GENERAL ASPECTS OF UPPER ATMOSPHERIC PHYSICS 
believed to be composed of ice erystals. According to 
some authorities the presence of the ice crystals in- 
dicates a temperature of 160K m this region [60]. 
Again, radio measurements show that the scale height 
(kT’/mg) for this region has a value which corresponds to 
a temperature of about 200K [19]. Meteor observations 
by Whipple, however, indicate a drop to only 250K 
near 80 km (see above). Work on atmospheric tides 
[104] also confirms the general trend of the drop in 
temperature in this region. 
Above 80 km a rise in temperature would be expected 
because of strong absorption by O2 molecules (1751 A). 
This is fully supported by the rocket observations. A 
rise from 180K to 335K is found between the levels 
80-120 km. 
Beyond 120 km there are several indirect evidences 
indicating that the rising gradient is maintained and a 
temperature near 1000K to 1200K is attained at the 
height of Region F,. These evidences are (1) increased 
seale height in Region F, as compared to that in 
Region E [3], (2) escape of helium from the outer 
atmosphere, which demands a temperature of above 
1000K [49, 98], and (8) large width of the green line of 
atomic oxygen in the spectrum of night-sky emission 
[8]. it should be mentioned, however, that in regard 
to the last-named evidence more recent measurements 
of the green oxygen line [102] show that the previous 
estimation of temperature was too large.) 
It appears that there is considerable variation, both 
spatial and temporal, in the value of this high temper- 
ature within the ionospheric Regions H, F,, and Fy, 
80 to 400 km [95]. In the F.-layer the temperature is 
found to vary (both spatially and temporally) from 
100K to 1000K. In lower levels the range is smaller. 
Tn the stratum from 200 to 300 km, in the noon merid- 
ian, two centres of high temperature are found, one from 
30°N to 50°N and the other at 35°S. The former is less 
peaked than the latter. It should be mentioned that 
these temperatures have been derived by Seaton [95] 
from the values of the recombination coefficient com- 
puted from ionospheric data for January 1947, collected 
at eighteen stations situated between 71°N and 43°S. 
Existence of such thermal patterns implies the ex- 
istence of quite strong wind systems in the ionospheric 
regions (see below). 
The existence of a high temperature in the upper- 
most regions of the atmosphere can also be inferred on 
strong theoretical grounds. The primary effect of the 
absorption of solar radiation might be dissociation, 
ionization, or excitation of the constituent gases of the 
atmosphere; but, like all other forms of energy, the 
absorbed energy must ultimately degrade into thermal 
energy of molecular agitation, causing a rise of tem- 
perature. (The applicability of the concept of temper- 
ature for the very high regions of the atmosphere has 
been discussed by the author [72, pp. 507—510].) 
Study of Meteors. It has already been mentioned 
that the existence of a region of high temperature in the 
middle atmosphere was first inferred from a study of the 
heights of appearance and disappearance of meteors. 
Winds in the upper atmosphere have also been studied 
253 
from measurements of the drift and distortion of long- 
lived meteor trails [47, 82, 99]. Study of meteors, in 
fact, provides a valuable means of investigating the 
physical characteristics of upper atmospheric regions. 
It is, therefore, very satisfactory that a new technique— 
the radar technique—has been adapted for this study, 
and developed in the research laboratories of different 
countries [46, 55, 58, 63]. Not only is the ionization pro- 
duced by meteoric impacts being systematically studied, 
but methods have also been developed for measuring 
the velocity [34] and other characteristics of meteors 
(e.g., height, range, and radiant point). Accurate deter- 
mination of the meteor velocity is very important in 
the application of the theory of meteors in deducing 
various upper atmospheric data. 
Ozonosphere—Origin of High Temperature in the 
Middle Atmosphere. The high temperature of the mid- 
dle atmosphere mentioned above is a consequence of 
absorption of solar radiation by atmospheric ozone. 
Because of this absorption the solar spectrum ends 
abruptly at about 2900 A. The absorption bands re- 
sponsible for this are known as Hartley absorption 
bands. 
The ozone content of the atmosphere may be de- 
termined by the measurement of the variation of the 
intensity of solar radiation near the ultraviolet end of 
the spectrum as the zenith distance of the sun changes 
{20]. A more accurate method is spectrophotometric 
study of sunlight scattered from the zenith sky. The 
intensities of two spectral regions near the absorption 
edge, one of which is rather strongly and the other 
weakly absorbed, are compared when the sun is setting 
[41]. This makes possible, with the help of the so-called 
Umkehr effect, an approximate estimate of the distri- 
bution of ozone with height. Contemporary improve- 
ments in the technique of ozone measurement, utilising 
photoelectric multiplier tubes should be mentioned. At 
the 1948 Oslo meeting of the Union of Geodesy and 
Geophysics it was claimed that with the improved tech- 
nique, the scattered light from the moon and the stars 
sufficed for making measurements of the ozone content 
at night. 
It has been found that the proportion of ozone to air 
by volume is a maximum at a height of about 35 km. 
The atmospheric ozone near the region of 50 km acts 
as an enormous reservoir of heat. This height is much 
above the centre of gravity of ozone because the solar 
ultraviolet radiation responsible for heating is almost 
completely absorbed in the top layer [87]. 
The ozone content shows a diurnal variation, being 
greater at night or at least never less than that durmg 
the day. There is also a seasonal variation; the maxi- 
mum occurs in spring and the minimum in autumn in 
both hemispheres. This annual variation is greatest in 
the high latitudes and least near the equator. 
It is to be noted that solar radiation is responsible 
for both production and destruction of ozone. It is 
produced by absorption by molecular oxygen in the 
range of the so-called Runge-Schumann absorption. 
bands (1760-1925 A) when excited O, molecules are 
