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Adv. Space Res. Vol. 29, No. 10, pp. 1409%1416.2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved

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PII: SO273-1177(02)00203-X

TOTAL SOLAR IRRADIANCE VARIATIONS SINCE 1978

Claus Frdhlich

Physikalisch-Meteorologisches Observatorium Davos, World Radzation Center, CH-7260 Davos Dorf, Switzerland

ABSTRACT

A composite record of solar total irradiance compiled from measurements made by five independent space-based radiometers since 1978 is the basis for an evaluation of the influence of solar activity on total solar irradiance. An empirical model that parameterizes the combined influences of dark sunspots and bright faculae features on solar irradiance is able to explain more than 95% of the variance. After removing the magnetic influence with the model, the remaining ‘quiet sun’ shows no trend over the whole period, indicating that the sun has not changed over the past two solar cycles. The inclusion of p-mode frequency changes in the model does not explain more than already expained by the magnetic parameters. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

INTRODUCTION

The past 20 years provided us with direct observations of the total solar irradiance (Z’SI) from space with different instruments as shown in Fig. 1. The radiometric accuracy of irradiance measurements made by individual instruments, of the order of 0.2%, is insufficient to determine long term changes of only about 0.1% that occur during the 1 l-year modulation. While the instrument repeatability is adequate to monitor short term changes, the long term behavior can only be retrieved by careful tracing of one experiment database to the other, incorporating good knowledge of the degradation of radiometers operating in space, Fortunately several time series exist from different platforms made by different radiometers. This allows the construction of a composite time series having improved long term precision, thus yielding an unbiased estimate of TSI variability during the last two solar cycles as described in Frohlich and Lean (1998a); Frohlich and Lean (1998b); henceforth referred to as FL98. In the following we summarize (from FL98) the TSI measurements used for the construction of the composite together with the corrections which were applied to homogenize this record. Moreover, the record is updated through June 1998, the time when SoHO took its vacation.

REVIEW OF TSI OBSERVATIONS AND CONSTRUCTION OF A COMPOSITE TSl

Five separate radiometric instruments onboard a variety of spacecraft have monitored the Sun’s total irradiance over selected intervals of time since late 1978. Fig.1 presents these individual datasets from which the data from HF, ACRIM I & II and VIRGO were used for the composite; the ERBE data have together with HF been used to adjust ACRlM II results to the scale of ACRIM I. The data set from the Solar Variability (SOVA) experiment onboard the European Retrievable Carrier (EURECA) are not used for the composite. Instrumental sensitivity drifts which are inevitable during long space missions have been identified and corrected. They depend mainly on the rates of exposure to high energy solar radiation and particles of the interior surfaces of the receiver cavities, and on fluctuating thermal and aspect environments on board their spacecraft platforms. There are also other effects which may depend on sudden changes in the instrument performance and/or the operational procedures. In the following the corrections used by FL98 are shortly described.

. The Hickey-Frieden (HF) radiometer of the Earth Radiation Budget (ERB) experiment on the NIMBUS-7 spacecraft operated from November 16,1978 until January 24, 1993 (Hoyt et al. 1992), performing measurements for a few minutes of each orbit during the passage of the Sun through the angle of view at the southern terminator of the spacecraft. Several

1409

1410 C. Frlihlich

Days(EpochJan0,1980) 0 2000 4000 6000 I" 1 I ” r I ” I ‘/

13721 T E L. 1370 8 s j 1368 k i

13641

RIM II + 0.120% -

1362t 1 I I I I I I I I I I I I / I I I I / I

787980818283848586878889909192939495969798 Year

Figure 1 Compared are daily averaged values of the Sun’s totalinadiance TSI. Each dataset is corrected for in-flight sensitivity changes and other instrumental effects, as described in the text.

corrections to the published record of HF data from Hoyt et al. (1992) have been identified and corrected. Data prior to the end of 1980 are adjusted downward cotresponding to a slip in the NIMBUS-7 orientation relative to the sun. Two other glitches occur around October 1, 1989 and May 8, 1990 when decreases of about 0.3 Wm-2 were observed each time in comparison with independent data from the Earth Radiation Budget Satellite (ERBS) and also by comparison with ground based evaluations (Lee III et al. 1995; Chapman et al. 1996). The assessment by FL98 confirmed these two glitches but with slightly different offsets of -0.26 and -0.32 Wmp2 respectively. Corrections for the HF radiometer degradation prior to 1982 were not possible in the original analysis by Hoyt et al. (1992) because of lack of information from a backup instrument. For this information FL98 utilized data from the Active Cavity Radiometer Irradiance Monitor I (ACRIM I) after correcting for early degradation (described below), and considered also the degradation of the PMO6- V radiometers (Anklin et al. 1998) of the Variability of solar IRradiance and Gravity Oscillations (VIRGO) experiment, presently flying on the Solar and Heliospheric Observatory (SOHO) (Frohlich et al. 1995; Frijhlich et al. 1997). The PM06 and HF radiometers have similar geometry of the receiver cavity and use the same black paint, thus the well established characterization of the degradation of the PMO6-V radiometers was used to estimate the behavior of HF.

The ACRIM I operated on the Solar Maximum Mission (SMM) spacecraft from February 14, 1980 until June 1, 1989 (Willson and Hudson 1991). The total degradation during 1980 was determined as 340 ppm (FL98) which is much more than the 180 ppm deduced by Willson and Hudson (1991). This difference is due to the fact that degradation measured after the repair of the SMM spacecraft in 1984 happened effectively in 1980, since the exposure time during the spin mode phase (Dec. 1980 - April 1984) was about hundred times less than during normal operation, and thus was equivalent to only about 15 days. This yields a downward adjustment of 160 ppm during 1980.

The Solar Monitor on ERBS, which employs a radiometer similar to that used in ACRIM (Lee III et al. 1987), has operated since October 25,1984 with a four month gap in 1993 when problems with the spacecraft batteries required the instrument to be switched off. As with the HF observations on NIMBUS-7, ERBS acquires solar irradiance data as the Sun drifts through the angle of view of the instrument, but only once in two weeks. No corrections to the published data have been applied which may be justified by the very short overall exposure time.

ACRlM II operates on the Upper Atmosphere Research Satellite (UARS) since October 1991 (Willson 1994), and the published data are used without corrections. Lie ACRIM I, the ACRIM II data derive from comparative measurements made by three radiometers with different rates of exposure to solar radiation. The scaling of ACRIM II to ACRIM I, as shown in Fig. 1 is performed by minimizing simultaneously the ratios to both the NIMBUS-7 and ERBS data (Frohlich 1998). However, we regard ACRIM II data during the first 8 months (until June 1992) as less reliable than the later ones since they seem to be influenced by the early exposure to space environment.

Total Solar Irradiance Variations Since 1978 1411

1369 1

1368 1 T E

z 1367 8 c $ 1366 2 i

1364

1363

Days (Epoch Jan 0,198O) 0 2000 4000 6000

I--‘I”‘l ’

78 79 80 al a2 a3 a4 a5 a6 a7 aa a9 90 91 92 93 94 95 96 97 98 Year

Figure 2 A prominent 1 l-year cycle is evident in the composite record of T SI compiled from detailed cross-calibrations of the valious mdiometric measurments (Fig. 1) from the end of 1978 to July 1998. the beginning of the SoHO vacations. The absolute scale of the composite record corresponds to the Space Absolute Radiometric Reference (SARR of Crommelynck et al., 1995).

. VIRGO radiometers (DIARAD and PMO6-V, see Frohlich et al. 1995) have operated on SOHO since January 18, 1996. The VIRGO data used are composed of the PMO6-VA values ‘adjusted’ to the long-term behavior of the DIARAD-L. The analysis of the degradation of the VIRGO radiometers by Anklin et al. (1998) emphasizes the difficulty in attaining levels of 10 ppm (0.001%) repeatability, even in the environment of minimal thermal, electrical and aspect perturbations secured on SOHO.

It is evident in Fig. 1 that no single solar radiometer has monitored solar irradiance throughout the last two decades. Furthermore, significant differences exist among the absolute levels of the various space based datasets because of uncertainties in their absolute radiometric calibrations which typically amount to ‘only’ about f0.2%. For this reason, reliable deductionof true solar irradiance variability from extant observations requires the construction of a composite record utilizing overlapping data among the various radiometer datasets for their cross calibration. The composite (Fig.2) uses ACRIM I values during 1980 and after 1984, following the spin-mode period of SMM, and the scaled ACRIM II after mid 1992. Until the advent of VIRGO on SOHO, ACRIM was the only experiment which secured a reliable in-flight determination of the degradation of its operational radiometer by virtue of regular comparisons with its second and third redundant receiver. These selected ACRIM data are augmented with measurements by the HF radiometers prior to the commencement of SMM in March 1980, during its spin-mode period from 1981 to 1984, and in the gap between the ACRIM I and II measurements. After observations commenced on SOHO VIRGO data are used in the composite. The absolute value of the composite TSI is adjusted to the Space Absolute Radiometer Reference (SARR, defined by Crommelynck et al. (1995)) with the SARR factor of ACRIM II. This adjustment allows comparison with other space experiments, but it should be noted that the uncertainty is not decreased by the procedure. The relative changes needed to adjust the different experiments to the composite are shown in Fig.3 which also illustrates very well the difficulties encountered for reliable long term monitoring.

COMPARISON WITH A MODEL

To further assess solar cycle irradiance changes an empirical model that parameterizes the combined influences of dark sunspots and bright faculae features on solar n-radiance since 1976 was developed, following the earlier approach of Foukal and Lean (1986)and described in FL98. Sunspots and faculae are magnetic features that when present in the solar atmosphere respectively reduce and enhance the overall level of radiation from the Sun. Their varying occurrence throughout the solar activity cycle is the primary cause of total irradiance variability ( Lean 1997; Frohlich and Lean 1998b; Foukal and Lean 1986). A daily time series of sunspot darkening of irradiance, called the Photometric Sunspot Index (PSI, see e.g. Foukal 1981; Frohlich et al. 1994),

1412 C. Frahlich

Days(EpochJanO,l980) 0 2000 4000 6000

2.01, 1 ( ' * g 1 ' ' ' 1 '_I

1.0 T

5 0.5

S i' A 5

0.0

a -0.5 ACRIM I-0.099% ACRIM II +0.148%

-1st I I I I I I I I I I I I I I I I L I I I I 787980818283848586878889909192939495969798

Year

Figure 3 Adjustments of the 5 TSI observation relative to the composite. There is a shift in absolute value indicated by the percentage correction, but also sudden and/or smooth adjustments in time due to degradation and/or operational effects. Note also the variation between different instruments as e.g. between VIRGO and ACRIM II during the period VIRGO is used as basis for the composite.

is constructed from sunspot data (area, position and extent) published in the Solar Geophysical Data catalogue by NOAA with assumptions on their contrast and limb-darkening function. For representing irradiance brightening by faculae the chromosphere Mg II index (ratio of the emission from the core of the Fraunhofer line near 280 nm to that from the wings) which is known to vary primarily in response to bright faculae (both in active regions and the surrounding network). A composite Mg II index was constructed as described in FL98 by using data from the Solar Backscatter Ultraviolet instruments (SBW) from 1978 to 1992 (Donnelly 1988), the Solar Stellar Intercomparison Experiment (SOLSTICE) thereafter (de Toma et al. 1997). and the He EW from 1976 to 1978 ( Harvey and Livingston 1994). ‘Ibe composite Mg II index is placed on the scale of the SBW Mg II index and extended by linear relationships determined from the data in periods of overlap. Although chromospheric proxies track bolometric brightness changes relatively well over solar cycle and active region time scales (FrGhlich and Lean 1998a; Foukal and Lean 1986), differences in center-to-limb variations and filling factors of the sources of variability in the chromospheric proxies limit their ability to track photospheric faculae brightness changes on times scales of days to months.

The fluctuations of sunspot darkening and facular brightening are shown in Fig.4 a linear combination of which produces a reconstruction of solar total irradiance that tracks closely the variations in the composite total irradiance record. The MgII index is a good proxy for solar W and EW emissions, but it is not this pecularity which is used to model TSI variations. On the short-term scale it is used as a proxy for the area of the faculae and the network related to active regions, similar to CaK images showing the projected area of plages. On longer time scales it can be used as an index for global changes of TSI related to the network outside active regions, and also including the influence of the bright bands evolving through the solar cycle by starting at high latitudes, migrating and extending towards the equator as the solar cycle progresses (e.g. Kuhn et al. 1988; Kuhn and Libbrecht 1991). Recognizing this basic difference, the MgII is separated into a smoothly varying longer term component related to the solar cycle and a shorter term component associated with rotational modulation, shown in Fig.4B. The data are first smoothed with a boxcar running average with a width of 217 days (-8 solar rotations) yielding (1%Zg11)*~,; the negative excursionsofMgII- (MgII)217, designated as neg (Mgl I - (A4gl Ijz17) , are again smoothed with the same filter and the two components are then calculated according to Eq. 1 and 2.

(1)

Mgll, = Mgll- MgIIl for MgII - Mglll P 0 = 0 for Mgll - Mglll ‘: 0

The factor f is determined such that Mgll~ becomes the lower envelope to the original MgII time series.

(2)

Total Solar Irradiance Variations Since 1978 1413

Days (Epoch Jan 0,198O) 2000 4000 6000

787980818283848586878889909192939495969798 Year

Figure 4 Shown in a) is the time-dependent contribution of facular brightening to solar total irradiance variability, parameterized by a composite of the Mg II index (from Fr&lich t Lean, 1998a). This total facular brightening (above the zero line) is compared with the calculated sunspot darkening (below the zero line, PSI) contribution to the total irmdiance variability. In b) the longer term of the facular brightening (smooth dark line,MgZZL) and daily components (MgZZs) are separated.

In FrGhlich and Lean (1998b)multiple linear regression analysis was used to determine the scaling of the sunspot darkening PSI, slowly varying Mg Il index MglIi and short term Mgll, facularproxies to represent the irradiance composite TSI. The multiple regression analysis

y = b + u+ru + + UkXC + + a,_lx,_l (3)

calculates the coefficients b and a~ from the m dependent time series Xk (in our case PSI, Mgl I,, MglIl) and the independent time series y (TSI). The square of the multiple correlation coefficient R amounts to

M-l M-l

R2= c ak Ci nk,iyi

k=,, Ci Y’ (4)

with the individual M terms in the sum describing the contributions of the partial correlation Pz of each time series to the total correlation R2. The correlation coefficient ryk between the time series xk and y is calculated by neglecting the influence of the other Xi as in the case of a single x versus y regression analysis and sXr and sY are the standard deviations. For the time period from 1978 until July 1998 this empirical 3-component model yields a correlation coefficient R2 = 0.828 with a share of 0.040, 0.160 and0.629 for PSI, Mgl I, and Mglll respectively. Note that the share of PSI for the explanation of the variance is only about 5%, whereas the longterm MgII index explains three quarters of the total explained variance (82.8%). The coefficients and their lo standarddeviations are for PSI 1.065 f 0.008,for Mgll, 88.1f1.4and for MgIZZ 121.1f0.9. The coefficient for the intbrence of PSI is close to one which means that the physical basis for the assumptions used to calculate PSI are quite adequate. The scaling of the long-term is N 1.4 times the one of the short-term component of MgJJ which may indicate that the physical effects controlling the influence of activity on TSI are different.

With all this in mind, it might be better to remove first the influence of the sunspots by adding PSI (multiplied by 1.066, the scaling Tom the 3-omponent model) to TSI. This empirical 2-component model yields a correlation coefficient R2 = 0.9 13 with a share of 0.251 and 0.663 for Mgll, and MglIl and is now explaining 91.3% of the variance of TSI + PSI. The determined coefficients (88.1f1.2 and 121.2f0.8) are unchanged because we use still the same parameters as for the 3-component model, only the uncertainty is slightly improved due to the higher correlation. The flux contribution of faculae and network on the irradiance has a strong angular dependence (limb-brightening, see e.g. Umuh et al. 2000) which is not reflected by the MgJJ proxy as it represents a sort of projected area of the faculae and does not mimic the angular distribution. As illustrated in Fig 5b

1414 C. FrBhlich

dayO=lODec1996

-20 -10 0 Days

10 20

Figure 5 The figure shows the influence of Eltering on the TSI + PSI and MgZZ,t time series. The different kernels applied are shown in a). The ‘dmmedar’ is used to filter Mgl&t and the optimal ‘camel’ for TSZ + PSI. The optimal Elter is determined by applying the Elters 1. . .6 and searching for the maximum as described in the text. Panel (b) shows the observations and the filtered time series for a petiod in May 1996 when only a very small and vanishing sun spot present and (c) for a period in November 1996 with a larg sun spot.

the limb-brightening leads to a double peak modulation of the irradiance during the passage of an active region; even if there is no sunspot in the region (as in May 1996). the irradiance is lower at the meridian transit than at 4-5 days before and after the transit. As the MgII index is behaving lie a projected area its peak influence is at the central-merdian transit. To account for these differences the time series of TSI + PSI and the short-termMg II-index are convoluted with a double-peak (‘camel’) and a triangular (‘dromedar’) filter of 17-day base width as shown in Fig. Sa, which indeed corresponds quite well to the behavior during the May event in Fig. 5b. For the camel filter we have different forms and the best filter is searched by applying filters i = 1. . .6 (Fig.Sa) to the time series TSI + PSI yielding (m)i; then we calculate the multiple linear regression of (m) with the dromedar filtered Mg I I, and Mg I II (the latter does not need to be filtered as it is the long-term part of the time series I The correlation coefficient for each of the 6 camel filtered time series is then fitted to a parabola to determine the maximum. In order to asses the improvement by the filter we compare the maximum correlation with the one of the dromedar filtered (m),, series as reference (the same filter for both time series). The result for the time-series from November 1978 until July 1998 is R& = 0.9422 compared to the dromedar filtered Ri = 0.9408. This difference seems rather small; but compared to the standard deviation of the parabolic fit of 4.5.10P5 the improvement is about 300. As shown in Fig. 5a the optimal filter is between 3 and 4 and the corresponding coefficients are 82.6f1.2 and 127.3f0.6. As the filtering of time series renders neighboring points dependent the correlation coefficient increases inherently; thus the increase from 0.913 to 0.942 does not mean that the explained part in the filtered time series has increased by about 3%. But the existance of a well delined maximum for the optimal filter indicates that the arguments for the different filters for TSI + PSI and MgII, are confirmed. The comparison of the original and filtered series as illustrated in Fig. 5b and c for an active region without (May 1996) and with (Nov. 1996) an important sunspot shows that in the latter case the maximum of TSI + PSI is not coincident with the central

Total Solar Irradiance Variations Since 1978

Days(EpochJan0,1980) 0 2000 4000 6000

1369t' 1 z 1 ’ s ’ straght line through both Wims (trend: -24 ppm/i la)

1368 b

2 6 1365 i? 3 l-

1364 TSI + PSI ---- -

TSI + PSI - Mgll. TSI + PSI - Mgll. - Mgll, -

1415

1363F 1.1 I I I I I I I I I I I I I I I II 787980818283848586878889909192939495969798

Year

Figure 6 The figure shows the influence of filtering on the TSI + PSI and Mg1Z.y time series. The different kernels applied are shown in a). The ‘dromedar’ is used to filter MgZZs and the optimal ‘camel’ for TSI + PSI. The optimal filter is determined by applying the filters 1. .6 and searching for the maximum as descxibed in the text. Panel (b) shows the observations and the filtered time series for a period in May 1996 when only a very small and vanishing sun spot was present and (c) for a period in November 19% with a large sun spot.

meridian passage of the spot and that the amplitude of the Mglls as determined from the whole time series is too small during this period of solar minimum. This indicates that most probably different scalings for the MgIl proxy is needed for different phases of the solar cycle, which may be evidence for another component modulating TSI related to the surface magnetic fields, but not directly connected.

The final result of this analysis is shown in Fig. 6 with the original TSI, the TSI + PSI, TSI + PSI - MgIIs and TSZ + PSI - MgZ I, - MgIll timeseries plotted. The latter represents a sun with the iniluence of acitiviy removed, a kind of ‘quiet sun’. A straight line through the mean of TSI + PSI - Mgll, - MgI Ii during each minimum indicates a trend of E* 2 pp&a. so the Sun may be regarded as constant after removing the influence of solar activity as described by PSI and the MgIl index. There are, however, significant deviations from this strait line which may - at least partly - be due to instrumental noise of the radiometers, and also noise in PSI and the MgII index. Since the last minimum in 1996, however, an important deviation from the strait line is observed as a much steeper increase of TSI is observed than the model would predict. This increase is also confumed by the independent ACRIMII and ERl3S observations; thus it is most probably not due to some yet undetected behavior of the VIRGO radiometers in space from which the composite TSI is deduced during this period of time.

CONCLUSIONS

The composite record of solar total irradiance since 1978 is used to evaluate the influence of solar activity on total irradiance. An empirical model that parameter&s the influences of bright faculae and the magnetic network on solar irradiance is able to explain about 91% of the variance of the spot comected irradiance TSI + PSI time series. After removing the magnetic influence with the model, the remaining ‘quiet sun’ shows essentially no trend over the whole period, indicating that the sun has not changed by more than about 50ppm during the past 20 years of observations The onset of the new cycle, however, looks quite different as the observed increase of the irradiance is much steeper than the model would suggest.

1416

ACKNOWLEDGEMENTS

C. FrGhlich

The VIRGO team is gratefully acknowledged which made these exceptional data possible. VIRGO is an experiment on SOHO, a space mission of international cooperation realized by ESA and NASA. PMOD/WRC is grateful to the Swiss National Science Foundation for financial support of the solar research.

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