AURORAE AND MAGNETIC STORMS 
the normal stratification of the ionosphere—the H-, 
F,-, and F.-layers. In addition to the ultraviolet rays 
from the sun, corpuscular rays emitted by the sun may 
also have an ionizing effect when they penetrate the 
earth’s atmosphere. The penetration of these corpuscu- 
lar rays is made visible by the appearance of aurorae. 
A marked increase in the ionization of the layers during 
auroral displays might therefore be expected, and this 
is what actually happens. The effects of aurorae on the 
ionosphere are treated below. 
The Appearance of the Abnormal E-Layer during Au- 
roral Displays. We must here distinguish between the 
effects of faznt and strong auroral displays. Durmg a 
faint auroral display the electrically charged corpuscles 
will emit light visible as aurorae, and in addition they 
will produce ionization by impact which will increase 
the electron density of the layers, especially the H-layer. 
In polar regions there is a very close connection between 
the appearance of faint or medium aurorae, small mag- 
netic storms, and a simultaneous increase in the elec- 
tron density of the H-layer. This increase of the electron 
density of the layers can be followed by radio-echo 
observations which measure the critical penetration 
frequencies of the layers. From the critical penetration 
frequencies one can easily calculate the maximum 
electron densities of the layers. Figure 6 shows how 
the maximum electron densities of the F.- and E-layers 
change at a polar station during a small magnetic 
storm accompanied by a faint aurora overhead. 
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Fra. 6.—Appearance of abnormal H-layer during a small 
magnetic storm accompanied by faint aurorae (observed in 
Tromso). Magnetic declination (D), horizontal intensity (H), 
and vertical intensity (V) of the magnetic field are shown 
above. (MET = Middle European Time.) 
During stronger magnetic storms and aurorae the 
conditions are more complex, and the effect of ab- 
sorption is of the greatest importance. During strong 
auroral displays, the radio echoes reflected from the 
ionosphere are usually weak, and there may be a com- 
plete cessation of radio echoes during the greatest dis- 
303 
plays. At the same time there may be a complete break- 
down of medium- and short-wave commercial radio 
transmission. The explanation of these two effects— 
the increase in electron density in the E-region during 
small storms and faint aurorae, and the complete cessa- 
tion of radio echoes during strong magnetic storms—is 
the following: During small storms the impact of the 
electrically charged particles imcreases the ionization 
of the E-layer down to a height of 100-110 km. This 
increase In ionization is measured by the increase in 
critical penetration frequencies of the layer. During 
strong magnetic storms and aurorae, this increase in 
ionization is displaced farther down in the atmosphere, 
to a height of about 80 km. Here the density of the air 
will be considerably greater than in the H-layer, and 
this will increase the collision frequency between the 
free electrons and the surrounding gas molecules. The 
increase in collision frequency will cause a strong ab- 
sorption of radio waves in the medium- and short-wave 
bands. The irregular disturbances in radio communi- 
cations show very close connections with the appearance 
of magnetic storms and aurorae, and it is possible, 
to a certain degree, to forecast the disturbances by 
carefully observing magnetic storms and aurorae. 
The propagation of long and very long radio waves 
is only slightly influenced by irregular changes in the 
ionosphere. The long waves are propagated mainly as 
ground waves over short distances. Over longer dis- 
tances there will be a fraction—about 10-20 per cent 
of the field strength at the receiving station—which 
is propagated as a reflected wave. For such long waves 
the ionosphere acts almost as a reflecting mirror. An 
increase in the electron density in the lowest part of the 
E-layer only increases the reflective power of the layer 
for these waves, and receiving conditions for very long 
radio waves are therefore usually more favorable during 
disturbed periods than during quiet periods. 
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Fic. 7—Critical frequencies of the F.-layer and magnetic 
character numbers (observed in Troms). During periods of 
great magnetic activity the critical frequencies decrease. 
The Effects on the Fs-Layer. The effect of magnetic 
storms and aurorae on the F2-layer is more complex, 
since disturbances at this height (about 250 km) are 
accompanied by changes in the structure of the layer. 
