AURORAE AND MAGNETIC STORMS 
By L. HARANG 
Norwegian Defense Research Establishment 
Introduction 
In this survey of our present knowledge of aurorae, 
the first two sections which follow will be concerned 
mainly with the appearance, or morphology, of the 
auroral forms. The first section contains material ob- 
tained for the most part from observations at a single 
station, while in the second section the material (mainly 
height determimations) was obtained from two or more 
stations. The principal mstrument used was the auroral 
camera. The third section gives a survey of spectral 
investigations, and the fourth section outlines the cor- 
puscular theory of aurorae and magnetic storms. Al- 
though this theory must be regarded as only a first 
approximation to the real conditions, it is of the greatest 
value in discussing observations and in developing new 
points of view for further research. Two sections are 
devoted to a discussion of the connection between 
aurorae and magnetic storms, and a new field of re- 
search—the scattermg of radio waves in the VHF- 
band from aurorae—is briefly mentioned. We can as- 
sume that new and important knowledge of the details 
of the auroral processes will emanate from the fields 
of research mentioned in these two sections. The final 
section lists some promising lines for further research. 
Appearance of the Aurorae 
Forms. Vhe variety of auroral forms and the gradual 
shift from one form to another have made necessary a 
general classification, involving international coopera- 
tion. The atlas of auroral forms [10] published under 
the auspices of the International Union of Geodesy 
and Geophysics (Prague, 1927), is based mainly on the 
photographs and descriptions presented by Stormer. 
In this atlas the forms are divided into two main 
classes: forms without ray-structure and forms with 
ray-structure. An additional class, flaming aurorae, can 
be added. 
I. Forms without Ray-Structure. 
1. Homogeneous Quiet Arc (HA). This form may 
appear near the horizon, usually in the northern sky. 
It is diffuse along the upper border, sharp along the 
lower. This is the most stable of all auroral forms, and 
may maintain its shape for a period of time ranging 
from several minutes to half an hour. A double are 
often appears, or several arcs may simultaneously be 
seen in the sky. 
2. Homogeneous Bands (HB). These forms are ir- 
regular in shape and often move rapidly. As with HA, 
the upper border is diffuse while the lower is sharp. 
The homogeneous bands often change into bands with 
ray-structure. 
3. Pulsating Are (PA). This form is seldom visible. 
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The are or parts of an are show more or less regular 
pulsations in luminosity. The period of pulsation ranges 
from about twenty seconds down to a few seconds. 
In some cases PA may disappear almost completely 
and then reappear, at approximately the same place, 
at regular time intervals. 
4, Diffuse Luminous Surfaces (DS). These appear 
in the form of a diffuse veil over parts of the sky. They 
have no distinct limitations. These surfaces often ap- 
pear after intense displays of rays and curtains. The 
appearance of DS is therefore usually restricted to the 
later hours of the night, after brilliant auroral displays. 
5. Pulsating Surfaces (PS). These appear in the form 
of diffuse patches which come and go rythmically at the 
same place, retaining the same irregular form. They 
usually appear after a display of flaming aurorae. 
6. Feeble Glow (G). This form resembles the dawn. 
It often appears as the upper part of an are whose lower 
border is below the horizon. 
II. Forms with Ray-Structure. 
These consist of short or long rays which can be 
arranged in various ways. 
1. Bands with Ray-Structure (RB). These resemble 
HB, but consist of a series of rays close to one another 
along the band. When a band is near the magnetic 
zenith it may have the form of a corona. 
2. Draperies (D). If the rays along a band become 
longer, the form resembles a curtain or drapery, the 
lower border of which is often very luminous. With the 
exception of the homogeneous arc, the drapery is the 
most common auroral form. When it appears near the 
magnetic zenith, perspective may give D a fanlike form. 
3. Rays (R). These forms may be isolated, or parallel 
in bundles. In the latter case they often resemble D. 
The vertical extension may vary considerably, rang- 
ing from short rays to the very long, sunlit rays. 
4, Corona (C). When rays or draperies approach 
the magnetic zenith they appear to converge, because 
of the perspective, and a corona is formed. This corona 
is often incomplete, and only one-half of it may be 
developed. It can also be formed by bands which con- 
verge towards the magnetic zenith. 
Ill. Flaming Aurorae (/). 
This is a form consisting of strong waves of light 
which characteristically move rapidly one after the 
other in the direction of the magnetic zenith. The waves 
may take the form of detached ares which move up- 
wards towards the magnetic zenith. Flaming aurorae 
appear frequently after strong displays of rays and 
curtains and are often followed by the formation of a 
corona. 
