GENERAL ASPECTS OF UPPER ATMOSPHERIC PHYSICS 
By S. K. MITRA 
University of Calcutta 
INTRODUCTION 
The present article is intended as an introduction to 
the articles on the different aspects of the upper atmos- 
phere which follow. The main topics of this contribution 
will be a discussion of the physical origin of the upper 
atmosphere, methods of investigation and a general 
survey of the contemporary state of our knowledge of 
the upper atmosphere, and an account of the problems 
which still remain unsolved. There will, of necessity, be 
references to many subjects which are dealt with in the 
articles that follow. These references will be included 
at the risk of a certain amount of repetition, as the aim 
of the present article is to give a general idea of the 
physical aspects of the upper atmosphere. The reader 
who desires detailed knowledge in any particular sub- 
ject should seek it in the specialised articles that follow. 
It should also be mentioned that the treatment of some 
of the topics in the present article will follow closely 
that given in the author’s recent book [72]. 
The region of the earth’s atmosphere denoted by the 
term wpper atmosphere is not yet well defined. To the 
meteorologist, upper atmosphere may mean the regions 
investigated by the conventional sounding balloons; 
to the geophysicist studying the aurora or the iono- 
sphere, upper atmosphere may mean the high regions 
near and above 100 km. In the present article, the 
words wpper atmosphere will generally refer to the 
regions above the troposphere. However, for purposes 
of reference the whole atmosphere may sometimes be 
divided into three regions: the lower atmosphere (tropo- 
sphere); the middle atmosphere (stratosphere); and 
the upper atmosphere, extending above 100 km. The 
lower stratosphere, as reached by sounding balloons, 
will be generally left out of consideration in the present 
discussion. 
In studying the upper atmospheric region it is help- 
ful to bear in mind its high tenuity. At 15 km, near the 
base of the stratosphere, the atmospheric density is 
about one-eighth of that at sea level. Above this the 
density falls off rapidly, and at 100 km it is only a 
millionth of that at sea level. The pressure is thus about 
10 mm, which is of the same order as in the so-called 
“vacuum” of ordinary electric bulbs. At 300 km the 
pressure is of the order 10-° mm. This is the pressure 
attamed by only high quality modern vacuum pumps. 
Tf we consider the mean free path of a molecule, we note 
that its value near sea level is 10-° cm, at 15 km it is 
10% cm, at 100 km it is 10? cm, and at 300 km it is 
10° cm. 
It may seem surprising that regions of such extreme 
tenuity in the upper atmosphere could be the seat of 
any phenomenon of geophysical importance or of in- 
terest for any aspect of our daily life. Nevertheless 
such is the case. But for the presence of the ionospheric 
245 
regions at heights of 200 km and above, long distance 
radio communication at night would be impossible. 
Auroral displays which illuminate the long winter nights 
in the polar regions occur with greatest frequency near 
a height of 100 km, and auroral streamers sometimes 
extend up to heights of 1000 km and beyond. The com- 
mon phenomenon of shooting stars appears most fre- 
quently in the region 50 to 150 km. During World 
Wars I and II the sound of cannon fire in France could 
be heard in England because the wave of explosion 
was bent downward by refraction at heights of about 
35 km. 
Many upper atmospheric phenomena are also of 
great scientific interest because they occur under con- 
ditions and on a scale which cannot be reproduced in 
laboratories. As a matter of fact the upper atmospheric 
regions may be regarded as constituting a vast physical 
laboratory where Nature carries out experiments on a 
gigantic scale on such phenomena as bombardment of 
air masses by charged and uncharged particles, electric 
discharge, magnetic double refraction, ionization by 
collision, photochemical reaction, and recombination of 
ions and electrons. In laboratory experiments the ioniza- 
tion track of a charged particle in a Wilson cloud 
chamber may be only a few centimetres long; in the 
upper atmosphere it may be hundreds of kilometres 
long. In a laboratory, rarefied gas for study of discharge 
phenomena has of necessity to be confined within a 
glass vessel. The walls of the vessel are responsible for 
the quick disappearance of electrons and ions and the 
consequent extinction of the discharge glow. In the rare- 
fied upper atmosphere there are no glass walls. Un- 
hindered by the wall effect, the ions and the electrons 
and the luminescence persist for a long time. 
One further point of interest in connection with 
upper atmospheric phenomena may be mentioned. Un- 
expected associations (some still unexplained) have 
been found to exist between physical conditions in the 
upper atmosphere, tens or hundreds of kilometres above 
the surface of the earth, and weather conditions in the 
tropospheric regions. 
Finally, accurate knowledge of the physical proper- 
ties of the high regions of the atmosphere is of utmost 
importance to those engaged in the development of the 
many new types of high-flying aircraft. 
THE UPPER ATMOSPHERE 
We may now consider the origin of the observed 
structure of the upper atmosphere. If the earth’s at- 
mosphere were at rest, undisturbed by any external 
agency, conduction of heat from one part to another 
slow as it is—would, after a sufficient length of time, 
produce uniform temperature throughout the mass. 
Further, if the atmosphere consisted of more gases than 
