Measurements of solar total irradiance and its variability
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MEASUREMENTS OF SOLAR TOTAL IRRADIANCE AND ITS
RICHARD C. WILLSON
Jet Propulsion Laboratory, Calif. Inst. of Technology, Pasadena, CA 91103, U.S.A.
(Received 20 March, 1984)
Abstract. The developmen t of electrically self calibrated cavity pyrheliometric instrumentation that occurred in the early 20th century provided the technological base for experiments to detect variability of the solar total irradiance. Experiments from ground based observatories, aircraft and balloons during the 1st half of the 20th century were unable to achieve sufficient accuracy or long term precision to unambiguously detect irradiance variations of solar origin. Refinements in pyrheliometric technology during the 1960's and 1970's and the accessibility of extended experimental opportunities above the Earth's atmosphere in recent years have provided the first direct observations of solar total irradiance variability and provided the cornerstone observations of a long term database on solar irradiance. A program of solar irradiance monitoring has evolved to sustain the database over at least 22 years, corresponding to a single cycle of solar magnetic activity, and the shortest well identified cycle of climate variation. Direct links between total irradiance variations, solar magnetic activity and the solar global '5 rain' oscillation phenomena have been derived from recent space flight observations by the SMM/ACRIM I experiment.
The nature of the emission of radiation by the Sun is of overriding importance to all life on Earth. The Earth's weather and climate are completely determined by the amount of incident solar radiation and its interactions with the Earth's atmosphere, oceans and land masses. Sustained variations in the. total solar energy received by the Earth from the Sun, the solar total irradiance, could have significant effects on both. Variation of solar total irradiance may have caused some of the many past changes in the Earth's climate.
In spite of its importance, solar irradiance has not been systematically measured in the past. Total irradiance observations with the short term accuracy and long term precision required to unambiguously detect solar variations and provide a quantitative database for climatology and solar physics investigations did not begin until 1980 with the launch of the Solar Maximum Mission (SMM) by the National Aeronautics and Space Administration (NASA).
Because of the long thermal time constants of the Earth's oceans and polar ice caps, many years of such data will be required to understand the climatological significance of solar variability. The principal usefulness of the solar monitoring data- base on shorter timescales is in the field of solar physics. The discovery of solar total irradiance variability by the SMM and its relationship with the evolution of solar active regions has provided new insight into the physics of the outer solar atmosphere.
Space Science Reviews 38 (t984) 203-242. 0038-6308/84/0383-0203506.00. 9 1984 by D. Reidel Publishing Company.
204 RICHARD C. WILLSON
The detection of the total irradiance signature of solar global '5 min' oscillation by SMM will aid solar physicists in understanding the behavior of the Sun's interior.
The format of this review is a series of critical discussions of the salient research topics relevant to measurement of solar irradiance variations: the development of the present state of the art in solar total irradiance measurement technology, the historical evolution of experimental efforts to measure solar variability, a description of the results of the first such observations, a discussion of their potential significance for solar physics and climatology and an assessment of the experimental work required to sustain and improve the solar variability data base in the future.
2. Solar Irradiance Measurement Technology
The most accurate technology for measuring solar total irradiance is a specialized form of radiometry referred to as pyrheliometry in which the heat produced by the absorption of solar radiation is compared with the heat produced on the same detector by the dissipation of a known amounli of electrical power. At the solar total irradiance level (1368 W m- 2) pyrheliometric detectors are the most accurate means of defining the radiation scale.
Two principal types of pyrheliometric detectors have been used in solar observa- tions. The first is an absolute or electrically self-calibrating detector which can relate radiation measurements to the International System of units (SI) by its stand-alone operation. The second basic type is a relative detector that simply provides a signal in response to solar heating and must be calibrated by comparison to an electrically self-calibrating pyrheliometer to produce a quantitative result in SI units.
2.2. THE ]kNGSTROM AND SMITHSONIAN INSTRUMENTATION AND SCALES
Several radiation 'scales' have been used during the 20th century for referencing solar irradiance measurements to SI units. (A summary of these scales, their relationships and references to these early works is presented in FrOhlich, 1973.) Each has been based on the performance of one or more electrically self calibrated pyrheliometers. The Angstrrm scale, defined in 1905 and based on the performance of Angstr/3m electrically self calibrated pyrheliometers developed in Sweden in the late 19th century, has been widely used to reference meteorological solar irradiance measurements. Electrically self calibrated cavity pyrheliometers developed in the early 20th century at the Smithsonian Institution in the U.S. were the references for the 1913 Smithsonian scale used as the standard for their long term program to detect solar variability.
Solar observations on the two scales differed by about 5~o until a downward revision of the Smithsonian scale in 1932 reduced the difference to 2.5~o. The Inter- national Pyrheliometric Scale of 1956 (IPS 56), was adopted by international agree- ment in that year to provide a common reference for solar irradiance measurements during the International Geophysical 'Year' (1957-1958). Although intended to be a
MEASUREMENTS OF SOLAR TOTAL IRRADIANCE AND ITS VARIABILITY 205
compromise between the Smithsonian 1932 scale and the ,~ngstrOm 1905 scale, it was subsequently found to be much closer to the latter (Fr~Shlich, 1973, 1977). The IPS 56 was employed as the principal international reference for solar observations from 1956 until the mid 1970's.
2.3. MODERN PYRHEL IOMETRY AND RADIAT ION SCALES
A new generation of electrically self calibrating cavity pyrheliometers was developed in the latter half of the 1960's and early 1970's at several laboratories in the US and Europe. Development of new cavity pyrheliometers began in the US at the Jet Propulsion Laboratory (JPL) of the California Institute of Technology (CIT) in the mid 1960's (Haley, 1964; Plamondon and Kendall, 1965; Kendall, 1968, 1969; Willson, 1967, 1969, 1971a; Sydnor, 1970; and Kendall and Berdall, 1970). New pyrheliometers were under development by the late 1960's at the Physical Meteorological Observatory at Davos (PMOD), Switzerland (Brusa and Fr~Shlich, 1972), at the U.S. National Bureau of Standards (USNBS) (Geist, 1972), and at the Royal Institute of Meteorology in Brussels (IRMB), Belgium (Crommelynck, 1973).
A series of radiation scale experiments were conducted during 1968-1970 at the Table Mountain Observatory (TMO) (California, U.S.A.) using the new JPL cavity pyrheliometers. These instruments, capable of defining the radiation scale with less than + 0.5~o uncertainty found a systematic - 2.2~o error in the International Pyrheliometric Scale of 1956 (IPS 56) (Willson 1969, 1971a, 1971b, 1972a, 1972b). The TMO result was subsequently confirmed during the third and fourth International Pyrheliometric Comparison experiments (IPC's III and IV), conducted under the auspices of the World Meteorological Organization (WMO) at the Swiss PMOD (FrOhlich et al., 1973b; Brusa and FrOhlich, 1975).
An international solar irradiance reference scale, the World Radiometric Reference (WRR) was defined by the average performance of five of the new pyrheliometers (as shown in Table I) during the 1 st International Comparison of Absolute Pyrheliometers conducted as part of IPC IV. The WRR was again reproduced during the IPC V at the PMOD in 1980 by a subset of the original five instruments (FrOhlich, 1981a). This scale, 2.2~o higher than the IPS 56 and within 0.5~o of the 1932 Smithsonian Scale, is believed to represent SI units with less than 0.3Yo uncertainty.
By the late 1970's continued improvements in the cavity pyrheliometers developed by the JPL, PMOD, and IRMB groups had produced instruments theoretically capable of defining the radiation scale at the solar total irradiance level with an uncertainty approaching +0.1~o (Brusa, 1983; Willson, 1973a, 1973b, 1975, 1979, 1980a; Crommelynck, 1981a, 1981b). A principal thrust of present research is the experi- mental demonstration of the level of SI uncertainty achieved by the advanced pyrhelio- meters developed by these groups. An approach developed at the USNBS (Geist, 1972) and adapted in various forms at the PMOD and IRMB use stabilized lasers in laboratory experiments to characterize (quantify in SI units) sources of uncertainty in cavity pyrheliometers (Brusa, 1983; Crommelynck, 198 lb). Another characterization approach using the Sun as the irradiant source has been implemented at JPL's TMO
206 RICHARD C. WILLSON
Results of the First International Comparison of Absolute Cavity Radiometers at the Physical Meteorological Observatory, Davos (1975). The relative performance for the five cavity pyrheliometers whose average weighted results are used to define the World Radiometric Reference Scale (WRR) are shown. The WRR has replaced the IPS56 as the solar irradiance scale of reference sanctioned by the World Meteorological Organization. Solar total irradiance measurements on the WRR are 2.2% higher than on the IPS56. The SI uncertainty of the
WRR is + 0.3 %.
Instrument Developer Performance relative to WRR (%)
ACR 310 Willson + 0.04 PMO 2 Frohlich + 0.10 ACR 311 Willson + 0.11 PACRAD III Kendall - 0.19 CROM Crommelynk - 0.20
The affiliations of the developers of the instruments are: Kendall and Willson - Jet Propulsion laboratory, California Institute of Technology, U.S.A.; FrOhlich - World Radiation Center, Physical Meteorological Observatory, Davos, Switzerland; Crommelynck - Royal Meteoro- logical Institute of Belgium.
(Willson, 1981a). A combination of these experimental techniques, applied to the leading sensor technology, should lead to the definition of an international reference scale at the total solar irradiance level with _+ 0.1% uncertainty by the late 1980's.
A radiation scale experiment based on a promising new technology, a cryogenic pyrheliometer operating at liquid helium temperatures, has been under development at the National Physical Laboratories of the U.K. (Quinn and Martin, 1982). This approach is capable of defining the radiation scale with 0.01% uncertainty, nearly an order of magnitude smaller than current conventional (uncooled) pyrheliometers.
The cryogenic technology represents a major advance in the ability to define the radiation scale in a laboratory environment, but it is not clear what level of uncertainty it can achieve in solar irradiance measurements. This type of instrument must reside in a vacuum with the radiant source to be measured to achieve full accuracy. The obvious application for cryogenic pyrheliometers would be in space flight experiments.
3. The Evolution of Solar Irradiance Experiments
3.1. THE SMITHSONIAN SOLAR IRRADIANCE MONITORING PROGRAM
The task of routinely monitoring the total solar irradiance at the mean Earth-Sun distance (1 Astronomical Unit or 1 AU) was begun in the US by the Smithsonian Institution at the turn of the century (Abbot et al., 1908). They conducted a systematic,
MEASUREMENTS OF SOLAR TOTAL IRRADIANCE AND ITS VARIABILITY 207
long term program (1902-1962) to detect solar variability using measurements made from mountain top observatories. The record of 1 AU total irradiance derived from their observations was limited to a long term precision near 1 ~o by fluctuations in atmospheric transmittance, and provided no clear evidence of solar variability. (A comprehensive revue of the Smithsonian observations has been written by Hoyt (1979).)
Recognizing the possible uncertainties imposed by atmospheric transmittance varia- tions, the Smithsonian also conducted the first flight experiments to measure the total irradiance. Following a trial flight in 1913, observations by an automated version of the silver disk instrument on a balloon at 25 km altitude in 1914 was used to derive a 1 AU solar total irradiance result of 1326 W m -2, about 1.5~o lower than their average ground based result (1346 W m -2) (Abbot etal., 1922).
3.2 . FL IGHT EXPERIMENTS US ING NON-SELF-CAL IBRAT ING DETECTORS
The next flight experiments to measure solar total irradiance were not attempted until nearly a half century after the Smithsonian balloon flights. Experiments on high altitude balloons in the U.S.S.R. from 1962-1968 (Kondratyev and Nikolsky, 1970) and on aircraft in the US during 1967-1968 (Drummond and Hickey, 1967) did not achieve significantly higher accuracy or precision than the Smithsonian ground based measurements despite the decreased uncertainties of atmospheric transmittance in their results. Principal sources of uncertainty for both these investigations were the ground-based, preflight calibrations required to relate the observations of their non- self-calibrating flight instruments to SI units. The results of Drummond and Hickey included an additional large source of uncertainty due to the errors in determining transmittances for the aircraft windows in their B57 and CV990 flights and the trans- mittance of the instrument window for the X15 flight (FrOhlich, 1981b).
Kondratyev and Nikolsky initially reported a total irradiance variation over 6 years in excess of 2~o and attributed it to variations in solar activity. They later concluded the variation resulted from changes in atmospheric extinction brought about by secular changes in the concentrations of stratospheric aerosols and gases (Nikolsky, 1978).
High altitude balloon experiments conducted in the US by the University of Denver were reported to show a 0.4~o increase in the 1 AU total solar irradiance between their first set of experiments inn 1967-1968 and two later ones in 1978 and 1980 (Murcray etal., 1969; Kosters and Murcray, 1981). The claim that the increase is resolvable by the series of experiments has been questioned by FrOhlich (1981b).
The University of Denver flight instruments were not self calibrating so the traceability of their results over more than a decade relies on gr...