16 RADIATION 
other than sunspots must produce these ionospheric 
variations; only on the average is the ultraviolet in- 
tensity correlated with sunspots. We should expect 
therefore that the meteorology of the troposphere would 
not be correlated with sunspots on a daily basis if 
ionospheric variations were used as a criterion. What 
about longer periods? The wave lengths involved are 
of the order of 1000 A or less, so that the amount of 
energy is small and the pressure (about 10 mb or less) 
and density of the absorbing layers are so small that it 
seems unlikely that heating in the ionosphere by the 
increased ultraviolet energy can directly affect the me- 
teorology of the lower atmosphere; at least that is the 
view of one school of thought [71, p. 504]. Therefore 
for longer periods also, if only the ionosphere were in- 
volved in variable ultraviolet absorption, no tropo- 
spheric effect would be noticed. 
At present, one cannot specify the height at which 
anomalous heating in the upper atmosphere can affect 
the troposphere through dynamic processes. However, 
if in addition to an increase in the radiation which heats 
the ionosphere, increases also occur in the ultraviolet 
energy which can penetrate to lower layers, for example, 
to the top of the ozone region (40-50 km) where the 
pressure is of the order of 1 to 3 mb, then dynamic 
pressure changes caused by additional heating are more 
likely [44; 71, p. 504]. And indeed such ultraviolet 
energy increases may occur, but it is not certain at 
present that significant increases occur at wave lengths 
which can heat the ozonosphere. 
There are several solar-induced effects in the iono- 
sphere, as revealed by magnetic and radio data, and 
from the ozone-heating viewpoint at least one of these 
ionospheric phenomena deserves further mention. That 
phenomenon is the radio fade-out or sudden ionospheric 
disturbance (SID). SID’s are caused when radio waves 
transmitted upward from the ground are strongly ab- 
sorbed in the D-layer, so that they cannot be reflected 
back to the ground by the higher ionospheric layers. 
The increased absorption of the radio waves is caused 
by a sudden increase in the ionization of the D-layer; 
the increased D-layer ionization is caused by a sudden 
large increase in the solar ultraviolet energy which 
reaches and is absorbed by the layer. In short, SID’s 
are caused by sudden increases in ultraviolet solar 
radiation and occur simultaneously with the appear- 
ance of visible bright solar flares on the sun (chromo- 
spheric eruptions). 
Moreover, it is found that during SID’s the F-layers 
are practically unaffected and the H-layer is only 
slightly affected. Hence the upper ionospheric regions 
are apparently transparent to the ionizing radiations 
in this case, while the lower D-region absorbs them 
strongly. The duration of SID’s is of the order of a few 
minutes to a few hours, and their intensity is of course 
variable. 
Here then is a phenomenon which produces short- 
lived intense ionization, and hence heating in the vicin- 
ity of 60 km. Since the upper layers are unaffected, the 
radiation must be of wave lengths such that the air 
above 80 km is transparent. Wulf and Deming [79] 
offer an interesting explanation of this ionization. Ozone 
absorbs very strongly in the region 2300 A to 2800 A, 
but this spectral region is transmitted readily by the 
atmosphere above 60 km. They suggest that partial ab- 
sorption at the top of the ozone layer ionizes the ozone 
there. Increased solar emission at those wave lengths 
may therefore be responsible for the increased D-region 
ionization and hence for SID’s. It should be pointed 
out however that the recent V-2 rocket measurements 
[47] could not detect any ozone above 55 km. Probably 
an amount of ozone smaller than could be detected by 
the rocket exists above 55 km and this is sufficient to 
produce the observed ionization.1 
According to some writers, the increased emission 
during solar flares is contributed largely by specific ele- 
ments, hydrogen and calcium, for example [25], al- 
though emissions from other elements have been meas- 
ured. Hydrogen does not radiate in the wave lengths 
2300-2800 A. Calcium and some of the other elements, 
on the other hand, do emit monochromatic radiation 
there, so that some increase in J, occurs during solar 
flares in this region—how much of an increase is still 
unknown. 
Mitra [62] discusses some other SID explanations and 
prefers 973 A as the wave length of the ionizing radia- 
tion. Heating by such radiation in a narrow spectral 
band may be small and should not extend very far into 
the ozonosphere. But if, as Wulf and Deming suggest, 
increased radiant energy in the broad band 2300-2800 
A is emitted, then the resulting increased heating, not 
only at and above 60 km but also lower in the ozono- 
sphere, may have important implications for tropo- 
spheric meteorology [44]. It would therefore be highly 
desirable to measure the distribution of spectral in- 
tensity in this region at frequent intervals. Hulburt [47] 
reports that the solar spectrum near 2200 A was de- 
tected at 34 km from the V-2 rocket, and Brasefield [16] 
has described some temperature measurements from an 
unmanned balloon up to 140,000 ft (42 km). If such 
balloons could be equipped for constant-level flights at 
40 km or higher and designed to carry spectral measur- 
ing equipment, it would be possible to determine the 
solar spectrum near 2200 A and for }.> 2700 A at 
frequent intervals during days of both disturbed and 
undisturbed solar conditions. Such measurements may 
determine whether current ideas regarding solar con- 
trol of weather through heating of ozone have any basis. 
Near Ultraviolet. In a series of optical measurements 
during the years 1924-32, Pettit [68] determined the 
intensity at 3200 A relative to the tensity at 5000 A 
and extrapolated in the usual manner to the “top” of 
the atmosphere. Presumably the intensity at 5000 A 
changed rather little, so that the variation in the ratio 
reflects the time variation in the intensity at 3200 A. 
His averaged data show a rather good agreement with 
the smoothed sunspot numbers, low sunspot numbers 
1. At the January 1950 meeting of the American Mete- 
orological Society in St. Louis, Missouri, R. Tousey of the 
Naval Research Laboratory reported a rocket measurement of 
small amounts of ozone up to 70 km. 
