254 
produced. It is destroyed by absorption in the near 
ultraviolet in the Hartley bands (2100-3000 A). 
Observations show that the variation of the atmos- 
pheric ozone content has little if any association with 
the 11-year solar cycle. It appears, however, that there 
are 27- and 15.5-day periods of variation with small 
amplitudes. 
‘Ionization in the Upper Atmosphere—Radio Explor- 
ation. Atmospheric constituents from 70 km upwards 
are more or less ionized by the action of the extreme 
solar ultraviolet rays. Under certain ideal conditions 
each atmospheric constituent ionized may be supposed 
to produce a distinct layer or region of ionization. Such 
a region of ionization is called a Chapman region [24]. 
There are several ionized regions and these are known 
collectively as the ionosphere. The two main regions of 
maximum ionization, Region E and Region F, are at 
average heights of 100 km and 275 km respectively. 
The average maximum ionization densities of these two 
regions, in the epoch midway between the maximum 
and minimum of solar activity, are of the orders 10° 
and 108 electrons em~', respectively. During daytime, 
Region F splits up into two regions, F; and F2. A sub- 
sidiary Region E» (above E, which is sometimes called 
E;) also appears during the daytime. The average 
heights of the maximum ionization of F; and E, are 200 
km and 140 km, respectively. The ionization below 80 
km (Region D), present during the daytime, causes 
absorption of radio waves of medium length. 
As may be expected, the ionization densities of the 
various regions vary with the hour of the day, the 
season of the year, and also with the solar cycle. It 
should be noticed, however, that while Regions E and F; 
follow, in the main, the simple -V cos x law (7.e., intensity 
of ionization is proportional to +/ cos x, Where x is the 
zenith angle of the sun), Region F, refuses to do so. 
Region F; is, in fact, notorious for its erratic behaviour. 
Besides the regions mentioned above, which are more 
or less permanent features of the ionosphere, mention 
should also be made of what is known as sporadic E. 
Part, at least, of sporadic E is ascribed to ionization 
produced by meteoric impacts [55]. 
The knowledge of upper atmospheric ionization sum- 
marized above has been gained mainly through ex- 
ploration by radio waves. The radio wave is now the 
most powerful experimental tool for studying upper 
atmospheric ionization. The most important method of 
radio exploration is as follows: Packets or “pulses” of 
radio waves are sent upwards. These are generally 
scattered if they meet a region of ionized atmosphere. 
If the ionized region is extensive—of dimensions much 
larger than the wave length—then, depending upon the 
frequency, the wave packet may be totally or partially 
reflected. Or it may penetrate through, or be absorbed 
by, the ionized region. 
The technique of the method has now been greatly 
perfected. Records of ionosphere characteristics are 
now kept in many stations of the world as regularly as 
weather and magnetic data. Predictions of ionospheric 
conditions, well in advance, for determining the maxi- 
mum usable frequencies for communication over given 
THE UPPER ATMOSPHERE 
distances, are also made regularly by many ionosphere 
stations. 
The other physical properties of the upper atmos- 
pheric regions which have been determined from the 
ionospheric studies are the following: 
1. The absorption phenomena of radio waves enable 
us to estimate the collisional frequencies and the molec- 
ular densities in the different regions. Thus, it is found 
that for Region H, the pressure is of the order 10% 
mm and for Region F 10-° mm. 
2. Analysis of the records of diurnal variations of the 
heights and the maximum ionization densities of the 
Regions E and F, has confirmed the existence of tidal 
motions in the high atmosphere. Observations on ab- 
sorption in the lowermost ionospheric region—the D-re- 
gion—has revealed the existence of lunar tidal oscilla- 
tion effects in this region also [5]. 
3. The study of the magnetic splitting of the radio 
waves in the ionosphere affords a means of estimating 
the intensity of the earth’s magnetic field at great 
heights above the surface of the earth. 
It has now been established that the different iono- 
spheric regions are produced by the ionization of the 
different atmospheric constituents at different levels. 
According to one hypothesis [16], Region F is produced 
by ionization of atomic oxygen, Region F, by ioniza- 
tion of N2, Region Eby ionization of O, at second 
ionization potential, and Region D by the ionization of 
the same molecule at its first ionization potential [74]. 
According to another hypothesis, Region F; is produced 
by ionization of atomic oxygen in the normal manner 
and Region F; by splitting up of this region into two 
regions [78]. According to still another hypothesis, some 
of the ionized regions may be produced by radiation 
from the solar corona [107]. In regard to Region E it 
may be noted that for a long time it had been difficult 
to explain theoretically the location of its height of 
maximum ionization, because the height at which it was 
expected was much above the observed height. The diffi- 
culty has been reconciled by the hypothesis that the 
E-layer is located in the region (90-110 km) where the 
density of 02 molecules diminishes rapidly with height 
on account of the dissociative action of solar ultraviolet 
rays [67]. Further, it is believed that in this region the 
QO» molecules are pre-ionized, rather than directly ion- 
ized, by solar radiation of appropriate wave length [81]. 
Luminescence of Upper Atmospheric Regions. On 
dark moonless nights, the portions of the sky devoid of 
stars are found to emit a faint light. Part of this light 
is due to starlight scattered by the atmosphere and part 
due to other sources. But a certain proportion, about 
40 per cent, has been found to be due to the self- 
luminescence of the upper atmospheric gases. Spectro- 
scopic study of the night-sky radiation shows the pres- 
ence in the upper atmosphere, besides O2 and No, of O 
and Na atoms and of H.0 and OH. 
The most prominent lines and bands im the night-sky 
spectrum in the visible region are lines due to O atoms 
and bands due to N2 molecules. The luminescent region 
emitting these lines and bands may possibly be identi- 
fied with Region F of the ionosphere (200-400 km). It 
