SOLAR RADIANT ENERGY 15 
or more. It should be pointed out, however, that the 
black-body energy is so small at those wave lengths— 
even during disturbed sun conditions—that radiation 
from a black body at 10°K can contribute only a very 
small amount of thermal energy by comparison with 
the principal solar energy [39]. 
The Unmeasured Radiation. We turn now to an im- 
portant region of unmeasured radiation, namely, the 
ultraviolet energy below 2200 A. 
From a consideration of the ionized states of certain 
elements observed in the sun and from the considera- 
tion of the state of ionization of the earth’s ionosphere, 
several investigators have concluded that in the region 
below 1000 A the sun radiates energy corresponding to 
temperatures above 6000K. Greenstein [37] mentions 
the uncertainty regarding the need for assuming such 
excess ultraviolet radiation and concludes that at least 
for \ > 1215 A the temperature corresponding to solar 
energy is probably less than 6000K and is near 5000K. 
For } S 900 A he discusses a suggestion by Kiepen- 
heuer and Waldmeir that corona temperatures of 10°K 
enhance the photospheric radiation by a factor of 2 at 
900 A and by 2 X 10° at 600 A. For \ > 900 A no ap- 
preciable radiation is contributed by the corona. Super- 
posed on the average radiations, measured and un- 
measured, are the radiations from the “cool,” dark 
sunspots (temperature about 4800K [4]), and the in- 
creased radiation from the bright areas, that is, from 
faculae and flocculi. 
In addition to the electromagnetic radiation de- 
scribed above, the sun also emits particles which are 
responsible for such effects as the aurorae and some 
types of magnetic and ionospheric storms. 
Summary. It appears then that in the optically 
measured spectrum, from about 0.45 p» to 24 w the sun 
radiates as a black body whose temperature is close to 
6000K, perhaps increasing to 7000K towards the higher 
wave lengths. For shorter wave lengths, at least down 
to 0.22 u, the measured radiation corresponds to a con- 
siderably lower temperature of about 4000K to 5000K. 
For 1000 A < \ <2200 A this lower temperature may 
also apply [37]. But for \ < 1000 A the hot corona may 
dominate, resulting in much higher effective black- 
body radiation. In the comparatively long wave lengths 
from 1 cm to 30 m, high temperatures (10°K) are again 
indicated. In the region from 24 » to 1 em, no measure- 
ments seem to have been made. 
Fluctuations of Emitted Radiation. The bright and 
dark markings and other manifestations of changes on 
the sun can be expected to produce spectral emissions 
which differ from the average solar emission. These 
spectral variations are indeed observed directly or in- 
directly in nearly all parts of the solar spectrum. 
Far Ultraviolet. That there are marked changes in the 
far ultraviolet emission from the sun is evident from 
measurements of the reflection of radio waves by the 
ionospheric layers in the earth’s upper atmosphere. 
These layers are regions where solar ultraviolet energy 
(A < 1000 A) has ionized the atmospheric gases, with 
the result that they have the property of reflecting 
radio waves of certain frequencies. The approximate 
heights [88, 62] and pressures of those layers are given 
in Table I. 
TABLE I. APPROXIMATE HEIGHTS AND PRESSURES OF [ONIZED 
LAYERS 
Laver Fen Approminate pressure 
Fr, 300 5 X 107 
Fy 200 i <elOm 
100 5 X 105% 
D 80 to 50 5 X 10 to 1 
The highest frequency radio signal which can be re- 
flected is called the critical frequency f, for the reflecting 
layer, and it can be shown that under certain assump- 
tions [64, p. 59] 
iio cc N;, (1) 
and that 
N? a In, (2) 
where NV; is the maximum ion density of an ionized 
layer. Hence on combining equations (1) and (2), 
In & fo. (3) 
The critical frequency for, of the F»-layer fluctuates 
considerably from day to day. When the measured 
daily value of for, at noon is plotted against the daily 
sunspot number, no significant correlation can be found 
[69]; on a monthly basis a small correlation may be 
present [13]. However, if a twelve-month running aver- 
age of noon for, is plotted against the twelve-month 
running averages of sunspot numbers, a linear relation 
results with very little scatter of the points [64], and 
for, increases by a factor of about 2 from sunspot 
minimum to sunspot maximum. Similar results appear 
for the F,- and H-layers; but for these latter layers, fo 
increases by a factor of about 1.2 from sunspot mini- 
mum to sunspot maximum [9]. The D-region variation 
is apparently similar to the E and F, variation [64]. 
If equation (3) is applied to the smoothed data over 
the sunspot cycle, Jo, (which may be in a different wave- 
length region for each layer) increases by a factor of 
approximately 2 for the H- and Fy-regions [9, 64]. But 
since for, increases by a factor of 2, Zo, would increase 
sixteenfold in the F.-region [64]. However, other rela- 
tions have been mentioned for the F.-layer. Mitra [62] 
indicates that instead of equation (2), V; « Jo, applies 
here; this leads to J, « fc. Allen [9] prefers I, « fo. 
Mitra’s relation leads to a fourfold increase in Jp,, while 
Allen’s relation leads to a twofold increase. We may 
conclude therefore that Zo, which produces the F»- 
layer, surely increases from sunspot minimum to sun- 
spot maximum and that the amount of the increase is 
at least twofold. 
What is the significance of ionospheric heating for 
tropospheric meteorology? Since very little direct, rela- 
tion exists between the critical frequencies and the 
unsmoothed sunspot data, some effect (possibly solar) 
