248 
It is to be noted that if the sun is assumed to be 
radiating like a black body, then the number of emitted 
quanta decreases rapidly with the decrease of wave 
length. As such, the available number of quanta may 
not be sufficient to produce the observed effects in the 
region of extreme ultraviolet. It is therefore believed 
that the sun must be sending out continuously (with 
occasional outbursts) line radiations in the extreme 
ultraviolet, for example, principal series of H, He and 
Het. Besides, there may also be ultraviolet radiation 
Tasie I. Hrrects 
THE UPPER ATMOSPHERE 
INumination of the Upper Atmosphere by Solar Rays. 
In connection with the solar control of the upper at- 
mosphere it is important to remember that the hours at 
which the sun’s rays first strike in the morning or dis- 
appear in the evening from the high atmospheric regions 
are quite different from those at the surface of the 
earth. For example, the high atmosphere above 100 
km as far from polar regions as lat. 55°, is illuminated 
at midnight by solar rays during high summer. The 
method of calculating the hours of sunrise and sunset at, 
OF SOLAR ULTRAVIOLET RADIATION ON UprrrR ATMOSPHERIC GASES 
Spectral region Reaction 
Remarks 
3000-2100 A 
(Hartley absorption bands of Os) 
Oz + hv — O2 + O* (excited) 
Very strong absorption by ozone (50-60 
km). 
1925-1760 A 
(Runge-Schumann absorption bands)| O2 + hy — O, (excited) 
OF + O2 > O + O3 
O+0:+ M0; + M 
Comparatively weak absorption. 
Production of ozone. 
(Note: The last reaction is a three-body col- 
lision process, in which J is the so-called 
third body which takes away the excess of 
momentum and energy.) 
1751-1200 A 
(Runge-Schumann continuum) 
O. + hy > O + O* (excited) 
Very strong absorption. 
Dissociation of O2 above 80 km. 
1012-910 A Os + hv > O; (normal) + e Weak absorption. 
‘ First ionization potential of Os. 
Production of D-region (?) (50-80 km). 
910-795 A 0+ h— O*t+e Very strong absorption. 
Ionization of O. 
Production of F,- and F5-regions (?) 
(200 km upwards). 
795-755 A No + hv Ne (normal) + e Comparatively weak absorption. 
First ionization potential of No. 
Production of Hs-region (?) 
(140-160 km). 
744-661 A O2 + hy > On (excited) + e Strong absorption. 
: Second ionization potential of O2. 
Production of Ei-region in the transition 
region 02 > O + O (90-120 km). 
661-585 A No + hy Ne (excited) + e Very strong absorption. 
Second ionization potential of N.. 
and soft X rays coming out of the corona which is 
regarded as having a very high temperature of the order 
of some million degrees. 
The phenomenon of radio fade-out strikingly illus- 
trates the solar control of the upper atmosphere. During 
an intense solar flare, when bright spots of hydrogen 
light appear on the sun, the magnetic disturbances and 
the ionization in the absorbing D-region of the iono- 
sphere both show an abnormal and simultaneous in- 
crease. This causes total stoppage of radio traffic over 
the sunlit portion of the earth [64]. 
different atmospheric levels and the results obtained for 
a few typical cases are given below. 
The shadow cast by the earth in space is of cylindrical 
shape. We have from Fig. 2, 
1), 
1 
i= ea 
— 
where H is the height at which the cylinder cuts the 
zenith, a is the radius of the earth, and @ is the de- 
pression of the sun below the horizon. 
