THE IONOSPHERE 
Since 1925 only electromagnetic-wave propagation 
has given adequate information about the ionosphere. 
In 1925 there were two laboratories, one near London, 
England, and one near Washington, D. C., at which 
direct measurements of the ionosphere were being made. 
In 1949 the reasonable hope of direct measurement by 
instrument-carrying rockets presented itself. In 1949 
over sixty ionospheric measuring stations scattered 
throughout the populated regions were in operation. 
Despite these advances an adequate number of ob- 
serving stations does not yet exist. There is a serious 
lack of knowledge over a twelve-degree-wide belt in 
equatorial regions, and both north and south polar 
areas are virtually unexplored from an ionospheric 
standpoint. Regions of especial interest are where geo- 
magnetic and geographical equators cross, where they 
are farthest apart, and polar regions, particularly in 
the zones of greatest auroral activity. Intermediate 
locations are also necessary. 
Because the electromagnetic wave has so far been 
of greatest usefulness in exploring the ionosphere, theo- 
retical developments have occurred allowing the alter- 
ations suffered by the wave in its passage through an 
ionized atmosphere to be interpreted. In 1912 Eccles 
laid the foundation for determining the effect of charged 
particles upon the propagation of radio waves. The 
theory was incomplete and in 1924 Larmor supplied 
improvements. The Eccles-Larmor theory forms the 
foundation for understanding electromagnetic-wave 
propagation in the ionosphere, if the effect of the earth’s 
magnetic field is neglected. The problem becomes much 
more complicated with inclusion of the effect of col- 
lisional friction and of the earth’s magnetic field tra- 
versing the ionosphere. The magneto-ionic theory de- 
veloped by Appleton, Nichols and Schelleng, Hartree, 
Goldstei, and others, together with the theories of 
Lorentz, Booker, and contemporaries, give the broad 
foundations for interpretation of information collected 
experimentally. There are still uncertainties, and with- 
out doubt additional elegant theoretical work and ex- 
perimentation must be done before our knowledge ap- 
proaches completeness. But notable progress has been 
made since 1880. 
Elementary Concepts 
Originally the ionized region of the upper atmosphere 
was called the Kennelly-Heaviside layer. Later, when 
stratification was indicated, each investigator named 
his “layer.” The ensuing arguments over names be- 
came so troublesome that by mutual consent the whole 
structure was designated the zonosphere, with letters 
and subscripts to indicate the principal regions within 
it. There are three principal maxima of free-electron 
concentration: the E-region at about 100 km, the Fi- 
region at about 225 km, and the F,-region at about 
350 km. The normal E- and F-regions develop maxi- 
mum electron density with greatest solar altitude and 
exist only by day. Behavior of the F,-region is more 
complicated. With low solar altitude F,- and F,-regions 
merge to form a general F-region which persists through- 
out the night. In the H-region sporadic ionization oc- 
curs with quite large free-electron concentrations. Spo- 
330 
radic H-region phenomena are not understood but the 
ionization appears to be patchy and does not follow 
solar altitude control. Figure 1 is drawn to a distorted 
scale in order to permit visualization of the principal 
ionospheric regions. 
NORTH POLE 
E-REGION 
F, - REGION 
F>— REGION 
REGION 
SOUTH POLE 
Fra. 1—Principal ionospheric regions. 
In addition to diurnal variations dependent upon 
solar altitude there is a pronounced change related to 
solar activity as measured by sunspot numbers. Argu- 
ments in favor of some ionization being caused by 
charged particles bombarding the atmosphere are called 
up by behavior of the F,-region. Because the earth’s 
magnetic field extends far into space, charged particles 
entering the upper atmosphere from space will be de- 
flected by this field. Average maximum F>-region ioniza- 
tion, studied with respect to the geomagnetic coordi- 
nates, shows values in accordance with action of charged 
particles under*the influence of the magnetic field. 
However, the origin of the particles, their charge, mass, 
and velocity, are not known. It has not been definitely 
shown, either, that the observed behavior is surely 
caused by charged particles. If particles do influence 
the ionospheric layers, it is not known whether they 
come from space or whether they are parts of our own 
atmosphere thrown high up by thermal agitation and 
returning under gravitational influence. 
Typical Data 
In exploring the ionosphere by means of the elec- 
tromagnetic wave at radio frequency, it is usual to 
emit short wave-trains approximately 10~* sec long 
of known radio frequency and polarization. The time 
of departure of each wave packet is noted. One or more 
wave packets may be expected to return from the iono- 
sphere when the radio frequency emitted lies below a 
value limited by the number of free electrons and de- 
scribed approximately by 
N = 1.24 (10)4f2, (1) 
where N is the number of electrons per cubic centimeter 
and f is the radio frequency in megacycles per second. 
The returning wave packets are altered during their 
flight. The elapsed time between transmission and re- 
ception, magnitude of returned energy, polarization of 
the returned wave, splitting of the original wave packet 
