GENERAL ASPECTS OF UPPER ATMOSPHERIC PHYSICS 
has been found that the night-sky brightness due to 
these lines and bands fluctuates erratically when the 
ionospheric conditions in Region F do so [69]. The reac- 
tion which excites these spectra is most probably due 
to the mutual neutralization of N$ and O- ions [67], 
thus, 
Ns + O- — Nz (excited) + O (excited). 
In the infrared region there are strong emissions due to 
OH bands [65]. The region from which these are emitted 
is still uncertain. 
There is a luminescent layer (60-80 km) emitting 
sodium D lines [12] and also possibly one emitting O» 
bands at the height of Region H. 
The intensity of the night-sky radiations varies with 
the season of the year and with the solar cycle and 
shows how upper atmospheric conditions.are controlled 
by solar radiation and by the corpuscular emission 
from the sun. 
Winds in the Upper Atmosphere—Atmospheric Tides. 
There is evidence that up to the height of Region F the 
atmosphere is subject to winds, caused partly by tem- 
perature gradient and partly by tidal effects. Tentative 
curves depicting the variation of wind with height, for 
summer and winter in the middle latitudes, are drawn 
after Sheppard in Fig. 7 [96]. The data for the region 
\—=> 
WARM TOWARD POLES 
+ 
WARM TOWARD EQUATOR 
—}— 
WARM TOWARD POLES 
HEIGHT (Km) 
nS 
fe) 
> 
at 
10 
WARM TOWARD EQUATOR 
ea! Lt ea ee) 
150 100 50 O 50 100 150 
EAST WEST 
WIND SPEED (m/sec) 
Fia. 7.—Probable variation of wind (east and west 
components) with height in middle latitudes for summer and 
winter. The height is given in logarithmic scale as this enables 
one to infer directly the approximate mass flow. The merid- 
ional temperature gradient for quasi-geostrophic winds is 
shown on the right. (After Sheppard [96].) 
around 30 km are from sounding-balloon [44] and smoke- 
shell observations [50]. For higher regions, the evidence 
is from observations on noctilucent clouds and from 
meteor trails. For the latter (in the region of 120 km) 
an optical technique has been developed for measure- 
ment of wind from the drift and distortion of long- 
lived meteor trails by Stérmer in Norway [99], Olivier 
in the United States [82], and Hoffmeister in Germany 
and Southwest Africa [47]. Radio observations on the 
movements of what are called zon clouds also confirm 
the existence of winds in this and in still higher regions 
[14, 36, 77, 79]. 
Regarding the tidal effect it is to be mentioned that 
there exist in the atmosphere, just as in the ocean, tidal 
motions due to the pulls of the sun and the moon. These 
255 
tidal effects have been observed up to the highest 
regions of the ionosphere—the F-region (250-400 km). 
A direct effect of the tidal oscillations is seen in the 
rhythmic variation of the barometric pressure. An in- 
direct effect is the rhythmic variation of the terrestrial 
magnetic elements on a magnetically quiet day. The 
amplitudes of these variations are small—only about a 
few thousandths of the total value—but they are singu- 
larly persistent. 
The tide-raising force of the moon is about 2.5 times 
as strong as that of the sun. But, contrary to expecta- 
tion, it is found that the solar tidal effect is very much 
more prominent than the lunar one. The origin of this 
anomaly is traced to a resonance effect. It was first 
pointed out by Taylor and Pekeris that, because of the 
peculiar temperature distribution in the middle atmos- 
phere (a2 temperature rise in the neighbourhood of 50 
km followed by a cold top), the atmosphere has a mode 
of oscillation with a 12-hour period [86, 100]. Hence 
the solar tidal effects are greatly enhanced by resonance. 
The ionized regions of the upper atmosphere are also 
necessarily subject to the tidal oscillations. The oscilla- 
tions of the lowest of these regions are responsible for 
the quiet-day magnetic variations as follows: As the 
conducting ionized region, in its horizontal tidal mo- 
tions, cuts the magnetic lines of the earth, emf’s are 
developed in these regions. These emf’s produce world- 
wide electric current systems in the upper atmosphere. 
The magnetic field of these current systems produces 
the quiet-day variations of the terrestrial magnetic ele- 
ments. It is believed that the most probable location of 
these current systems is the lower regions of the iono- 
sphere (D- and E-regions) where the mean free paths of 
electrons and ions are small [64]. 
Aurorae—Entry of Fast Charged Particles into the 
Upper Atmospheric Regions. The luminescence of the 
aurora is due to the excitation of air molecules by colli- 
sion with the fast charged particles emanated from the 
sun and incident on the upper atmosphere. Besides the 
limes and bands observed in the night-sky light (with 
different intensity distribution) a special feature of 
the auroral spectrum is the great intensity of the so- 
called negative bands of nitrogen due to N¢ ions. It also 
appears that nitrogen atoms are present in the aurora. 
Identification of forbidden lines of atomic nitrogen in 
the auroral spectrum has been reported by more than 
one worker. Bernard [13] claims to have identified the 
forbidden ultraviolet doublet of N, 2P — 4S (3466 A) 
while Dufay, Gauzit, and Tcheng Mao-lin [33] an- 
nounce that they have observed the forbidden green 
doublet, ?D — 48 (5199 A) in low-latitude aurorae. 
It should be mentioned that according to quantum- 
mechanical calculation by Pasternack [85], the lifetime 
of the excited state for 5199 A is 10 hours. According to 
Gétz [42], the strength of this radiation in the case of 
low-latitude aurorae (which generally lie at greater 
heights) may continue undiminished for hours after the 
aurora ends. From the intensity measurement made on 
a particular aurora, the concentration of excited N 
atoms, at an altitude somewhat above 200 km, was 
estimated to be of the order 10° em~. 
