3900 
limit precisely for all forms. In Fig. 3 the lower limits 
of all forms of polar aurorae are shown. The measure- 
ments are the result of three series of observations. 
HARANG 
D 
0 50 100 200 0 50 100 200 O 50 100 I50 
Fie. 3.—Height statistics. Lower limits of polar aurorae, all 
forms. (The number of points appearing in 2-km height inter- 
vals are used as abscissae.) 
The lower limits lie between 80 and 140 km, with a 
pronounced maximum at about 106 km. Stérmer has 
made an extensive study of the lower limits of different 
forms and has also compared the lower limits of the 
polar aurorae and aurorae appearing at more southerly 
latitudes. There appear to be only small systematic 
changes in the lower limits for the various types and 
latitudes. 
There is a wide range of variation in the vertical 
extent of the luminosity. In polar regions the rays 
extend to heights up to 200-250 km. In lower latitudes 
the vertical extent may increase to as much as 800- 
1000 km. Aurorae appear only at these latitudes during 
very intense magnetic storms, and radio-echo measure- 
ments indicate that there is a considerable increase in 
the reflection heights of the ionosphere during the 
storm phase. The increases in vertical extent of the 
aurorae at lower latitudes may therefore be due partly 
to the special conditions of the ionosphere during strong 
magnetic storms. 
Sunlit Aurorae. Height measurements in arctic re- 
gions near the auroral zone have shown that the rays 
and draperies in the dark atmosphere very seldom 
attam heights over 250-300 km. Stormer has shown, 
however, that faint rays, often of a grayish color, appear- 
Fic. 4.—Aurorae situated in the border region between the 
dark and sunlit atmosphere. The horizontal line indicates the 
shadow line. (After Stérmer [13].) 
ing over southern Scandinavia at a considerable dis- 
tance from the auroral zone, may attain heights up to 
800-1000 km. Aurorae appearing at such great heights 
THE UPPER ATMOSPHERE 
are always situated in the sunlit atmosphere and, at 
these low latitudes, appear only during great magnetic 
storms. Figure 4 shows how the lower limits concen- 
trate along the shadow line between the dark and 
the sunlit atmosphere. Spectrographic observations by 
Stormer have shown that the nitrogen band in sun- 
lit aurorae is strongly enhanced relative to the green 
auroral line. 
Spectrum of the Aurorae 
The intensity of the auroral luminosity is low, and 
if spectrographs are to be used, they must be of con- 
siderable light-gathering power. The spectrum con- 
sists of a number of lines and bands; more than one 
hundred are listed in the wave-length tables. They 
extend from 8100 A (infrared) to 3100 A (ultraviolet) 
where ozone absorption cuts off the spectrum. 
Wave Lengths and Average Intensities of Spectral Lines. 
By exposing a spectrum over a number of nights, an 
average spectrum with intensities approximating the 
values given in Table I is obtamed. Here only the 
stronger of the auroral lines, together with their identifi- 
cations, are listed. The spectrum is dominated by the 
nitrogen band systems, the negative group (NV G) and 
the first and second positive groups (1 P G and 2 P G), 
which are all well known from the study of gas dis- 
charges. The visible color of the aurorae, however, is 
produced by the atomic oxygen lies 5577 A in the 
green (Fig. 5) and the doublet 6363, 6300 A in the 
red. In addition to the stronger lines a considerable 
number of weak lines and bands have been listed, some 
of which seem to be atomic lines of O and N. Of special 
interest is the appearance of the forbidden nitrogen 
line Ni @S — ?P) at 3466 A. 
4708 
4278 , 
3914 
ty 
9) “ 
2D 
a 
Fie. 5.—Spectrum of aurorae. (After Vegard [15].) 
Intensity Effects. The study of intensity variations 
within the auroral spectrum is of great importance, 
but because of the rapid movements and low intensity 
it is difficult to carry through. Here one has to use 
small spectrographs of the highest light-power, per- . 
mitting short exposures, and the measurements are 
therefore limited to the stronger lines. The following 
intensity measurements have been made: (1) intensity 
variations within a single auroral form from the lower 
edge to the upper limit; (2) intensity variations from 
one auroral form to another; and (8) latitudinal varia- 
tions between polar aurorae and aurorae at lower lati- 
tudes. 
