AURORAE AND MAGNETIC STORMS 
Variations in Latitude. The most remarkable fact 
about the geographical distribution of the aurorae is 
that the greatest frequency, in both arctic and antarctic 
349 
and Geddes [19] have drawn up a similar zone of maxi- 
mum frequency for the antarctic region (Fig. 1). How- 
ever, the records are too few for a complete construction 
ZONE OF 
MAXIMUM 
AURORAL 
FREQUENCY 
°PLACES OF 
OBSERVATIONS 
30° Ww 
Fic. 1—The geographical distribution and zone of maximum auroral frequencies in the north and south polar regions. (After 
Fritz [4], and White and Geddes [19].) 
regions, occurs along a zone lying about 20°-25° from 
the earth’s magnetic axis point. For the arctic region 
this was stated as early as 1881 by Fritz [4]; White 
BOSSEKOP 
p=70°N X=23°E GR. 
— CORONA 
--- DRAPERIES 
CAP THORDSEN 
$=78.5N \=16°E GR. 
GODTHAAB 
A=52°W GR. 
— CORONA 
$=64°N 
--- DRAPERIES 
KINGUA FJORD 
$=67°N A=67°W GR. 
FORT RAE 
$=62.5°N =116°W GR. 
— CORONA 
--- DRAPERIES 
24h 4h gh 12h 
LOCAL TIME 
at it LOCAL MAGNETIC MIDNIGHT 
Fie. 2.—Diurnal variation of the aurorae, observed during the 
Polar Year 1882-83. (After Vegard |15].) 
20h 
of the lines of equal frequencies over the antarctic 
region, and additional observations from conveniently 
distributed points would be of great mterest. 
Variations in Time. The mean diurnal appearance 
of the aurorae seems to be correlated with local magnetic 
time. The maxima apparently occur about 1.3 hours 
before magnetic midnight (Fig. 2). (Magnetic mid- 
night is here defined as the time at which the plane 
through the point of observation and the magnetic 
axis passes the sun.) 
Position in Space of the Aurorae 
The Method of Height Determination. Reliable height 
determinations can be made only by means of paral- 
lactic photography. If objectives of high lght-power 
(about f 1:1.5 and less) and sensitive plates are used, 
the time of exposure for aurorae of medium intensity 
is even less than a second. Graphical methods, chiefly 
developed by Stérmer [12],! are used for reduction of 
the plates. The base line should not be less than 20-30 
km. 
Direction of the Arcs and Position of the Radiation 
Point. The position and the direction of the ares are 
closely connected with the direction of the earth’s 
magnetic field at the place of observation. The position 
of the radiation points of coronae is similarly dependent 
on the inclination and declination of the earth’s mag- 
netic field at the place of observation. 
Height Statistics of Aurorae. The positions in space 
of the auroral forms are usually characterized by the 
lower and upper limits of the luminosity. In the photo- 
graphs of such forms as ares, bands, and draperies, the 
lower limit is usually easy to determine, since a sudden 
decrease in the luminosity is apparent. The determina- 
tion is more difficult with such forms as rays and pulsat- 
ing surfaces. It is always difficult to determine the upper 
1. Methods for the reduction of a pair of parallactic photo- 
graphs are described in detail in [5]. (See also [16].) 
