S O L A R T O T A L I R R A D I A N C E O B S E R V A T I O N S BY A C T I V E

C A V I T Y R A D I O M E T E R S *

R I C H A R D C. W I L L S O N

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif. 91103, U.S.A.

Abstract. Pyrheliometry, definition of the radiation scale in the International System of Units and monitor ing the variability of solar total irradiance have been a focus of research at the Jet Propulsion Laboratory since the mid 1960's. A series of automated, electrically self-calibrating, cavity pyr- hel iometers known as Active Cavity Radiometers (ACR's) was developed as part of this program. A series of ground based exper iments in 1968-69 led to the discovery of a systematic error in the International Pyrheliometric Scale. A C R ' s were among the instruments used to define the World Radiometr ic Reference in 1975.

A C R flight exper iments have been conducted to determine the 1 A U total solar irradiance and monitor its variability in time. A 1969 balloon exper iment yielded a 1366 W m -2 result. The value from a 1976 sounding rocket exper iment was 1368.1 W m 2. The results for two additional rocket experiments in 1978 and 80, revised in accordance with recent calibrations of A C R response to elevated pressures during these flights are: 1367.6 and 1367.8 W m -2, respectively. An A C R experiment (ACRIM) on the Solar Max imum Mission satellite has shown continuous variability of the total solar flux below the • level and two large, temporary decreases of 0 .1-0 .2% lasting more than a week. The mean 1 A U total flux for A C R I M ' s first five months ' observations was 1367.7 W m -2. Inflight comparison of A C R rocket and satellite measurements in May, 1980 demonst ra ted agreement to within :t:0.05%. The 1 A U total solar irradiance results f rom A C R rocket and satellite experiments between 1976 and 1980 differ f rom their mean of 1367.8 W m -2 by no more than • The less precise 1969 balloon result is 0.1% lower. Al though no observations were made from 1970-75, if solar behaviour in those five years was similar to that observed since 1976 then the upper limits of long term solar total irradiance variability are • for the 1969-1980 period and • between 1976 and 1980, based on the set of A C R observations.

1. Introduction

Interest in absolute radiometry and the solar constant at JPL began in the mid 1960's when the thermal behavior of JPL spacecraft departed from pre-flight predictions. It was not known whether the knowledge of the solar constant was in error, if the instrumentation used to calibrate the simulated solar irradiance during pre-flight spacecraft testing was inaccurate, or a combination of both. The answers were sought through development of new cavity pyrheliometer for absolute measurement of solar irradiance both in pre-flight testing of spacecraft and in flight experiments to measure the total solar flux. (Kendall et al., 1965; Plamondon and Kendall, 1965; Kendall and Berdahl, 1969; Willson, 1969, 1973a, 1979; Plamondon, 1969).

A series of instruments developed as part of this program for automated flight observations of solar total irradiance are electrically self-calibrated cavity pyr- heliometers whose mode of operation is characterized by the name Active Cavity Radiometer. The temperatures of their cavity sensors are servocontrolled, actively

* Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16-19 September 1980, Scheveningen, The Netherlands.

Solar Physics 74 (1981) 217-229. 0 0 3 8 - 0 9 3 8 / 8 1 / 0 7 4 1 - 0 2 1 7 $01.95. Copyright (~ 1981 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U,S.A.

218 R I C H A R D C. W I L L S O N

maintained at constant temperatures relative to their heat sinks by electrical heating. The solar irradiance is related to the International System of Units (SI) by measuring the difference in amount of electrical heating required with the cavity sensor alternately shaded and exposed to the Sun. A description of these instruments can be found in Willson 1969, 1973a, 1979),

2. Radiation Scale Definitions

Radiometer comparison experiments were conducted at the Solar Test Facility of the Jet Propulsion Laboratories' Table Mountain Observatory during 1968-69. Synchronous solar irradiance observations of several types of instrument were compared, including PACRAD's, ACR's and an Eppley Pyrheliometer (number 8420), the latter used as a U.S. reference for the International Pyrheliometric Scale of 1956 (IPS 56). The results demonstrated systematically lower irradiance observa- tions on the IPS 56 by 2.2% relative to the ACR (Willson, 1972) and by 1.8% relative to the PACRAD (Kendall and Berdahl, 1969). The SI uncertainties of the ACR and PACRAD instruments were less than +0.5%, making the IPS 56 differences significant.

Comparison experiments were conducted at the World Radiation Center in Davos, Switzerland during 1974-75 to investigate the apparent error in the IPS 56 and to define a new and more accurate scale to replace it, The synchronous solar irradiance observations of five electrically self-calibrated, cavity pyrheliometers (PMO-2, CROM-2, PACRAD III, ACR 310, ACR 311) during these comparisons were used to define the new scale, known as the World Radiometric Reference (WRR). The new scale is 2.2% above the IPS 56. (Brusa and Fr6hlich, 1975).

3. Balloon Observations

The first balloon experiment in August, 1968 used ACR type II sensors whose elevated-temperature mode of operation required a vacuum environment. These sensors were flown in evacuated containers at altitudes above 25 km and viewed the sun through quartz windows. Care was taken to calibrate the transmittance of these windows, but because of contamination during the flight their uncertainty became the limiting source of error for the experiment. The resulting mean solar constant value, after corrections for Earth-Sun distance and the transmittance of the atmos- phere and quartz windows, was 1369.5, but with a large SI-uncertainty of nearly • (Willson, 1971).

A second balloon experiment was conducted in August, 1969 with an ACR (type III) sensor capable of operating at all pressures, eliminating the vacuum requirement and quartz window. The 1969 observations at36 km altitude produced a 1 AU total solar irradiance value of 1366 W m -2 with an SI uncertainty of • (Willson, 1973b).

S O L A R T O T A L I R R A D I A N C E O B S E R V A T I O N S 219

Fr6hlich later adjusted this value to the WRR, using ground comparison results, yielding a 1369 W m -2 result (Fr/Shlich, 1977). Recent advances in the technology

of cavity reflectance measurements at the U.S. National Bureau of Standards has provided a more accurate value for the effective absorptance of the balloon's diffuse, black, cavity, detector (Zalewski et al., 1979). The new value (0.99777) is 0.19% higher than that used in the original computation of the balloon results and more accurately determined. The 1 A U total solar flux from the 1969 observations is then 1366.4 W m -2 on the WRR.

4. Rocket Experiments

A series of solar irradiance rocket flight experiments was begun in 1976 to provide independent, in-flight comparisons with satellite solar total flux observations. The rocket payload included three types of self-calibrating cavity pyrheliometers: a P A C R A D , an ESP and two ACR's. Its first flight on June 29, 1976 provided a direct comparison with the Nimbus 6 / E R B solar sensor (Hickey, 1976). The mean 1 A U solar total flux for the self-calibrating rocket sensors was 1367 W m -2, 1.6% lower than the Nimbus 6 / E R B value (Duncan et al., 1977). The ACR rocket result was 1368.1 W m -2 with an SI uncertainty of less than +0.2% (Willson and Hickey, 1976; Willson, 1978).

A second ACR solar irradiance rocket experiment was conducted in Nov., 1978 to provide a comparison with the Nimbus 7 / E R B / H - F experiment (Hickey et al., 1980). The 1 AU solar total flux result originally reported for the ACR was 1373.4 W m -2 (Willson et al., 1980). Results for the other rocket sensors have not been published. Ground comparisons conducted in air prior to the 1976 and after the 1978 rocket flights indicated an ACR repeatability over this period near the +0.1% level, giving apparent significance to the 0.39% increase in ACR flight results obtained in the two experiments.

The data from the first two rocket flights indicates different payloa.d pressures in the two years: Less than 10 -4 Torr in 1976 and greater than 1.0 Torr in 1978. The significance of the pressure difference on the observations was not understood at that time. A recent experiment conducted to explore the rocket ACR's pressure sensitivity (discussed in Section 7, below) shows that the higher 1978 result was principally an artifact of the higher pressure during that flight (Willson, 1980). Corrected to the high vacuum pressure range of the 1976 rocket experiment, the 1978 result is 1367.6 W m -2, 0.60% lower than the Nimbus 7 / E R B / H F result, and 0.04% below the 1976 ACR rocket result.

The solar irradiance rocket payload was flown again in May, 1980 to provide a comparison with the ACR experiment (ACRIM) on the Solar Maximum Mission. The mean 1 AU total irradiance for the ACR, P A C R A D and H-F rocket sensors was reported as 1374 W m -2 (Duncan et al., 1980). The ACRIM result was 1367.1 W m -2 for the day of the rocket flight (May 22, 1980), 0.50% lower than the rocket result and 0.46% lower than the preliminary ACR rocket value of

220 R I C H A R D C, W I L L S O N

1373.5 W m -2. The pressure in the rocket payload was above 1.0 Torr in 1980 and the ACR result higher than ACRIM's by about the same amount as the difference between the 1976 and uncorrected 1978 rocket results. The 1980 rocket value, corrected for its pressure dependence, is 1367.8 W m -2, less than 0.05% higher than SMM/ACRIM's 1 3 6 7 . 1 W m -2 result but 0.51% lower than the 1374.8 W m -2 mean of the P A C R A D and H-F rocket sensors.

5. Solar Maximum Mission Experiment

The Active Cavity Radiometer Irradiance Monitor (ACRIM) experiment was launched on the N A S A Solar Maximum Mission (SMM) spacecraft in February 1980 to make regular observations of the solar total irradiance. The principal goals of the experiment were: (1) To begin a climatological data base on solar irradiance variability to be extended over at least one solar magnetic cycle (about 22 years) with +0.2% long term precision and an SI uncertainty of less than +0.5% ; and (2) to provide a shorter term data base (minutes to months) with maximum precision and accuracy for studying aspects of solar variability significant to solar physics investiga- tions.

The AC R I M instrument employs three Active Cavity Radiometer type IV sensors. (Willson, 1979) (see Figure 1). Each A CR sensor is an independent, electrically self-calibrated cavity pyrheliometer, capable of defining the radiation scale at the solar total irradiance level with a precision and accuracy not previously achieved in satellite observations. The sensors view the Sun through a circular five degree field of view defined by their common heatsink. Wavelength sensitivity is nearly uniform from the UV through the far IR with a cavity absorptance of 0.9995 (Zalewski et al., 1979). Separate shutters on each sensor facilitate their operation with different frequencies for all possible combinations in either automatic or

manual modes. The three sensors are used in various combinations to provide periodic cross ~eferences on the system's performance. This phased use of the three channels is designed to sustain the precision of ACRIM's observations within the +0.1% level for at least one year. Planned flight comparisons with the rocket and space shuttle payloads should sustain the multi-year precision of ACRIM within

the +0.1% level for the life of the Solar Maximum Mission. The ACRIM observations over the first 153 days of the Solar Maximum Mission

are presented in Figure 2. Shown are the mean values for each orbit as measured by channel A, adjusted to give the total solar irradiance at 1 AU, plotted as a percentage variation about the mean value for the 153 day period.

Each orbital mean represents an integration over the solar observing portion of an orbit. The 32 individual samples in each of the shutter open periods are integrated and the results averaged to form the orbital means. In one orbit there are a maximum of 28 shutter-open solar observation periods providing as many as 896 individual measurements.

BAFFLED VIEW LIMITING HEA

S O L A R T O T A L I R R A D I A N C E O B S E R V A T I O N S

THERMISTOR TEMPERATURE / ~ HEAT Sl

PRIMARY APERTURE ~ S E N S O R ~ / ACR IV DUAL CAVITY DETECTOR

221

FIELD-OF-VIEW DEFINING APERTURE

REFLECTIVE

SOLAR

Fig. 1.

S HUTTERS~ ~ ~ ' ~ ~ ELECTRONIC CIRCUIT �9 I J C,,CU,TBOAROS OPTICAL ALIGNMENT

The active cavity radiometer irradiance monitor instrument on the Solar Maximum Mission.

The irradiance measured by ACRIM is corrected for the following effects (in order of significance): (a) Normalization to 1 AU distance, based on linear interpolation from the ephemeris distance plus the projection of the satellite orbit on the radial direction to the Sun; (b) correction for the slow decrease in channel A's sensitivity between days 62 and 163; (c) temperature-dependent corrections for radiation lost through the aperture and for the temperature coefficient of resistance of the cavity heating elements; (d) correction for relativistic radiative effects due to the Sun- satellite relative velocity; (e) correction for the cosine of the angle between ACRIM's line-of-sight and the Sun's center. These corrections are small and only the first two exceed 0.01%. At the present level of correction for systematic effects, the lo- standard error of the relative measurements is frequently as small as 0.001% for a one-orbit average.

The weighted mean value of the 1 AU total solar irradiance for the first 153 days of the SMM is 1367.7 W m -2 with an SI uncertainty of less than -+-0.2%. This value, although derived principally from channel A observations, is reported on the scale defined by the mean performance of all three ACRIM sensors in flight. It includes the effects of all observed variations including the large, temporary decreases in

222 R I C H A R D C. W I L L S O N

1 9 8 6 D A T E =

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1 8 0 1 9 S

8.185

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- 8 . 1 8

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Fig. 2. The 1AU total solar irradiance measured by the ACRIM channel A sensor is shown as a percentage variation about the weighted mean for the first 153 days of the Solar Maximum Mission. The individual tic-marks represent the mean irradiance for the sunlit portion of one orbit. The vertical bars through each tic-mark are the (• 1~) standard errors of the orbital means. The tic-marks with associated 'B' and 'C' designations are the channel B and C measurements during comparisons with channel A.

irradiance centered at days 99 and 146. The weighting factors used in forming the mean are the inverse squares of the standard errors.

Three principal-in-flight intercomparisons of A C R I M sensors were conducted, the first on day 62 after allowing two weeks for the spacecraft and its instruments to come to equilibrium, and the second and third on days 163 and 199. The comparison on day 62 shows a range of disagreement between ACRIM's three

independently calibrated sensors of less than +0.05% from their mean. The change in detector sensitivity for channel A (the continuously operating channel) from day 62 to 163 was less than - 0 . 0 2 % . This drift stabilized by day 163 and no further

change was detected on day 199. A slow degradation of channel A's cavity

absorptance, an anticipated effect of solar U V and particle fluxes, is calibrated by channels B and C, removing its effect on ACRIM's long term data base to within the 0.0015% change in the C/B ratio over the period. The 1 A U irradiance record of Figure 2 has been corrected for channel A's drift.

The most obvious features of the A C R I M observations are the two large tem- porary decreases in early April and late May, 1980. The events, centered at days 99 and 146 appear to be related to the evolution of specific groups of sunspots. An analytical approach developed by Hudson (Willson and Hudson, 1980a, b; Willson et al., 1980b) to study the relationship between sunspot behavior and total flux shows a high correlation between the flux deficits it predicts for the April decreases and McMath regions 2370 and 2372, and between the May decrease and McMath

S O L A R T O T A L I R R A D I A N C E O B S E R V A T I O N S 223

regions 2469 and 2420. In both cases the sunspot groups were initially in a phase of rapid growth. It has not been possible to accurately model the excess flux

contributed by white light faculae. The dominance of the sunspot signal in the April and May irradiance events has

significance for convective energy flow in solar active regions, at least in these two cases in which short term energy balance between sunspot deficit and facular excess does not occur. The conclusion is that the sunspot-deficit energy is stored and /or redistributed within the convection zone through the effects of the magnetic fields associated with the sunspot groups involved. Subsequent development of faculae associated with these regions may provide the mechanism for radiating the stored

energy away. The second interesting feature of the ACRIM irradiance record is its continuous

variability at smaller amplitudes than those of the big dips. Preliminary time series analyses of daily means shows results typical of a band-limited noise spectrum (Figure 3). The most significant feature is a cut-off of spectral power density at about seven days. This is consistent with development of sunspot groups and associated facular areas occurring at unrelated intervals and locations and taking at least seven days to complete their evolution.

The two dominant peaks of the power spectrum bracket the mean solar rotation period. The spectrum of the PSI adjusted flux (Figure 3b), which has most of the effects of the two big dips removed, shows a persistence of these same two peaks near 43 and 26 days, but with less relative intensity, indicating that a solar rotation periodicity may be resolved by a longer data base.

6. Comparison Experiments

Pyrheliometers provide the most accurate means of defining the radiation scale at the solar total flux level in the International Systems of units (SI). Comparisons of simultaneous observations of solar irradiance are a useful test of the design of pyrheliometers and the status of radiation scale definition.

A series of comparisons were conducted prior to the launch of the ACRIM experiment on the Solar Maximum Mission to relate the performances of ACRIM's sensors to various rocket and reference sensors. These comparisons, with all the instruments at ambient atmospheric pressure, are summarized in Table I as the ratio of the performance of each instrument to the rocket ACR's channel A detector (ACR402A). ACRIM's three sensors agreed to within +0.2% about the mean of their ratios.

In-flight comparisons of ACRIM's sensors in the vacuum environment of the Solar Maximum Mission demonstrated agreement to within +0.05% of the mean of their observations (Willson et al., 1980b). The in-flight ratio of A C R I M / A C R 4 0 2 A was initially determined to be 0.9953 by a May 22, 1980 rocket experiment, prior to correction of ACR402's pressure dependence.

224 RICHARD C. WILLSON

.

0 .

0 . 0 9

42.7 18.3 11.6 8.S 6 . 7 S.6 4 .7

SMM/ACRIM IRRADIANCE VARIATIONS POWER SPECTRUM OF DAILY MEANS

128 .0 2 5 . 6 14 .2 9 . 8 7 .S 6.1 5 .1

PERIOD (DAYS) :

4 . 4

Fig. 3a. Power spec t rum of the A C R I M ' s daily mean irradiance record for 1980 days 52 through 179.

I N T E N S I T Y

0 .

0 .

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O.OC"

42 .7 18.3 I t .6 8.S 6 .7 S.6 4 .7

SMM/ACRIM IRRADIANCE VARIATIONS POWER SPECTRUM OF DAILY MEANS

1 2 8 . 0 2 5 . 6 1 4 . 2 9 . 8 7 . 5 6 . 1 S . I

PERIOD CDAYS) : Fig. 3b. Power spec t rum of the PSI-adjusted irradiance for the same period.

4 . 4

SOLAR TOTAL IRRAI)IANCE OBSERVATIONS 225

TABLE I

Ground-based intercomparisons of ACR and other pyrheliometers shown as their ratio to the ACR 402A rocket instrument. A discussion of the ACR's can be found in Willson (1973a, 1979) and of the other instruments in Kendall and Berdahl, (1969), Fr6hlich,

(1977), and Crommelynck (1981).

Instrument Principal use Developer Ratio to ACR 402A

PMO 2 PMO 6-G CROM 2 L PACRAD VI PACRAD R ESP

A C R ' s 402B 401 403 404 405 ACRIM A ACRIM B ACRIM C

Reference sensor Fr6hlich/Brusa 0.99471 Reference sensor Fr/Shlich/Brusa 0.99857 Reference sensor Crommelynck 0.99288 Reference sensor Kendall 0.99158 Rocket sensor Kendall 0.99956 Rocket sensor Hickey 1.0006

Rocket sensor Willson 0.99955 Reference sensor Willson 1.00176 Reference sensor Willson 1.00198 Reference sensor Willson 1.00220 Reference sensor Willson 1.00251 Solar maximum sensor Willson 1.00441 Solar maximum sensor Willson 1.00069 Solar maximum sensor Willson 1.00340

7. Pressure Dependence of the Rocket ACR

T h e large change in the A C R I M / A C R 4 0 2 A ra t io b e t w e e n the a m b i e n t a t m o s p h e r i c

p re s su re e n v i r o n m e n t of the pre- f l igh t compar i sons (1,0028) and the high v a c u u m

of s p a c e f i g h t o p e r a t i o n (0.9953) was an u n e x p e c t e d resul t . Tha t p y r h e l i o m e t e r s

expe r i ence a change in sensi t ivi ty b e t w e e n air and v a c u u m was a wel l k n o w n effect

of the p o w e r exchange b e t w e e n sensors and thei r su r round ings by air conduc t ion

and convec t ion tha t is absen t in high vacuum. I t was as sumed , howe,0er, tha t all

s ignif icant p re s su re d e p e n d e n t effects were r e m o v e d f rom i r r ad iance obse rva t ions

by e i the r the shu t t e r ed act ive m o d e of o p e r a t i o n e m p l o y e d by the A C R , P M O , and

C R O M ins t ruments or by empi r i ca l cor rec t ions in the case of pass ive ly o p e r a t e d

in s t rumen t s such as the P A C R A D , ESP, and H - F . Ye t res idua l p ressu re effects have

been r ecen t ly d i scove red for severa l of these p y r h e l i o m e t e r s tha t a re signif icant at

the + 0 . 1 % level of unce r t a in ty and prec i s ion (Cromme lync k , 1981; F r6h l i ch and

Brusa , 1981).

Tes ts were r ecen t ly c o n d u c t e d at the J P L Tab le M o u n t a i n Solar Tes t Fac i l i ty to

d e t e r m i n e the p re s su re d e p e n d e n c e of A C R 4 0 2 ' s p e r f o r m a n c e . The resul ts showed

a i r / v a c u u m i r r ad iance m e a s u r e m e n t ra t ios of 1 .0042 and 1.0037 for t h e . A and B

sensors , respec t ive ly , wi th an unce r t a in ty of less than +0 .0005 . Fu r the r , mos t of the

change occurs wi th in the 0.5 to 0.05 To r r p re s su re range , wi th no signif icant change

a b o v e 1.0 T o r r or b e l o w 10 4 Tor r .

226 R I C H A R D C. W l L L S O N

The pressure dependence of the ACR402 shutter-closed, cavity heater power determined in the Table Mountain tests facilitates a determination of the pressure experienced by the instrument during the three rocket flights. Analysis of the shutter-closed flight data shows clearly that the 1976 observations were made with the ACR402 sensors at pressures lower than 10 -4 Torr, and in 1978 and 1980, at pressure above 1.0 Torr. The effects of the air/vacuum correction must be applied to the 1978 and 80 results to determine the 1 AU fluxes that should have resulted had the observations been made at pressures comparable to those of the 1976 rocket and SMM/ACRIM experiments. These corrections produce an improvement in internal consistency of the set of SMM/ACRIM and rocket results that is immedi- ately apparent from Table II by comparing ratios of originally reported and corrected values to the 1976 rocket result. The agreement of'the results of all four experiments within +0.02% of their mean is strong support for our conclusion that residual pressure effects increased the results of the second two rocket experiments in a highly systematic way. The 1 AU total solar irradiance results from the ACR402 rocket instrument and the SMM/ACRIM flight comparison on May 22, 1980 would have agreed closely had the rocket instrument's sensors operated in a high vacuum environment. The irradiance increase of 0.4%, previously reported on the basis of the first two rocket flight experiments (Willson et al., 1980a), was an artifact of the higher pressure environment in the second flight.

8. Discussion

The cavity pyrheliometers developed at the Jet Propulsion Laboratory over the past 15 years have been used in various experiments to decrease the SI uncertainty of the radiation scale for total solar irradiance observations both within and outside the Earth's atmosphere. The World Radiometric Reference scale is within • of the level first established by experiments at JPL's Table Mountain Observatory

in 1968-69. Detection and calibration of the air/vacuum performance ratio for the rocket and

solar Maximum Mission A C R instruments, the more accurate knowledge of the balloon ACR's cavity absorptance and the series of instrument intercomparisons have significantly decreased the bounds of possible solar total irradiance variability over the 1969-1980 period. The smallest bounds are for the 1976-80 period of the rocket and Solar Maximum Mission experiments. The 1 AU total solar irradiance derived from these measurements are all within +0.02% of their unweighted mean (1367.8 W m-2). The slope of a line fitted to these four observations by linear regression is less than 0.01%/yr, too small to be significant at their level of long term precision of about +0.1% over the four year period.

The decreased uncertainty of the 1969 balloon ACR's cavity absorptance and, therefore, its 1 AU solar total irradiance result makes its inclusion in the solar variability study of renewed interest. No direct comparisons were possible between the ACR III No. 3 instrument of the 1969 balloon experiment and the ACR402A

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228 R I C H A R D C. W I L L S O N

rocket instrument, but their measurements can be related through intercomparisons

with a common third instrument: Kendall 's P A C R A D III. The uncertainty of this

approach will be greater than direct comparisons of the flight instruments but should

provide a long term comparability of their flight results with a precision of ~:0.2%

or less. The ratios determined in air for the balloon A C R / P A C R A D III and the

rocket A C R / P A C R A D III were 1.001 and 1.0048 during tests at the JPL Mountain

Solar Test Facility in 1969 and 1976, respectively. These, together with the

A C R 4 0 2 A air/vacuum ratio (1.0042) indicate that the radiation scale defined by

the 1969 balloon A C R was less than 0.05% lower than that defined by the

A C R 4 0 2 A in vacuum. The difference is not significant with respect to the precision

limit of the comparisons and no correction is applied to the balloon result

(1366 W m-2). The unweighted mean 1 A U total solar irradiance from 1969-1980 is 1367.4 W m -2 (using February-July, 1980-ACRIM mean result of 1367.7 W m -z)

and all observations are within +0.1% of that value. A linear regression performed

with these five experimental results shows a positive slope of 0.01 .% in solar output

over the period that is not significant at the 0.1 to 0.2% level of long term precision

of the measurements.

In summary, while variations of a few tenths of a percent in solar total irradiance

lasting no more than a week or two have been detected, no long term trends are

identified from the A C R observations that are significant at the +0.1% level. This

does not rule out climatologically important solar variations over time scales of

decades or longer, but their detection will require a vigorous experimental program

with precision and accuracy equal to or better than the S M M / A C R I M observations.

Acknowledgement

The S M M / A C R I M experiment is supported by the National Aeronautics and Space

Administration under grants NAS-7-100 at the Jet Propulsion Laboratory.

References

Brusa, R. W. and Fr6hlich, C.: 1975, Scientific Discussions of IPC IV, WRC Publ. No. 534, WRC, Davos, Switzerland.

Crommelynck, D.: 1981, Solar Phys. 74, 509 (this volume). Duncan, C. H., Harrison, R. G., Kendall, J. H., Willson, R. C., Hickey, J. E., and Thekaekara, M. P.:

1977, 3-. AppL Opt. 16, 2690. Duncan, C. H. et al.: 1980, '1980 Rocker'Measurements of the Solar Constant', NASA/GSFC,

Greenbelt, Maryland, in press. Fr6hlich, C.: 1977, in O. R. White (ed.), The Solar Output and Its Variation, University of Colorado

Press, Boulder, Colo. Fr6hlich, C. and Brusa, R.: 1981, SolarPhys. 74, 209 (this volume). Hickey, J. R. et al.: 1976, 'Extra-Terrestrial Solar Irradiance Measurements from the Nimbus 6 Satellite',

Proc. Joint Conference on Sharing the Sun, Winnipeg, Manitoba, Canada. Hickey, J. R., Stowe, L. L., Jacobowitz, H., Pellegrino, P., Masckoff, R. H., House, F., and Vonder Haar,

T. H.: 1980, Science 208, 28i. Kendall, J. M. Sr. and Berdahl, C. M.: 1969, J. Appl. Opt. 9, 1082.

SOLAR TOTAL IRRADIANCE OBSERVATIONS 229

Kendall, J. M. Sr., Haley, F., and Plamondon, J.: 1965, Cavity Type Absolute Total Radiation Radiometer, Proc. Instr. Soc. Am., Oct. 4-7, Los Angeles, Calif.

Plamondon, J. A.: 1969, JPL Space Programs Summary, 37-59, Vol. III, p. 162, Jet Propulsion Lab, Pasadena, Calif.

Plamondon, J. A. and Kendall, J. M. Sr.: 1965, JPL Space Prog. Summary, 37-35, Vol. IV, Jet Propulsion Lab, Pasadena, Calif.

Willson, R. C.: 1969 JPL Tech. Rept. 32-1365, Jet Propulsion Lab, Calif. Willson, R. C.: 1971 J. Geophys. Res. 76, 4325. Willson, R. C.: 1972 Nature 239, 208. Willson, R. C.: 1973 3-. AppL Opt. 12, 810. Willson, R. C.: 1973 Solar Energy 14, 203. Willson, R. C.: 1978 J. Geophys. Res. 83, 4003. Willson, R. C.: 1979 J. Appl. Optics 18, 179. Willson, R. C.: 1980. 'Results of the Active Cavity Radiometer Experiment on the Solar Maximum

Mission', Worksho ~ on Solar Variability, NASA-GSFC, Greenbelt, Maryland. Willson, R. C. and Hickey, J. R.: 1977, The Solar Output and Its Variation, University of Colorado Press,

Boulder, Colo., p. 111. Willson, R. C. and Hudson, H. S.: 1980a, Adv. Space. Res. 1, 285. Willson, R. C. and Hudson, H. S.: 1980b, 'Variations of Solar Irradiance', accepted for publication by

the Astrophys. J. Letters. Willson, R. C., Duncan, C. H , and Geist, J.: 1980a, Science 207. Willson, R. C. et al.: 1980b, Science 211, 700. Zelewski, E., Geist, J., and Willson, R. C.: 1979, Proc. Soc. Phot. Opt. Instr. 196, 152.