250 
from 20 km to 40 km is rich in atmospheric ozone. 
The total quantity of ozone is small—only about 0.25 
em thick if reduced to standard temperature and pres- 
FRINGE REGION 
7.3xl0° 
SUNLIT AURORAL STREAMERS 
OCCASIONALLY EXTEND BEYOND THIS HEIGHT 
REGION OF ESCAPE OF ATMOSPHERIC GASES 
1.4x10® 
3.4x10® 
Ss) 
(So) 
a 
WW 
a 
w 
6 & 
= 8.5xl0 5) 
= 
= a 
at 
= ie 
ro) 2.6x10" 
Ww fo) 
= a 
oO 
lIxio8 = 
=} 
Zz 
re 
8 = 
7.6x10% = 
Zz 
lJ 
(2) 
TEE RBH Stonauet 2.2x10'° 
HEIGHT ATTAINED 
13 
a NSITION O2>0+0] >* “*!0 
Ne LOWER LIMIT OF AURORA yey 
2.5x10!9 
Fia. 5. = musuatite some of wine physical features of the 
upper atmospheric regions. The ionospheric regions are shown 
by shading with dots, the depth of shading roughly indicat- 
ing the relative intensity of ionization. 
sure—but it suffices to cut off all solar radiation in the 
near ultraviolet below 2800 A. In the region round 
80 km noctilucent clouds occur. Meteors appear and 
disappear most frequently in the region from 50 to 150 
km. 
The regions above 70 km are ionized by the solar 
extreme-ultraviolet rays and are collectively called the 
ionosphere. The region from 80 to 130 km is the region 
of transition from an atmosphere consisting of N»2 and 
O2 to one of Nz and O, due to the dissociative action of 
solar ultraviolet rays. The region from 80 to 120 km is 
the region of the most frequent occurrence of aurorae. 
But auroral rays are sometimes known to extend up to 
a height of 1000 km and beyond. 
Atmospheric gas particles which are not ionized by 
solar ultraviolet rays escape from the region estimated 
to lie between 500 and 1000 km. Such particles as 
escape from the atmosphere but are unable to overcome 
the gravitational pull travel im closed orbits and form 
the exosphere or the fringe region of the atmosphere. 
The physical features depicted in Fig. 5 are, of course, 
only illustrative and should not be taken too literally. 
This remark is particularly applicable to the density 
distribution above 100 km. Here, because of the un- 
THE UPPER ATMOSPHERE 
certainty of the temperature distribution and of the 
mean molecular mass, the density distribution is un- 
certain. For example, if it is assumed that the region 
above 100 km is in diffusive equilibrium, the higher 
regions would consist almost entirely of oxygen atoms. 
But the spectrum of auroral rays shows that, up to the 
highest limits, the intensity of the negative bands due 
to ionized nitrogen molecules vies with that of the lines 
of atomic oxygen. 
METHODS OF EXPLORING THE UPPER 
ATMOSPHERIC REGIONS 
The methods of exploring the upper atmospheric 
regions may be classified under two heads—direct 
methods and indirect methods. 
Direct Methods. Under this caption we include those 
methods in which the physical agency employed for 
exploration is under the control of the investigator. 
A straightforward direct method is to send up craft 
carrying recording apparatus which may register tem- 
perature, pressure, humidity, wind, and any other 
measurable physical quantities. A variation of this 
method is the so-called radiosonde method in which the 
craft carries a small radio transmitter which auto- 
matically sends out signals of the recorded data. The 
great advantage of radiosondes is that instantaneous 
knowledge of the data is gained and it is not necessary 
to wait until the craft comes down and the registering 
apparatus is collected. 
Until recently the craft used for this purpose were 
generally sounding balloons filled with hydrogen gas. 
(Aireraft are also used for regions up to about 14 
km.) The height reached by such balloons seldom ex- 
ceeds 30 km. But a new departure was made in 1946 
by utilismg the V-2 rockets devised during the war. 
These reach enormous heights (up to 180 km) and even 
the very first trial flights have yielded many important 
results. 
A few words may be said here of the technique of this 
latest method of sounding the upper atmosphere. 
(General accounts of it are to be found in articles by 
Krause [52] and by Newell [80]; an excellent summary is 
also given by Sheppard [96].) The war head which 
contains the recording instruments bursts towards the 
end of the flight and the instruments are landed by 
means of a parachute. The height and the velocity of 
the rocket are tracked by radar throughout its flight. 
For the flight at White Sands, New Mexico, on March 
7, 1947, the highest velocity, 1600 m sec, was attained 
at the height of 127 km, 80 seconds after launching the 
rocket. The highest altitude attained was nearly 180 
km. Measurement of the velocity is important because 
it is necessary for evaluation of the temperature. Much 
of the recorded data was also telemetered to the ground 
during the flight. By various instruments and devices 
pressure down to 10-> mm was measured. The temper- 
ature was derived from the pressure-height curve and/or 
from the ratio of the ram pressure (the pressure at the 
stagnation point on the nose of the rocket) to the static 
pressure. The probable errors of measurement are still 
