PHOTOCHEMICAL PROCESSES IN THE UPPER ATMOSPHERE 
phere (sodium is an exception), as shown in Table IT; 
the electron detachment energies for O and O3 are 
also given. Table III gives various dissociation poten- 
Tasie II. Ionization PoTentTiats V ev 
Gas N O O- Na No O2 Or N20 
V ev 4! 5) || 1G || Bo |) Goll | WH |) es |) il PAB¢E 
TasweE III. Dissoctation PoTentiats V ev 
Gas Neo Oz O3 NaO He OH 
Vev 7.4, 9.8? 5.09 | 1.1 3° 4.5 | 4.3 
tials of interest in connection with the upper atmos- 
phere. 
The fraction of particles with thermal kinetic energies 
of the amounts shown in the two tables above is very 
small in the upper atmosphere (see § 14), and divi- 
sion by impact is produced mainly by fast-moving 
particles coming in from outside—meteors, cosmic rays, 
and (of most importance) the streams or clouds of gas 
emitted by the sun, which produce aurorae and mag- 
netic storms—and by atmospheric particles to which 
these external particles communicate sufficient of their 
energy. They dissociate N2 and Os, ionize them and the 
atoms NV, O, and excite No, N, Os, O, and their ions. The 
influence of such transient (though frequent) impact 
processes on the average composition of the air in auroral 
latitudes has received little attention as yet. But there 
can be little doubt that over most of the earth the 
chemistry of the upper atmosphere is mainly determined 
by the effect of sunlight. 
Among the chief of these effects is the ionization of 
the various layers of the ionosphere—D, H, Fi, and F2, 
in ascending order of height (respectively about 90, 120, 
220, and 300 km). Their detailed explanation still pre- 
sents difficulties [29]. It is uncertain whether the main 
ionized particles in the E- and F-layers are O2 and O 
respectively, or O and N. The high degree of F-layer 
No-dissociation implied in the latter case seems to con- 
flict with the evidence of the sunlit auroral spectrum 
indicating that there is undissociated nitrogen 200 km 
and more above the F-layer. There seems to be little Ny 
in the F-layer. The D-layer has been ascribed to atomic 
sodium. The formation of these layers filters out all or 
most of the sunlight of highest frequency, whose quanta 
exceed 12 ey. 
The light whose quanta have energy between about 
5 and 10 ev dissociates molecular oxygen, being ab- 
sorbed at levels between about 15 or 20 km and 120 km 
height. It is still uncertain to what extent molecular 
nitrogen is dissociated by part of this radiation, or 
radiation of somewhat greater frequency; the nitrogen 
molecule seems to be more readily ionized than disso- 
ciated by radiation. 
Other constituents of the upper atmosphere that are 
partly dissociated by absorption of sunlight are ozone 
269 
(O; + O2, + O), water vapor (H.0 — OH + #), and 
sodium oxide (VaO — Na + 0). 
16. Combination of Particles; Conservation of Energy 
and Momentum; Two- and Three-Body Collisions. The 
reverse of the division of one particle into two (dis- 
sociation or ionization) is the combination of two into 
one. This is called recombination in the case of ions and 
electrons, and attachment in the case of a neutral particle 
and an electron (for example, 0 + e — O-). The act of 
combination essentially involves juxtaposition, that is, 
a collision. The mean interval after division, before the 
two particles collide and may recombine, depends upon 
the density (see § 13), and therefore may have any value. 
Thus it differs essentially from the mean interval be- 
tween an excitation and a subsequent de-excitation 
with radiation, the lifetime, which depends solely on the 
nature of the particle itself and on its spontaneous 
processes. 
The results of separation can consequently persist for 
hours or even days in air of sufficiently low density; 
during this period the energy of dissociation or ioniza- 
tion is stored up in the gas as a kind of potential energy. 
In the upper atmosphere some of the energy absorbed 
from the sunlight during the day hours, causing disso- 
ciation and ionization, remains in this potential form 
after sunset, and during the night it is partly trans- 
formed slowly into radiation, which is observed as a 
faint luminosity of the night sky—the airglow. 
In general the two particles resulting from the divi- 
sion of any one particle do not recombine with each 
other. The parent particle gives birth to its progeny 
among a host of jostling neighbours, and the “new-born 
infants” at once lose each other in the crowd. Hach may 
combine in due course with some other particle of the 
same kind as its lost brother, or it may enter into a new 
partnership of a different kind. 
The division of a particle can take place in either 
of two ways: by absorption of light or by impact. The 
process that is the reverse of division by absorption 
of light is combination with emission of light, in a two- 
body collision. The process that is the reverse of the 
division of a parent particle AB into components A and 
B, by the impact of a particle X—a transformation 
symbolized by 
AB+X—-A+B4+X 
—is the combination 
A+B+xX—>AB+X 
produced by the three-body or triple collision of A and 
B and X, to form the two bodies AB and X. The two- 
body radiative combination is symbolized by 
A+ B-— AB + hb. 
In all types of collision there is conservation of both 
energy and momentum. In a radiative combination, by 
two-body collision, the initial velocities of the two 
combining particles determine the initial total momen- 
tum, which after the collision is divided between the 
single combined particle and the emitted photon; as 
