PHOTOCHEMICAL PROCESSES 
sky, and auroral. Intensity measurements at different 
altitudes have led to widely different estimates of the 
heights of emission of the sodium light. Bricard and 
Kastler have shown that for the twilight emission the 
spectral line is very narrow, whereas it is much wider 
in the night-sky emission. 
The problems connected with the sodium hight con- 
cern not only the levels and processes of emission; they 
concern also the amount of invisible sodium that may 
be present in the upper atmosphere, that is, the amount 
of sodium not in a state to emit the yellow (or other 
observable) light. Sodium is easily ionized by sunlight 
in the Hartley band, and must intercept some of this 
light, though most of it is absorbed lower down, in the 
ozone layer. The presence of some ionized sodium 
atoms must be expected in the high atmosphere, though 
their presence is not likely to make an important con- 
tribution to any of the ionized layers except possibly to 
the D-layer. Further, sodium forms oxides, NaO and 
NaO>, and these also would not be likely to give direct 
spectral indication of their presence. 
The proportions of Na, Nat, NaO, NaO, and any 
other forms in which sodium may exist in the upper 
atmosphere will differ with differing height, and the 
distribution and reaction processes of the sodium present 
a fascinating problem of atmospheric photochemistry, 
on which some speculation has been exercised, though 
as yet without any clear and definite conclusions being 
established. The oxides of nitrogen (as well as oxygen 
itself) may play a significant part in the reactions 
associated with sodium. The discussion of these oxides, 
as well as of the sodium itself, demands much knowledge 
that is still lacking, concerning, for example, energy 
data and reaction coefficients. As an illustration, one 
process suggested for the excitation of sodium atoms to 
the state ?Py from which they can emit the D light is 
Na+0+ X— Nad + X, NaO + 0 > Na’ + Os; for 
the excited sodium atom Na’ in the latter equation to 
be in the 7P» state, the energy 2.1 ev must be derivable 
from the oxygen dissociation energy gained (5.1 ev) 
less the energy needed to dissociate NaO; this is about 
3 ev, but it is not yet certain that it is not slightly too 
great for this process to provide the excitation energy, 
unless the colliding particles NaO and O have kinetic 
energies substantially greater than their average ther- 
mal energy (see § 14). This suggested process is based 
on the dissociation energy of oxygen stored up in the 
daytime from absorbed sunlight. Another possible proc- 
ess, involving only two-body collisions, is Na + 03; > 
NaO + O02, NaO + O > Na’+ Oj; it involves the same 
doubt as to the latter reaction. 
The photochemical problems briefly discussed here 
have an intrinsic interest and also some practical im- 
portance in that their solution is likely to aid in a full 
understanding of the phenomena of the ionosphere. 
At present the formation of the various ionized layers, 
themselves so vital to radio communication, is far from 
being properly understood, and every additional clue 
or sidelight concerning the electrical and chemical proc- 
esses in those regions is likely to prove valuable. 
IN THE UPPER ATMOSPHERE 273 
SUGGESTIONS FOR FUTURE WORK 
On the observational side the study of the photo- 
chemical processes in the upper atmosphere has vast 
scope. A basic necessity is the determination of the 
composition of the upper atmosphere as well as the 
height-distribution of temperature and density. Rocket 
Investigations may aid greatly in this field, particularly 
with regard to the composition, which indicates the 
extent to which diffusive separation occurs. Such investi- 
gations will also provide basic knowledge as to the 
solar radiation received at high levels in the atmosphere. 
Spectroscopic studies, from the ground and in situ 
(by means of balloons and rockets), have much to add 
concerning the nature and distribution of the con- 
stituents of the upper atmosphere, including water 
vapor, carbon dioxide, ozone, atomic oxygen and nitro- 
gen, the oxides of nitrogen, hydroxyl (OH), and sodium. 
The study of the absorption and emission by these and 
other gases in the high atmosphere, by night, at twilight, 
and during the day will enhance our understanding of 
the nature and processes of chemical, electrical, and 
energetic change there. For this work it is necessary to 
develop instruments of enhanced light-gathering power, 
spectral range, and dispersion, in particular for the 
auroral and night-sky luminosity. 
Radio investigations will greatly extend our knowl- 
edge of the energy absorption, photoelectric processes, 
and motions in the ionosphere, both at normal times 
and during magnetic disturbances, when corpuscular 
energy as well as wave energy is operative. 
Such observations need to be made not only at one or 
two places on the earth, but at a network of stations, 
so that the world distribution of the phenomena may be 
known—for its own interest, and also to aid in under- 
standing the processes occurring in any one place. 
Continued observations to determine seasonal! changes 
and changes throughout the sunspot cycle are desirable 
In many cases. 
On the theoretical side the subject is still young. 
Many additional purely chemical and physical data 
are required as a basis for atmospheric theories, namely 
data on the rates of various types of atomic and mole- 
cular processes, and in some cases on the temperature 
dependence of these rates. These data must be obtained 
partly by laboratory measurements, and partly by 
theoretical calculations in atomic and molecular physics. 
Such data are needed to elucidate the main processes of 
photochemical and photoelectrical change in the upper 
atmosphere, and to determine their height-distribution, 
distinguishing, among the vast number of possible re- 
actions and processes, those whose rate and number 
give them real significance. 
REFERENCES 
I. Books on chemistry, particularly photochemistry. 
1. Bonnorrrer, K. F., und Harreck, P., Grundlagen der 
Photochemie. Dresden & Leipzig, T. Steinkopff, 1944. 
2. Hinsuetwoop, C. N., The Kinetics of Chemical Change 
im Gaseous Systems, 4th ed. Oxford, Clarendon Press, 
1938. 
