Solar constants and radiometric scales

Transcription

1 Solar constants and radiometric scales B. W. Forgan The basic requirement for any type of measurement in which comparisons are made in a scale, relative or absolute. The values used by the Committee on Solar Electromagnetic Radiation to derive the solar constant were in different radiometric scales. Assuming that only difference in radiometric references produced the wide scatter of individual values, an attempt was made to convert all values to one radiometric reference. A radiometric scale based on electrical power equivalence was used as the reference. The resulting value was mw * cm- 2, as compared to the CSER derived value of mw cm- 2. This revised value lies within the error bounds of all the values used in the analysis. The revised estimate was then compared to values derived from more recent experiments. There appears to be a discrepancy between values derived in and out of the atmosphere. Introduction An absolute radiation scale can only be defined through the use of standard detectors or standard sources of electromagnetic radiation. For both types of instrument, knowledge of the fundamental physical properties of their operation and construction is essential. Some workersl 2 have discussed the difficulties involved in using standard sources as radiometric references; the definition of a radiometric scale with a standard detector does not suffer to the same extent. All standard detectors may be characterized as calorimeters in which the heating due to unknown irradiant flux is compared with that of a compensating electrical current. With knowledge of the area through which the detector is illuminated, the emissivity of the detector and the electrical power used to heat the detector, the incident flux may be measured on an absolute basis. Thus the accuracy of a radiometric scale relative to the fundamental physical concepts on which the detector performance is based cannot exceed the accuracy of the detector which defines the scale. A glance through the literature available on the various scales used in meteorological and astrophysical practice will show the effect of lack of knowledge of these fundamental principles. The Angstrom Scale of 1905 (A 1905) was based on the physical properties of the receiver strips of Angstrom Compensation Pyrheliometer A 70; an accurate knowledge of the so-called edge effects and thermal conductivity of the coating 3 resulted in errors of -2.0% being recognizedf4 6 The The author is with Flinders University of South Australia, Institute for Atmospheric & Marine Sciences, Bedford Park, South Australia Received 24 March revised Smithsonian Scale of 1913 (SS 1913) was also found to be in error by +2.5% from comparisons with more refined instruments. 7 Recently the definition of the International Pyrheliometric Scale of 1956 (IPS 1956), not being based on the output of a single absolute detector, was questioned 8 : it was based on a scale difference of +3.5% between the A 1905 and SS The above disparities lead to a confused scale position in the 's with the use of various scales even by experimenters in the same field. Using recent knowledge as to the relationship of the various scales, the effect is seen in the results obtained for one of the most important physical parameters in meteorology: the solar constant. The solar constant is defined as' the rate at which energy is received upon a unit surface normal to the sun's direction in free space at the earth's mean distance. Before 1965, observations were ground-based, and the main technique for deriving the solar constant was the Bouger-Langley method. Necessary for these earlier evaluations of the solar constant were additive corrections for the parts of the solar spectrum totally attenuated by the intervening atmosphere, namely, in the uv and ir. The magnitude of the solar constant oscillated from 1.90 to 2.05 cal cm- 2 min' in these early results prior to In the first half of this century, most work was done by the Smithsonian Astrophysical Observatory (SAO), data being evaluated from various high altitude stations in Africa, Asia, and South and North America on clear cloudless days. The SAO found that the solar irradiance is subject to small variations, the main cyclic variation a result of the earth's orbit being elliptical rather than circular. As a result of this in January at perihelion, there is a 3.4% increase in the extraterrestrial total irradiance, while in July at aphelion there is a similar decrease. The value obtained by the SAO in the 1628 APPLIED OPTICS / Vol. 16, No. 6 / June 1977

2 scale in actual use was given as cal cm'1 min-2 by Aldrich and Hoover. 7 Johnson 9 revised the SAO value by correcting the uv and ir additive corrections as well as adjusting the result into the R. Smithsonian Scale of The new solar constant value became 2.00 cal cm- 1 min- 2 and was widely used throughout the world until the early 1970's. Apart from Allen's 10 revision of Johnson's value to 1.98 cal * cm' min- 2, there was only one series of measurements, that of Stair and Johnston," 1 using a new standard of spectral irradiance. Using this standard a value of 2.05 cal cm- 1 min- 2 resulted. Thus, apart from Allen's revision of Johnson's value, it appeared that the solar constant was increasing with each successive measurement. Thekaekara 12 summed up this dilemma with the statement: The solar radiant flux is an important parameter in most problems of astrophysics and solar physics. It is indeed a disturbing situation that so important a physical constant has an uncertainty of a few parts in a hundred, when standard tables of other constants such as the velocity of light, electron charge, Planck's constant, etc., quote values within an accuracy of one part in a million or billion. Using a variety of observing platforms, 3 years of intensive investigation on the magnitude of the solar constant occurred between 1966 and The locations of the platforms differed from ground-based to extraatmospheric vehicles (Mariner Spacecraft), with the different experimental organizations developing a variety of measuring instruments to achieve their purpose. However, rather than consolidate the baffling radiometric scale measurements of the 50's and early 60's, the various instruments were calibrated in different scales depending on the type of device used. There were two major results of this series of measurements: (1) the value attributed to Johnson 9 was found to be high by approximately 2%; (2) all values from the new measurements lay between and mw -cm 2 (or cal -cm- 1 min- 2 ). An ad hoc committee was formed in 1969 to examine the data collected on the solar constant and then proposed a standard value. The Committee on Solar Electromagnetic Radiation (CSER) rejected all the ground-based data. All the high altitude data were investigated and various sources of error discussed. They resolved that there were four main reasons for error 13 : (1) the spectal irradiance standards vary between authors; (2) water vapor content is highly variable in the lower atmosphere (below 18 km), and hence allowance for the ir bands is uncertain; (3) the two extreme ends of the solar spectrum cannot be measured from within the atmosphere; (4) extrapolation to zero air mass by any technique is difficult. The Chairman of the CSER, Matthew P. Thekaekara, reported the findings of the Committee in 1970 and also However, the way the results were derived in the latter report differed from earlier reports From Thekaekara 14 the CSER analysis was said to be of eight values of total irradiance with each given a weight factor. The weighting factors were based on evaluations and criticisms by the members of the CSER. The balloon data of Willson were not, included. By 1972, due to objections on the weighting factors, Willson's value being included, the CSER used nine values obtained for the solar constant and noted their average. Both analyses gave the same result, namely, ' mw cm 2, which was first announced at the International Solar Energy Society Conference in Melbourne, Australia, This result was given to be in the International Pyrheliometric Scale of 1956, even though only five of the nine were reported in this scale. Table I shows the CSER analyses. The basic requirement for any measurement (either relative or absolute) is a scale to which results may be reduced. In radiometry, however, several scales have been used and only recently, through the work of Latimer1 7 and Frohlich, 4 have thorough comparisons been Table I. CSER Solar Constant as a Result of the and 1973 Weights Qyl Estimated error Solar Relative Weights ±mw- constant Author Source Instrument cm- 2 mw cm- 2 scale 0-10 scale 0 or 1 Murcray et al.qy 29 U. Denver Eppley-nip Thekaekara et al. 8 NASA GSFC A A Hy-Cal Cone Kondratyev and Nikolsky Qy 26 U. Leningrad Actinometer Plamondon" JPL TCFM Drummond and Hickey 2 l EPPLEY-JPL Radiometers WillsonQy"', 6 JPL Active cavity Resultant solar constant for different weights (mw cm- 2 ) Probable error (±mw.cm- 2 ) June 1977 / Vol. 16, No. 6 / APPLIED OPTICS 1629

3 made. Examining the CSER data we find that a minimum of three broad radiometric scales were used, broad in the sense that they were reported to be in a reference scale but not having the same references. Plamondon 18 and Thekaekara et al. 19 used Blackbody Cavity Scales, Willson8' 16 and Thekaekara et al. 19 used Absolute Scales, while the remainder used versions of the IPS The smaller variation in reported solar constant values in the late 1960's lead some experimenters to -'believe that their radiometric scales were similar. Labs and Neckel, 2 0 because of their close agreement in magnitude with the work of Drummond and Hickey, 2 1 stated that it "confirms the correctness of both our absolute scale and over-all spectral distribution." Yet Blevin et al. 2 2 in a comparison of the IPS 1956 as defined at IPC II in 1964 and with the Australian National Standards Laboratory radiometric reference found at 1.6% difference between them. In fact, the stability of the reference instruments for the IPS 1956 was critically questioned at IPC III in 1970, and instead of referring all instruments to the standard instrument, the mean of seven secondary standards was taken to represent the IPS 1956 now in use. This new definition varied from the previously used definition based on the supposed 3.5% difference in the Angstrom 1905 and Smithsonian Scale of 1913 by -1.1%. Thus the value given to be the solar constant by the CSER, from values in three broad radiometric scales, can only be considered as a rough approximation. Analysis In an attempt to bring all the CSER values into one radiometric scale, all nine values were examined individually. It was assumed that the extrapolation methods used to achieve the nine values-although different in each instant-gave a true result within a few fractions of a percent in the scale in which each value was represented. Five of the CSER values were taken in the IPS 1956, two in Absolute Scales (electrical power equivalents), and two in different Blackbody Cavity Scales. Representation by various countries in the IPS 1956 was traceable through the report on the IPC III, namely, Frohlich et al. 2 3 The variation of the Absolute and IPS Scales could be examined through reports of a series of independent comparisons by Frohlich, 4 Latimer, 1 7 and Willson during recent years. From these reports, the Absolute Scale defined by Frohlich 4 differed from the IPS 1956 Scale now in use by % with those in the Absolute Scale giving the higher radiant flux values. [This absolute scale will be called the Absolute Scale (1974) for the remainder of this paper.] Examining the CSER values individually, we have: (1) Kondratyev and Nikolsky 2 6 reported the results of a series of high altitude balloon measurements giving as the maximum values of the solar constant. The report on IPC III by Frohlich et al. 2 3 showed that the Leningrad standard to which all the values were converted gave readings 0.4% higher than the IPS 1956 now in use, while the IPC IV give it as being 0.6% higher. 2 7 This implies that the Kondratyev and Nikolsky 25 value for the solar constant is approximately 1.5% too low to be represented in the Absolute Scale (1974). (2) The two Eppley-Angstrom Compensation Pyrheliometers EA 6618 and EA 7635, used by Thekaekara et al., 19 were calibrated by A. J. Drummond of the Eppley Laboratory in the IPS 1956 Scale represented at the Eppley Laboratory after the observation flights. Through a careful examination of the combined Latimer,1 7 Frohlich, and Willson 2 5 comparisons, the deviation from the IPS 1956 now in use to the Eppley IPS 1956 is given as 0.3%, 28 the Eppley values being the smaller in magnitude. This makes a conversion factor of 2.3% to bring the two E-A values into the Absolute Scale (1974). (3) The University of Denver solar constant was derived from high altitude balloon measurements with Eppley Normal Incidence Pyrheliometers calibrated at the Eppley Laboratory. However, in 1970, a comparison of these E NIP's with Willson Active Cavity Radiometers representing an Absolute Scale showed a 2.9% difference, the Willson ACR's giving the higher values. Forgan 28 suggests that the Willson ACR's used in this comparison gave readings 0.1% lower than the Absolute Scale (1974), and hence a +3.0% correction of the Murcray et al. 2 9 values is in order. (4) The final value derived in the IPS 1956 was a result of data taken from a series of measurements in various NASA aircraft, including the experimental rocket vehicle, the X The Angstrom-type instrument to which the fast response multichannel radiometers were referenced was E-A 9000, which was calibrated against E-A 8420 and that in turn against the new Eppley reference E-A However, in the data analysis from the flights, the channel which was given the most weight was the 150 aperture, the more difficult to calibrate. Whereas the other IPS 1956 values mentioned previously were referenced to E-A 8420 directly, it is not the case with this value. (5) Of the remaining values, three had to be rejected because of the unavailability of comparison measurements: namely, the Cone Radiometer of Thekaekara et al., 19 supposedly in a form of an absolute scale; Plamondon's 18 value taken from the TCFM instruments on the Mariner 6 and 7 probes; and the result of the Hy-Cal NIP from Thekaekara et al., 19 both calibrated in Black Cavity Scales. The remaining value was that of Willson 8 which was reported as mw - cm- 2 in the ACR Scale, i.e., an absolute scale based on electrical power equivalence. Willson 3 l gave information concerning the comparison of ACR II's 2 and 4 used in the JPL 1968 balloon flights with PACRAD I; the Willson instruments showed a -0.1 and +0.1 differential, respectively. ACR III 3 used on the 1969 flight was developed from PACRAD, but utilized the differential measurement technique. By converting each instrument derived solar constant value to the Absolute Scale (1974) and taking an average, the Willson value used in the CSER analysis was increased by +0.15%. Table II (a),(b) shows the result of converting the six remaining values of the CSER solar constant. Table II (a) shows the result when the Drummond and Hickey 2 l value is converted using a 2.0% conversion factor 1630 APPLIED OPTICS / Vol. 16, No. 6 / June 1977

4 as suggested by Hickey 30 ; Table II(b) shows the result when the Drummond and Hickey 2 l value is not included. The values which are in greatest doubt in conversion are the University of Denver and Drummond results. The conversion factor used for the Murcray et al. 2 9 depends on the traceability of the Willson instruments used for the comparison in The accuracy of the Drummond results deserve further attention. J. R. Hickey of Eppley has reported at many meetings on the agreement between the JPL PACRAD radiometer and the Anstrom Pyrheliometer used in the latter part of the Eppley-JPL measurement series. 32 On the ground a difference of approximately 2% was found, while for the solar irradiance measurement flights, during which the PACRAD and Angstrom instruments were employed simultaneously, only a 1% difference was noted. The results of Table II indicate that with and without the Drummond value, the solar constant comes to or mw cm- 2. These values lie within all the error estimates of the six individual results used in the above analysis. Including the Drummond value gives a range of 1.7 mw cm- 2, while neglecting it only 1.0 mw cm- 2 separates all five values. Hence an improved estimate of the solar constant using five of the CSER values may be mw cm- 2, where the error term is the standard deviation of the estimated errors in the individual values. Conclusion Since the CSER analysis the results of the PACRAD determination of the solar constant have become available. 33 The value, based on 197 readings taken during nine flights between July and August 1968, has been given an over-all uncertainty of 1%. Referenced to the PACRAD scale, which gives values smaller by 0.1% than the Absolute Scale (1974), the value was mw. cm- 2. If revised to the Absolute Scale (1974), mw cm- 2, it agrees well with the Willson value of mw - cm- 2 and the suggested value of mw cm- 2. However, recent results from the ERB Nimbus 6 spacecraft, 34 which has a similar styled radiometer to the Eppley-JPL series of measurements, 35 have given in an absolute scale a value of mw cm 2 for the solar constant. All values have remained consistent to this value to within 0.2%. It is in excellent agreement with the previously revised Eppley-JPL result of mw- cm- 2. These two most recent results 3334 are separated by 2.1 mw cm- 2. Five of the six CSER revised values and the PACRAD value agree to within 0.5 mwcm- 2, all being a result of measurements within the atmosphere. The Eppley-JPL value was derived using measurements taken both in and out of the atmosphere, the X-15 value needing only correction for the reduction to mean solar distance. 2 1 While the main tool in the above analysis was the conversion of all values to one standard radiometric scale, the Absolute Scale (1974) defined as giving values 2.0% higher than the IPS 1956 now in use, 4 it was also assumed that the extrapolation techniques used to determine the solar constant values were correct. The Leningrad and GSFC values were obtained by generating probable attenuation curves and hence finding zero air-mass interception, while the remaining three values used in the above analysis were derived after describing the atmosphere as a bulk filter and then producing an atmosphere filter factor. Further corrections to the CSER values have been applied by Frohlich and Brusa 36 incorporating a recalibration of cavity absorbance and further reduced the revised value to mw * cm 2. It is therefore highly probable that until the actual devices used for measuring the solar constant outside the atmosphere can be either returned for calibration or calibrated in situ 3 7 that these differences will be resolved. Many workers in atmospheric physics have used differing solar constants and related solar spectral irradiance to derive components of atmospheric turbidity, rather than the difficult Bouger-Langley extrapolation technique. From the above discussion it should be obvious that the selection of a solar constant for turbidity analysis also implies that regard must be given to the radiometric scale to which both the actual atmospheric measurements and solar constant used, are referred. Table II. Results of the CSER Solar Constant Values Adjusted to the Absolute Scale (1974) Adjusted solar constant in Conver- Absolute Scale sion (mw-cm- 2 ) Source factor (a) (b) Kondratyev and Nikolsky Thekaekara et al.l 8 E-A E-A Murcray et al. Qy Willson Qy' Drummond and Hickey Qy MEAN The advice of Peter Schwerdtfeger is appreciated. The author thanks M. P. Thekaekara of GSFC, R. C. Willson of JPL, and J. R. Hickey of Eppley Laboratory for all the information given and certain comments from which the idea of this paper developed. June 1977 / Vol. 16, No. 6 / APPLIED OPTICS 1631

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