GENERAL ASPECTS OF UPPER ATMOSPHERIC PHYSICS 251 
rather large, averaging + 20C. The recorded data will 
be discussed im the next section. 
Another agency for direct study is pressure waves 
produced by explosion. These waves proceeding up- 
wards may be bent down if they meet a region of high 
temperature where the phase velocity is greater. Rec- 
ords of the downcoming wave yield information re- 
garding the density and temperature distribution. 
An analogous method is to use radio waves instead of 
sound waves. These waves penetrate beyond the high- 
est limits attained by V-2 rockets and when they come 
down, “‘reflected”” by the ionized regions of the upper 
atmosphere, they carry with them indelible messages 
regarding the physical conditions of such regions, which 
are no less useful than those recorded by instruments 
carried by sounding balloons or rockets. 
Indirect Methods. These consist in studying criti- 
cally such of the geophysical phenomena occurring near 
the surface of the earth or in the upper atmosphere as 
are known to have bearings on the physical state of the 
upper atmosphere. These phenomena include aurorae, 
lights from the night sky, meteoric flashes, noctilucent 
clouds, oscillations of barometric pressure, terrestrial 
magnetic variations, and spectra of direct or scattered 
sunlight. 
The various direct and indirect methods that have 
been employed in the study of the upper atmospheric 
regions and the main results obtained therefrom are 
shown in Tables II and III. Short accounts of the 
Tasie II. Dirnrcr MrrHops or STUDYING THE 
Upprr ATMOSPHERE 
Method Region explored |Atmospheric conditions inferred from the study 
Sounding Up to 30 
Troposphere is in quasi-adiabatic 
balloon km 
equilibrium; in the stratosphere 
the temperature distribution is 
nearly constant with height; in- 
formation about composition and 
wind systems. 
Smoke shell Existence of seasonal wind 
Up to 30 
km systems. 
V-2 rocket | Up to 120 | Generally confirms results on tem- 
km perature and density distribution 
as inferred from the indirect 
methods of study; the chemical 
composition of the atmosphere 
at 70-km level is practically the 
same as that in the troposphere; 
solar spectrum is extended be- 
yond the ozone absorption limit; 
Mgt doublet (2802 A) obtained 
in emission. 
Sound ex- | 35-60 km 
a Rise of temperature in the middle 
plosion 
atmosphere; existence of wind 
systems. 
Radio- 
wave ex- 
ploration 
Atmospheric constituents from 70 
km upwards are __ ionized; 
measurements of scale height 
H, intensity of magnetic field, 
recombination coeflicient. High 
temperature (approx. 1000K) in 
the region 250 km. 
“Bursts” of ionization are pro- 
duced along meteor trails. 
70-500 km 
methods together with a brief survey of the contem- 
porary state of ourknowledge of the physical state of 
the upper atmosphere will be given in the next section. 
TasLe II]. InprrEcr Mreruops OF STUDYING THE 
Upprr ATMOSPHERE 
Phenomena Region Atmospheric conditions inferred from 
studied explored the study 
Meteoric 40-150 km | Rise of temperature in the middle 
phe- atmosphere and correspondingly 
nomena greater density; drop in tem- 
perature at 80-km level; existence 
of seasonal winds. 
Ultraviolet |/20-60 km Contains ozone with maximum 
spectrum concentration at about 25-km 
of direct level; thickness of the ozone 
or scat- layer reduced to 8.T.P. is only 
tered about 0.25 mm, 
sunlight 
Noctilu- 70-90 km High-velocity wind and low tem- 
cent perature (approx. 200K). 
clouds 
Barometric |50-400 km | Tidal motions in the upper atmos- 
oscilla- phere; temperature rise in the mid- 
tions dle atmosphere with a cold top. 
Terrestrial |70-100 km | High electrical conductivity and 
magnetic world-wide electric current sys- 
varla- tems. 
tions 
Night-sky 60-500 km | Sodium atoms in the middle at- 
lumi- mosphere (60-80 km); atomic 
nescence oxygen in the higher regions 
above 100 km; nitrogen mole- 
cules are ionized by solar rays. 
Aurorae 80-1000 km | Entry of high-speed charged par- 
ticles; atomic oxygen and atomic 
nitrogen present. 
A SURVEY OF THE CONTEMPORARY STATE 
OF OUR KNOWLEDGE OF THE 
UPPER ATMOSPHERE 
Composition. Near the surface of the earth the at- 
mosphere consists of nearly 99 per cent by volume of 
the gases nitrogen and oxygen. Next in importance are 
argon 0.93, carbon dioxide 0.03, neon 1.8 X 107%, and 
helium 0.5 X 10-* per cents by volume [26]. 
Chemical analysis by laboratory methods of samples 
of air collected in stratosphere flights and by pilot 
balloons has shown that this composition is maintained 
up to a height of about 29 km [84]. Contemporary 
experiments made with samples of air collected by V-2 
rockets show that practically speaking the same com- 
position is maintained up to 70 km [23]. This is very 
satisfactory, because from various considerations the 
same conclusion had been arrived at long before the 
observations with V-2 rockets. (Ozone is present in the 
middle atmosphere in appreciable quantities. This will 
be discussed later.) 
The atmosphere above 80 km begins to change in 
composition because of the dissociative action of solar 
ultraviolet rays on molecular oxygen (A < 1751 A). 
The region 80-130 km is the region of transition from 
