At present, solar total irradiance measurements are made from four satellites using electrically self calibrating pyrheliometers, as a part of the earth radiation budget measurement programs. The Earth Radiation Budget (ERB) mission onboard Nimbus-7 spacecraft (Nimbus/ERB) started solar total irradiance measurements in November 1978, and is still obtaining irradiance data on every orbit, daily. The Earth Radiation Budget Experiment (ERBE) solar monitors onboard Earth Radiation Budget Satellite (ERBS), NOAA-9 and NOAA-10 started solar total irradiance measurements in October 1984, January 1985, and October 1986, respectively. Our knowledge of solar total irradiance and its variability has grown remarkably during the past few years, as a result of the above measurements, and the high precision data obtained from Solar Maximum Mission/Active Cavity Radiometer Irradiance Monitor-1 (SMM/ACRIM-1). The results from a comparative study of the solar constant data available from the above missions are presented. The solar constant value derived from the sensors agree within the uncertainty associated with absolute pyrheliometers available at present. An attempt will be made to correlate the solar irradiance variability with other solar parameters. The measurements from Nimbus-7/ERB started November 1978, as the solar cycle 21 was increasing in activity. The solar luminosity reached a maximum in the spring of 1979. The irradiance then decreased slowly to a minimum which lasted from 1984 through 1986. The irradiance is presently increasing towards a new maximum. It appears that the solar constant value follow an eleven year cycle

Kyle, H. L.

Lee, Robert B., III

Mecherikunnel, A. T.

English

NASA Technical Reports Server

SOLAR CONSTANT DATA FROM EARTH RADIATION BUDGET MEASUREMENTS 
A. T. Mecherikunnel, H. L. Kyle, 
NASA/GSFC, Greenbelt, Maryland 20771 
and 
R. B. Lee 111, 
NASA/Langley Research Center Hampton, Virginia 23665 
ABSTRACT 
At present, solar total irradiance measurements are made from four 
satellites using electrically self calibrating pyrheliometers, as a part 
of the earth radiation budget measurement programs. The Earth Radiation 
Budget mission onboard Nimbus-7 spacecraft (Nimbus-7/ERB) started solar 
total irradiance measurements in November 1978, and is still obtaining 
irradiance data on every orbit, daily. The Earth Radiation Budget Experiment 
(ERBE) solar monitors onboard Earth Radiation Budget Satellite (ERBS), 
NOAA-9 and NOAA-10 started solar total irradiance measurements in October 
1984, January 1985, and October 1986, respectively. The ERBE solar monitors 
are active cavity radiometers of similar design and mode of operation to 
the Active Cavity Radiometer Irradiance Monitor-1 (ACRIM-1) operated 
onboard Solar Maximum Mission (SMM) during 1980-1989. In the past, the 
ERBE missions obtained solar total irradiance measurements biweekly, 
mainly for the calibration check of earth viewing ERBE sensors. But, after 
the loss of SMM/ACRIM-1 in December 1989, the solar total irradiance data 
are taken daily from ERBE onboard NOAA-10, and weekly from NOAA-9. Our 
knowledge of solar total irradiance and its variability has grown remark- 
ably during the past few years, as a result of the above measurements, and 
the high precision data obtained from SMM/ACRIM-1. The results from a 
comparative study of the solar constant data available from the above 
missions will be presented The solar constant value derived from the 
sensors agree within the uncertainty associated with absolute pyrheliometers 
available at present. A-I attempt will be made to correlate the solar 
irradiance variability with other solar parameters. The measurements from 
Nimbus-7/ERB started November 1978, as the solar cycle 21 was increasing 
in activity. The solar luminosity reached a maximum in the spring of 
1979. The irradiance then decreased slowly to a minimum which lasted from 
1984 through 1986. The irradiance is presently increasing towards a 
new maximum. It appears that the solar constant value follow an eleven 
year cycle. 
INTRODUCTION 
The solar constant of radiation, is the most important of all physical 
quantities because of its influence on the life in the solar system. 
By the term Solar constant, we infer the intensity of the solar beam, in 
energy units per unit area normal to the beam, and at earth's mean 
distance (one Astronomical unit). The Earth's weather and climate are 
determined by the incomi3g solar radiation, and its interaction with the 
atmosphere, land surface nZd oceans. The radiation balance or the net 
https://ntrs.nasa.gov/search.jsp?R=19910003185 2020-03-24T06:54:40+00:00Z
radiation at the top of the atmosphere, resulting from the radiative 
exchange between the incoming solar radiation and the outgoing earth 
emitted thermal radiation, is the central determinant of the earth's 
climate. The radiation balance (R) can be expressed mathematically as 
follows . 
where, S is the Solar Constant, A is the planetary albedo, and F is the 
thermal radiation emitted to space from the earth-atmosphere system. The 
absolute value of the solar constant, is the key term that determines the 
Earth's albedo and the radiation budget. The determination of solar 
constant and its temporal variations is one of the prime goals of climate 
research. 
NIMBUS-7 Earth Radiation Budget - Mission 
The Earth radiation budget measurement programs determine the Earth Radiation 
Budget Components: the solar total irradiance, the earth reflected short 
wave, and the earth emitted thermal radiation. Because of its significance 
to weather and climate studies, the measurement of the Earth's radiation 
budget components from spacecraft platforms dates back to the early days 
of the NASA space program (House et al., 1986). The Nimbus-7/ERB, launched 
in November 1978, is the first ERB mission to make solar total irradiance 
measurements using an electrically self calibrating pyrheliometer (H-F or 
Hickey-Friedan sensor, also known as Channel 10C). The electrical self 
calibration provided an absolute radiance scale for comparison, while the 
cavity design reduced the sensitivity to degradation of the solar 
absorbing surface. The details of the sensor characteristics, data 
reduction and the results are published by Hickey et al, 1982, 1984, 1987), 
and by Hoyt and Kyle, 1990 (in this publication). The Nimbus-7 is a 
sunsynchronous polar orbiter, and the solar total irradiance measurements 
are made as the satellite crosses the southern terminator. There are 14 
orbits in a day and the orbital period is about 104 minutes. The solar 
disk is completely within the field of view (10 degrees) of the pyrhelio- 
meter for about 175 seconds, during each orbit. During the first four 
years, the Nimbus-7/ERB solar sensor operated on a 3 days on, 1 day off, 
duty cycle. In later years, the sensor has operated on a full time 
basis. The solar total irradiance measurements onboard Nimbus-7 continue 
to the present day, and the data set used in this study cover a period 
of 11 years and 3 months. The mean value of 1371.89 W m-2 is derived 
from the Nimbus-7/ERB for the period November 1978 through February 1990. 
The Earth Radiation Budnet - Ex~eriment (ERBE) 
The Nimbus-7 spacecraft, being a sunsynchronous polar orbiter, the earth 
radiant exitance measurements are confined to two local times, near noon 
and midnight; and the earth reflected data are confined to local noon 
only. This reduces the usefulness of the data for studies of the 
temporal variability of earth radiation budget components. The ERBE 
mission of the 1980's is designed to make improved measurements of the 
radiation budget components at various local times, to study the temporal 
and spatial variations of these components. In order to achieve this 
objective, the ERBE sense>rs make measurements from three spacecraft plat- 
forms: the National Aeronautics and Space Administration (NASA) Earth 
Radiation Budget Satellite (ERBS), and the two National Oceanic and Atmos- 
pheric Administration satellites NOAA-9 and NOAA-10, at different equator 
crossing times and orbital inclinations. The first of the three satellites 
carrying the ERBE sensors, the NASA ERBS, was launched into a 57 degree 
inclination orbit by Space Shutcle Challenger in October 1984. The NOAA 
operational meteorological satellites NOAA-9 and NOAA-10 were launched into 
near polar orbits in December 1984, and September 1986, respectively 
(ERBE Science Team, 1986). The ERBE solar monitors are identical electric- 
ally self calibrating pyrheliometers of active cavity type; and are similar 
in design and mode of operation to the SMM/ACRIM-1. In the ERBE measurement 
approach, the sun is allowed to drift through the field of view (13.6 degrees) 
of the solar monitors. The sensor characteristics, and mode of operation 
are given in detail by Lee et a1 (1987), and Lee (1990) in this publication. 
The Earth Radiation Budget Experiment (ERBE) sensor package consists of a 
solar monitor, and the earth viewing radiometers. The solar monitors make 
direct measurements of solar total irradiance, every two weeks, to provide 
data for earth radiation budget computations and to serve as a check on the 
radiometric calibration of earth viewing sensors. A mean value of 1365.15 
W m-2 for is obtained from ERBS/ERBE for the period October 1984 through 
December 1989. NOAA-9/ERBE gives a mean value of 1364.75 W m-2 for the 
solar constant for the period January 1985 through August 1989. The 
solar total irradiance data from NOAA-lO/ERBE are processed for a period 
of six months, October 1986-April 1987, and the mean solar constant value 
1363.24 W m-2, is obtained for this period. On April 1, 1987 the shutter 
mechanism of the solar monitor failed and the solar sensor remain open 
at all times. However, the solar total irradiance measurements are 
obtained daily from NOAA-10 spacecraft, from January 19, 1990, and from 
NOAA-9 every week. 
Intercomparison of Solar Constant Data 
Figure 1 shows the multiyear data set from the four satellites. The 
Nimbus-7/ERB solar data set covers a period of 11 years and 3 months 
(November 16, 1978 through February 1990). The SMM/ACRIM-1 data runs for 
a period of about 9 years from February 1980 through December 1988. The 
SMM/ACRIM-1 data is described by Willson et a1 (1984, 1988)) and Willson 
(1990) in this publication. The ERBS/ERBE data set is for a period of 5 
years and 2 months (October 1984 through December 1989), while the NOAA-9/ 
ERBE data covers the January 1985 through August 1989. It should be 
remembered that the data points consists of an average of 14 measurements 
per day for Nimbus-7/ERB; hundred of measurements for SMM/ACRIM-1, 'while 
ERBE measurements are the mean of two to three measurements on that 
particular day. The absolute calibration accuracy of the Nimbus-7/ERB 
Channel 10C, is quoted as +/- 0.5%, while the other experiments claim 
accuracy of 0.2% or better. The daily fluctuations in the irradiance 
values are observed in all the data sets. The maximum in the Nimbus-7/ERB 
irradiance value 1374.29 W m-2 occurs in March 1979, and shows a gradual 
decrease with time. It reached a minimum during 1984-1986 and then, 
increases gradually with time up to February 1990, the time the data sets 
are available at present. SMM/ACRIM-1 data set also shows a decreasing trend 
in irradiance values during 1985-1986, and then increases from the latter 
part of 1986. Both the ERBE measurements also show the decrease and 
increase observed for Nimbus-7/ERB and SMM/ACRIM-1. The increases and 
2 decreases are of the order of 0.1-0.28 (2-3 W m- ) .  The NOAA-9/ERBE data 
shows more scatter than the other data sets. The results of an earlier 
intercomparison of the Nimbus-7/ERB, SMM/ACRIM-1, ERBS/ERBE, NOAA-9/ERBE, 
and NOAA-lO/ERBE data sets are available in Mecherikunnel et a1 (1988). 
Figures 2 and 3 present the monthly and yearly mean value of the solar 
constant respectively, for the same period as shown for the daily mean 
values. The monthly mean irradiance value shows the same increasing and 
decreasing trend in all the four data sets. The minimum in solar constant 
value occurs in 1984-1985 for Nimbus-7/ERB. The SMM/ACRIM-1 and ERBS/ERBE 
show the minimum during 1985-1986. NOAA-9/ERBE shows a decrease in 
irradiance with time during 1986 to 1987, and then shows the increase 
starting 1987. The anomaly observed in the NOAA-9/ERBE data set is 
caused by factors other than solar, and needs further investigations. 
(Mecherikunnel et al, 1990). 
During the period 1985 through 1989 solar constant data are available 
from all four satellites for 71 days. It is to be considered that the 
data samplings vary for the 4 data sets. Table 1 lists the mean solar 
constant value, the standard deviation and the maximum and minimum observed 
during the 71 observation days. The solar constant value agree within 
the uncertainty associated with each solar sensor. 
TABLE 1 
SOLAR CONSTANT (IN W M - ~ )  FROM N-7ERB, SMM, ERBS AND NOAA-9 ON FOUR WAY 
MATCHING DAYS 1985-1989 
SENSOR SOLAR CONSTANT STD.DEVIATION MAXIMUM MINIMUM 
(MEAN VALUE) 
Figure 4 presents the Nimbus-7/ERB yearly mean solar constant, and the 
yearly mean sunspot number. Yearly mean value closely follow the 11 year 
sunspot cycle, as shown in the figure. The sunspot cycle 21 began in June 
1976, reached the maximum in 1979. Nimbus-7/ERB data shows the maximum 
value of solar constant in 1979 (March), and gradually decrease through 
the years reaching the minimum in 1984 with a gradual increase to 1990. 
But, the minimum of sunspot number occurs in 1986. It should be noted 
that the long term stability of the Nimbus-7/ERB data system is about 
0.3 W m-2. This is the first experimental evidence of an 11 year 
solar constant variability, which several investigators have speculated 
about for centuries. 
Solar Total Irradiance Variability and the Extraterrestrial Solar Spectral 
Irradiance 
The experimental results clearly indicate that there are fluctuations in 
the daily, monthly, and yearly mean solar total irradiance data. The 
irradiance variability f9llow the 11 year sunspot cycle as evidenced from 
Figure 4. How do the observed variations in the total irradiance affect 
the spectral distribution of the total irradiance? The solar spectral 
irradiance variations in the W and microwave regions are known and are 
being extensively studied. But very little is known about the variability 
in the visible and near infrared solar spectral region that contains more 
than 92% of the sun's energy. Almost all the radiation effects in the 
earth and atmosphere are wavelength dependent. Accurate data on solar 
constant and its spectral distribution in the W, Visible, IR regions are 
important in understanding various interactions in the earth-atmosphere 
system. This is especially significant in the Earth Observation System 
(EOS) investigations. 
CONCLUSION 
The comparative study of the solar constant data from the recent space- 
craft measurements indicate that the results are within the absolute 
radiometric accuracy of each sensor used in the measurements. The similar 
trends in solar irradiance variability is observed in all the data sets. 
The increases and decreases in the total irradiance observed are in step 
with the 11 year sunspot cycle. The results shows beyond any doubt that 
the variations observed are solar in nature. The Nimbus-7 ERB solar 
constant data set gives the first experimental evidence of an 11-year 
solar cycle in the total irradiance, analogous to the 11-year sunspot 
cycle. 
REFERENCES 
House, F. B., A. Gruber, G. E. Hunt, A. T. Mecherikunnel, History of 
Satellite Missions and Measurements of the Earth Radiation Budget (1957- 
1984), Reviews of Geophysics, Vol. 24, Pp. 357-377, 1986. 
Hickey, J. R., B. M. Alton, F. J. Griffin, H. Jacobowitz P. Pelegrino, 
R. H. Maschhoff, E. A. Smith and T. H. Vonder Haar, Extraterrestrial Solar 
Irradiance Variability two and one-half years of measurements from Nimbus- 
7, Solar Energy, Vol. 29, Pp. 125-127, 1982. 
Hickey, J. R., E. R. Major, and H. L. Kyle, User's Guide for the 
Nimbus-7 ERB Solar Analysis Tape (ESAT), TM 86143, NASA/Goddard Space 
Flight Center, Greenbelt, MD 20771, 82pp., 1, 1984. 
Hickey, J. R., B. M. Alton, H. L. Kyle and E. R. Major, The Solar 
Irradiance Measurements by the Nimbus-7 ERB Experiment, An Update of 100 
months, in Proceedings of a Workshop on Solar Radioative Output Variation, 
edited by P. Foukal, Pp. 189-194, Cambridge Research and Instrumentation, 
Cambridge, Massachussettes, 1987. 
Hoyt, D.V.,-and H. L. Kyle, An Alternative Derivation of the Nimbus-7 
Total Solar Irradiance Variations, 1990 (in this publication). 
ERBE Science Team, First data from the Earth Radiation Budget 
Experiment (ERBE), Bull. Am. Meteorol. Soc., Vol. 67, Pp. 818-824, 1986. 
Lee, R. B., B. Barkstrom, and R. D. Cess, Characteristics of the Earth 
Radiation Budget Experiment Solar Monitors, Applied Optics, Vol. 26, Pp. 3090- 
3046, 1987. 
Lee, R. B., Long-Term Solar Irradiance Variability: 1984-1989 
Observations, 1990 (in this publication). 
Mecherikunnel, A. T., R. B. Lee 111, H. L. Kyle, and E. R. Major, 
Intercomparison of Solar Total Irradiance Data From Recent Spacecraft 
Measurements, Journal of Geophysical Research, Vol. 93, No. D8, Pp. 9503- 
9509, 1988. 
Mecherikunnel, A. T., H. L. Kyle, C. R. Kondragunta, Principal 
Investigator Report, 27th ERBE Science Team Meeting, Orlando, Florida, 
April 19-20, 1990. 
Willson, R. C., Measurements of Solar Total Irradiance and Its 
Variability, Space Sciences Reviews, Vol. 38, Pp. 203-242, 1984. 
Willson, R. C., Solar Irradiance Variability During Solar Cycles 
21 & 22, 1990 (in this publication). 
Fig. 1. Daily Mean Solar Constant from: 1) Nimbus-7/ERB Nov. 1978- 
Feb. 1990; 2) SMM/ACRIM-1 Feb. 1980-Dec. 1988; 3) ERBS/ERBE Oct. 1984- 
Dec. 1989; and 4) NOAA/ERBE Jan. 1985-Aug. 1989. 
M o m  
NIMBUS-7 .+ SMM 0 ERBS A NOM-9 
Fig. 2. Monthly Mean Solar Constant from: 1) Nimbus-7/ERB Nov. 1978- 
Feb. 1990; 2) SMM/ACRIM-1 Feb. 1980-Dec. 1988; 3) ERBS/ERBE Oct. 1984- 
Dec. 1989; and 4) NOAA/ERBE Jan. 1985-Aug. 1989. 
Y m  
N-7 ERB +-  SMM O ERBS A NOM-9 
Fig. 3. Yearly Mean Solar Constant from: 1) Nimbus-7/ERB Nov. 1978- 
Feb. 1990; 2) SMM/ACRIM-1 Feb. 1980-Dec. 1988; 3) ERBS/ERBE Oct. 
1984-Dec. 1989; and 4) NOAA/ERBE Jan. 1985-Aug. 1989. 
+ SOL.CONSTA~T 
'(€ASS 
+ SUNSPOT NO 
Fig. 4. 11-year Solar Constant Data from Nimbus-7/ERB and the 11- 
year Sunspot Cycle. 


Solar Constant Data from Earth Radiation Budget Measurements

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