using all the knowncorrections for the di}erent measurements found no sig!ni_cant di}erence between the minimum levels of the twominima[ These contradictory results indicate the di.cultyof establishing the correct scaling factor to adjust datasets with existing gaps[

The results presented in this paper support the _ndingsof Fro�hlich and Lean "0887#[ Moreover\ if Willson "0886#were to be correct\ the Sun would have been brighter notonly during the minimum of cycle 11\ but also at the timeof its maximum and declining portion[ Neither the ERBSnor the Nimbus!6:ERB data support such a result\ asshown in Fig[ 0\ a further indication that there is nosubstantial change between the minima[

These results also indicate that the missing knowledgeof the long!term degradation behavior of the radiometershinders the establishment of a reliable amplitude valuefor the change of total irradiance from the maximum ofa particular cycle to its minimum and from one cycle tothe other[ This amplitude\ and its change from cycle tocycle\ are among the most important factors in~uencingthe terrestrial climate related to solar activity[ Withinthe accuracy and long!term precision of the availablemeasurements\ we conclude that total solar irradiance isabout the same during the minima of solar cycles 10and 11[ Our results also underscore the necessity of thecontinuous\ overlapping\ and redundant measurementsin order to maintain long!term irradiance data basis[

Acknowledgements

The authors gratefully acknowledge the past and ongo!ing e}ort of the VIRGO team to produce and interpretthe SOHO:VIRGO total irradiance data[ SOHO is amission of international cooperation between ESA andNASA[ The Nimbus!6:ERB\ SMM:ACRIM I andUARS:ACRIM II as well as the ERBE total irradiancedata were taken from the NOAA World Data Center\ aspublished in SGD[ The MPSI and MWSI data come fromthe Mt Wilson Data Archive\ the combined Nimbus!6:NOAA8 Mg c:w ratio data have been provided to thisstudy by Rodney Viereck of NOAA[ Thanks are extendedto Bob Lee\ Daryl Parker\ Roger Ulrich\ Ferenc Varadiand an unknown referee for their helpful comments[ Theresearch of JP was partially funded by the NASA Missionto Planet Earth and by NASA O.ce of Space Science andthat of CF by the Swiss National Science Foundation\ allof which are gratefully acknowledged[

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total solar irradiance variations

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