THE IONOSPHERE 
By S. L. SEATON 
Geophysical Institute, University of Alaska 
Introduction 
The ionosphere is that portion of the earth’s atmos- 
phere containing a sufficient free-electron population 
to noticeably affect propagation of electromagnetic 
waves in the radio-frequency spectrum. It might be 
described also as that region of our atmosphere in 
which the refractive index has a value perceptibly less 
than unity. 
The free-electron concentration is brought into being 
through detachment of electrons from negative ions 
and by ionization of neutral molecules and atoms. Con- 
temporary thought indicates that the principal elec- 
tron-liberating agent is ultraviolet light from the sun, 
but other factors are probably active, for example, 
bombardment by particles, freeing of electrons through 
molecular recombination processes, and so on. 
There is a tendency towards horizontal stratifica- 
tion of electron distribution. Important concentrations 
of free electrons exist from about 90 km to over 400 
km above sea level. In a broad sense, greatest electron 
densities are to be found at the subsolar point. Because 
of the high probability of collision and the estimated 
maximum energy falling on the atmosphere, it is be- 
lieved that large free-electron populations cannot exist 
continuously at altitudes less than about 10 km. Bal- 
loon-carried detectors indicate no large concentrations 
of electrons to 30 km, and radio measurements point 
to no continuous free-electron populations below about 
60 km. 
There seems to be no reason to restrict the upper 
limit of the ionosphere, although above about 600 km 
the atmospheric density is so low that the free-electron 
concentration is probably limited. Thus the upper and 
lower limits of the ionosphere are not well-defined. 
Historical 
As early as 1880 those studying terrestrial magnetism 
and its variations postulated an electrically conducting 
region high in the atmosphere in which electric currents 
flowed, inducing at the earth’s surface a small magnetic 
field. This postulate seems to have been stimulated by 
speculation upon the causes of the aurora polaris. How- 
ever, it was not until 1925 that direct evidence was ob- 
tained by Appleton and Barnett in England, and by 
Breit and Tuve in America, proving the existence of a 
conducting region in the earth’s upper atmosphere. 
These investigators identified electromagnetic waves 
returned from a reflecting region above their energy 
source and determined grossly the height of this re- 
flectng region. At first only one layer was deteeted 
but subsequently echoes were found to be returned from 
other heights, giving rise to the idea of separated strata. 
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More recent investigations point towards a continuous 
region with maxima and minima. 
In using the term free-electron density, it must be 
realized that a free electron is one which has become 
separated from its original environment as part of a 
negative ion, a positive ion, or a neutral particle. This 
free electron is likely to be captured soon by another 
similar environment, there to remain until set free 
again. Electrons per se, perhaps arriving from space, 
are usually neglected in considerations of the iono- 
spheric electron density, since the origin of specific 
electrons cannot be determined at present. Thus the 
free-electron concentration is made up of the average 
number of free electrons per cubic centimeter measured 
over convenient lengths of time, usually of the order 
of a fraction of a second or more. The contribution made 
by electrons other than those supposed to belong natur- 
ally to the environment has not been investigated, and 
indeed the means for such an investigation does not 
seem to be at hand. The same thing is true in regard to 
the flux of energy through the region. 
Neglecting philosophical utterances, which may have 
struck home by chance, the mathematician Gauss, 
followed by the physicists Balfour Stewart and Sir 
Arthur Schuster, showed from studies of terrestrial 
magnetic-field variations that there should be a region 
of high electrical conductivity in the upper atmosphere. 
Their works were known to only a few, and when Mar- 
coni succeeded in sending electromagnetic waves over 
long distances in 1900 it was not understood how these 
waves could bend round the earth. Unaware of the 
earlier suggestions, Professor A. EH. Kennelly of Harvard 
University, and the British engineer, O. Heaviside, 
independently proposed that an electrically conducting 
region must exist high in the atmosphere which would 
bend electromagnetic waves back to earth at a distance. 
Of these two approaches, that is, inference from ter- 
restrial magnetic variations, and propagation of elec- 
tromagnetic waves, only the latter has given direct 
evidence of the ionosphere. The former may have un- 
expected possibilities. The latter gives information 
about the former. 
Methods for Studying the Ionosphere 
In addition to the tools already mentioned, the in- 
strument-carrying rocket and examination of energy 
arriving at the earth from space promise useful infor- 
mation. Other means of exploring the ionosphere, for 
example, study of compressional wave propagation, 
light scattered from modulated searchlight beams, me- 
teorological variations, and cosmic ray changes, are 
worthy of serious consideration. Further seemingly more 
remote possibilities exist. 
