pia sie =e 
SOLAR RADIANT ENERGY 17 
corresponding to low ultraviolet radiation, as in the 
ionosphere, but at times the correlation was negative 
(June 1928 to June 1929). Since the intensity at sun- 
spot maximum was about 1.5 times that at sunspot 
minimum, he felt that in general the range of the ob- 
served variations at 3200 A was too great and that an 
atmospheric effect might have been partly responsible 
for the range. 
In support of Pettit’s doubts, Bernheimer [15] found 
an annual trend in Pettit’s data, and indicated that the 
observed variations may have been due to changes in 
atmospheric transmission, rather than to increased 
solar emission. As we shall see later, this difficulty of 
extrapolating through the earth’s atmosphere is also a 
continual worry in determinations of variations m the 
solar constant. 
Visible and Infrared Radiation. The fluctuations in 
ultraviolet radiation seem to be related to hot, bright 
areas, for example, faculae and flocculi [9]. These areas, 
which can be seen visually, cover rather small portions 
of the sun. As the temperature of a body increases, the 
intensity of the emitted radiation increases at all wave 
lengths, but the wave length of maximum intensity 
shifts towards shorter wave lengths. Hence, since the 
maximum intensity occurs at a wave length of about 
4700 A in the average solar spectrum (Fig. 1), it might 
be expected that, as the sun’s temperature increases, 
the relative increase of intensity at \ < 4700 A will be 
greater than in the longer wave length visible or infra- 
red regions. Such is indeed the case; the intensity of the 
radiation in the visible and infrared changes by small 
amounts in response to the dark and bright markings 
on the sun. 
Abbot [8, Vol. 6, p. 165] found that, on days with 
high solar constant, Zo at 0.35 u increased by about 5 
per cent over its value on days with low solar constant. 
At 1.7 p, Ip, decreased by about 1 per cent under the 
same circumstances. It should be noted, however, that 
solar-constant variations are not well correlated with 
sunspots [8]. 
Radio Waves. As stated earlier, radio-wave emission 
_ from the sun (from a few centimeters to a few meters in 
wave length) indicates temperatures of 10°K; under 
disturbed conditions, radiation corresponding to 10°K 
can be observed [60, p. 328]. However, the amounts of 
these energies are very small. 
Summary. There is good evidence from radio meas- 
urements that large fluctuations in solar radiation occur 
at A < 1000 A and at X = 10 to 10% cm. The variations 
seem to appear in daily measurements and also appear 
systematically in the sunspot cycle. In the meteorologic- 
ally important spectral region of 2000-2800 A there is a 
suggestion that at least short-lived large fluctuations 
may occur (SID) [79]; direct measurements of these 
fluctuations to determine their magnitude would be 
very desirable. At somewhat longer wave lengths, for 
example 3200 A [68], measurements indicate solar- 
controlled fluctuations, but doubts have been raised 
about the reality of their magnitude. That variations in 
the visible spectrum from parts of the sun occur can be 
seen from the bright and dark areas on the sun. How- 
ever, these areas are small and the variations in the 
visible and near-infrared radiation represent only a 
small percentage of the average radiation from the 
entire sun. 
The Solar Constant. So far we have discussed the 
relative spectral distribution of intensity im solar radia- 
tion. To describe the radiation it is also necessary to 
specify the amount of radiation on an absolute basis. 
The “‘solar constant” is a measure of the total amount 
of heat which reaches the outer atmosphere of the earth. 
Specifically, it is often expressed as the amount of 
energy which, in one minute, reaches a square centi- 
meter of plane surface placed perpendicular to the sun’s 
rays outside our atmosphere when the earth is at its 
mean distance from the sun. 
Methods of Measurement. To clarify the following dis- 
cussion let us review the basis for the fundamental (or 
“longe”) method of the Smithsonian Institution for 
measuring the solar constant [3, Vol. 6, p. 30]. The 
intensity J, of parallel monochromatic energy trans- 
mitted through the earth’s cloudless atmosphere is 
given by 
LH = Ine" = (4) 
I,.a, 
or 
In Qh = In Jy + m In w, 
where ka is the extinction coefficient, a is the atmo- 
spheric transmission with the sun in the zenith, and m 
is the optical air mass or the path length of the parallel 
light through the atmosphere measured in terms of the 
zenith path as unity.” 
Except for large zenith angles Z, the value of m is 
given by sec Z. Hence m can be readily determined; J, 
is measured in relative units. If a, remains constant, 
then by equation (4) a graphical plot of In J, against 
m will yield a straight line whose slope is In a and 
whose intercept for m = 0 (outside the atmosphere) is 
In Jo; J, is measured nearly simultaneously for the 
spectral region 0.34 1 < \ < 2.5 yu. This is repeated for 
several values of m, so that by the graphical method 
just mentioned Jo, can be evaluated at several wave 
lengths in the region. From the measurement and 
eraphical extrapolation, a plot of J, vs. \, and another 
plot of Zo, vs. \ can be made in relative units for the 
region 0.34 u to 2.5 4. We desire to find the areas 2>J,A) 
and Jp, AX, respectively, under these graphs in abso- 
lute units. Therefore, by means of a pyrheliometer, a 
nonspectral measurement in absolute energy units is 
made of the total radiation 7; J includes not only the 
energy which reaches the observer for \ between 0.34 u 
2. The value of m is ordinarily specified as unity for zenith 
path length at sea level. Values of m at sea level are given by 
sec Z for values of Z (sun’s zenith distance) up to 70°; for larger 
Z, Bemporad’s formula [56] is commonly used. To compute the 
air mass my for elevated stations where the pressure is p, the 
sea-level value of m is multiplied by p/po where po is the sea- 
level pressure. When J) is measured at one station, however, it 
is not necessary to correct m for pressure to find Zo, from 
equation (4). 
