304 
There is an inverse correlation between magnetic ac- 
tivity and critical penetration frequency, that is, the 
maximum electron density of the layer. During dis- 
turbed periods the maximum electron density decreases. 
This effect is commonly explained by assuming a verti- 
cal expansion of the ionosphere during disturbed peri- 
ods. Figure 7 shows the imverse correlation between 
the critical frequencies of the F>-layer and the magnetic 
character numbers, as recorded at a polar station. 
400 400 
200 200 
leg w 
lu c 
cS Ww 
w \= 
wW 
= = 
a ° 
= = 
oa =< 
400) 400 
200 200) 
ELECTRONS x 102 PER CmM® 
Fie. 8.—Changes in the structure of the ionosphere in the 
auroral zone from a quiet day to the following disturbed day 
(from records in Troms6). The magnetic records are shown 
above. The echo curves and the structure of the ionosphere are 
given below. 
The changes in the structure of the ionosphere during 
intense magnetic storms are evident from the echo 
records. A typical example indicating the conditions 
of the ionosphere on a quiet day and a following dis- 
turbed day at a polar station is given in Fig. 8. The 
structure of the ionosphere can be deduced from the 
character of the echo curves (virtual heights of echoes 
versus frequency). The expansion of the F.-layer, and 
the more pronounced stratification between the F»- 
and F\-layers, are evident. 
Scattered Reflections from Aurorae in the VHF-Band. 
It has already been mentioned that strong aurorae and 
magnetic activity are accompanied by an increase of 
ionization in the lower edge oi the E-layer. This in- 
crease in ionization at a level at which the density is 
comparatively high produces a strong absorption of 
THE UPPER ATMOSPHERE 
the radio waves in the usual high-frequency band 
1-15 me sec, which is usually in operation for iono- 
spheric radio-echo recording. Durmg strong auroral 
displays this absorbing region usually screens off the 
higher part of the ionosphere, and the echo records 
give no information about the conditions of the iono- 
sphere and auroral region during the phases of strong 
absorption. 
The amount of absorption will, however, decrease 
with mereasing frequency, and it thus becomes possible 
to use waves in the VHF-band for studying scattering 
effects from the dense ionization in the aurorae. Scatter 
from aurorae has been obtained by Harang and Stoffre- 
gen [6], using 41 me sec waves. Lovell, Clegg, and 
Ellyett [8] used frequencies of 72 and 46 me sec 
and obtained scattering effects. The use of VHF-waves 
for the study of scattermg in the ionosphere must be 
regarded as a new and promising field of research for 
obtaining information during the phases of strong ab- 
sorption. 
Concluding Remarks 
Auroral phenomena are the visual results of an ac- 
tion of a stream of solar particles on the upper layers 
of the earth’s atmosphere. Magnetic storms and changes 
in the structure of the ionosphere occur simultaneously 
with the aurorae. 
A quantitative theory of the aurorae, magnetic 
storms, and the irregular changes within the ionosphere 
would presuppose a knowledge of the nature of the 
solar particles and a detailed knowledge of the physi- 
cal condition of the upper atmosphere. 
In the preceding sections it has been pointed out that 
auroral phenomena give no definite indication of the 
nature of the solar particles producing the aurorae. 
Furthermore, the physical conditions of the upper at- 
mosphere—density, pressure, temperature, chemical 
composition, vertical and horizontal displacement of the 
air masses, and the variations of these quantities during 
the day and the year—have mainly been deduced in 
indirect ways. The picture of the physical condition 
of the upper atmosphere has therefore been put together 
like a mosaic, with information supplied by the various 
branches of geophysics. If one of these questions—either 
the nature of the solar corpuscles or the physical state 
of the upper atmosphere—could be solved independ- 
ently, an important step forward would have been 
taken in the theory of the aurora. Through V-2 rocket 
experiments, reliable values for densities and pressures 
up to heights of 90 km have now been obtained [7]. 
Further measurements will remove many uncertain- 
ties concerning the physical properties of the upper 
atmosphere. A direct experimental investigation of the 
nature of the solar particles should be possible by 
means of V-2 rocket flights through aurorae in polar 
regions. 
The position of the auroral zone and the distribu- 
tion of the auroral frequency with the latitude has 
hitherto been based on Fritz’ chart, published in 1881 
[4]. In the last few decades a vast amount of statisti- 
cal information has been gathered which should pro- 
