352 
of the electron stream can be made visible, and these 
model experiments gave a striking picture of the possible 
orbits for an incoming electron stream approaching the 
earth. These model experiments were later extended 
by Briiche and Malmfors (see [1]). 
The Movement of a Charged Corpuscle in the Earth’s 
Magnetic Field—Stérmer’s Calculations. The movement 
of an electron with a certain initial velocity is well 
known for a number of simple types of magnetic fields, 
such as movement in a homogeneous field parallel or 
transverse to the direction of the field. In a radial field 
which is produced by a single magnetic pole the orbits 
of the electrons will be like geodetic lines on a cone. It 
has not been possible to solve completely the more 
general case represented by the field of a dipole such 
as that of the earth. The differential equations for the 
movement of a particle in the dipole field are easily 
set down, but a general integration of these equations 
has not been possible. Stérmer has discussed the equa- 
tions in several papers and given many numerical solu- 
tions which he has applied to auroral problems. It 
may be added that the same problem appears in the 
theory of cosmic rays, where fast electrons from space 
enter the earth’s magnetic field. St6rmer’s calculations 
explain several of the effects associated with the appear- 
ance of the aurorae. According to theory, the electrically 
charged particles will impinge on the earth along two 
zones, symmetrical with respect to the magnetic axis 
points, which represent the auroral zones. Further fam- 
ilies of trajectories of the particles will end on the 
earth’s night side, thus making the appearance of the 
aurorae on the night side possible. 
Different views have been expressed concerning the 
motion of the electrically charged particles in an auroral 
form. In the case of a fine auroral ray it may be as- 
sumed that the earth’s magnetic field can be regarded, — 
to a first approximation, as equivalent to the field of 
a single pole. The motion should be along a geodetic 
line. When approaching the earth, the electrically 
charged particle will reach a certain minimum height, 
where the motion will be m a circle lyme at right angles 
to the field. The radius p of this circle is a measure for 
the stiffness (Hp) of the rays, given by the formula, 
Hp 
e 
where H is the magnetic field imtensity, and m and e 
are, respectively, the mass and the charge of a ray- 
corpuscle. Measurements of the width of fine auroral 
rays show that the stiffness (Hp) may attain values of 
10°. This corresponds to stiffness of the order of 6-rays 
or of fast positive rays in gas discharges. The measure- 
ments of Hp therefore do not give any definite informa- 
tion on the nature of the corpuscles producing the 
aurorae. But if, after entering the earth’s atmosphere, 
the primary rays are spiraling down along the lines of 
the earth’s magnetic field, the velocity must be con- 
siderable, and slow electrons or positive ions seem to 
be excluded. St6rmer’s mathematical theory treats only 
the highly idealized case, that is, the movement of a 
single electrically charged particle in the earth’s field. 
THE UPPER ATMOSPHERE 
In nature we have to consider the movement of a 
stream or cloud of charged particles, perhaps of both 
signs and of different masses and charges, leaving the 
sun and approaching the earth. During the passage, 
the cloud will induce changes due to electrostatic attrac- 
tion or repulsion. In addition, upon entering the earth’s 
magnetic field, the positive and negative charges will 
be displaced relative to each other and polarization 
effects will occur. Chapman [3] and, later, Alfvén [1] 
have extensively treated the passage of an ion cloud 
emitted by the sun and approaching the earth. Chap- 
man has paid especial attention to the terrestrial effects 
appearing as magnetic storms, while Alfvén has dis- 
cussed the auroral effects. 
The Aurorae and Magnetic Storms 
Physically, one must regard the aurorae and the 
polar magnetic storms as two effects with the same 
primary cause—an ionizing corpuscular radiation pene- 
trating the atmosphere to a level of 80-100 km. Mag- 
netic storms have been studied thoroughly and differ- 
ent types and phases within a storm have been classified. 
The appearance of the polar magnetic storm is of the 
greatest interest for auroral studies. This type of storm 
appears regularly and is mainly confined to the belt 
of the auroral zones. Physically it can be explained as 
the effect of a current sheet flowing along or parallel 
to the auroral zone at a height of 100-150 km. Birke- 
land [2] studied this storm type extensively, using 
synoptic charts. The records from the Polar Year 
1932-33 gave a unique opportunity of studymg this 
storm type in more detail than ever before. McNish 
[9], Vestine [11], Chapman [18], and others have made 
synoptic and statistical studies of the polar magnetic 
storm. Most mteresting in this connection is the study 
by Vestine. He showed that the mean position of the 
current sheet coincided almost exactly with the posi- 
tion of the auroral zone, both lymg at a distance of 
about 23° from the earth’s magnetic axis point. 
A most remarkable effect is the increase of the angu- 
lar diameter of the auroral zone which coincides with 
increasing strength of magnetic storms. During great 
storms the aurorae are displaced towards the south; 
at places along the auroral zone the aurorae are then 
usually seen towards the south. Stormer has explamed 
this widening of the auroral zone as due to the effect 
of a ring current appearing in the equatorial plane of 
the earth, but lying far outside the atmosphere. 
The Aurorae and the Ionosphere 
The normal polar aurorae appear at heights ranging 
from 80 to 300 km above the surface of the earth. At 
lower latitudes and during great storms auroral forms 
may appear at much greater heights. The ionosphere 
consists mainly of two regions of ionization, the H-layer 
at 120 km above the earth, and the F)-layer at about 
230 km. The normal ionosphere is produced by the 
ionizing effect of the sun’s ultraviolet spectrum. Owing 
to the stratification in the air’s composition at differ- 
ent levels, ionization maxima are produced when the 
sun’s rays pass through the atmosphere, giving rise to 
