THE IONOSPHERE 
by other distributions as analysis may indicate. Ap- 
pleton has detected tidal effects in the H-region heights, 
and Jones has recently found a lunar tide in F-region 
thickness of some 10 km for a station in high latitudes. 
It is to be noted that additional regions are indicated 
beyond and between those shown in Figs. 1 and 2. 
The distribution in Fig. 3 represents the most likely 
ionospheric situation according to contemporary 
thought. (There is a considerable variation in heights 
from this typical situation.) The penetration frequen- 
cies of Fig. 2 occur whenever the ionization gradient 
along the direction of wave travel becomes zero in 
Fig. 3. 
The D-Region 
Starting at the bottom in Fig. 3, the D-region is not 
well-defined. There is some evidence that very low 
radio frequencies are reflected at oblique incidence from 
heights in the vicinity of 60 km. It is also known that 
medium-to-high frequencies are absorbed at times in a 
region below 100 km. The latter information gives no 
evidence of the height of the absorbing area except that 
it must be where the mean free path of the electron is 
small. Wulf and Deming believe that formation of the 
D-region is the result of photo-ionization of ozone by 
ultraviolet light. Maximum electron concentration is 
probably of the order of 2 X 10° electrons per ce. Re- 
combination of electrons with positive ions near the 
60-km height is very rapid so that within a few seconds 
after the ionizing agent ceases to operate any existing 
free-electron population will disappear. It is im some 
zone below 100 km that the direct relationship between 
solar chromospheric eruptions and absorption of radio- 
wave energy appears. At such times some evidence is 
available poimting to enhancement of low-frequency 
oblique-incidence radio-wave propagation by virtue of 
reflections from a height of about 60 km. Height varia- 
tions and details of the distribution are unknown. Elec- 
tron density varies diurnally, seasonally, and with solar 
activity. 
The Normal E-Region 
Above the D-region there is to be found a zone in 
which changes in electron density follow solar altitude 
quite closely. In the E-region maximum free-electron 
concentration occurs shortly after local noon; reaching 
values of the order of 83 X 10° electrons per cc. For the 
normal H-region, ionization disappears within a few 
minutes after ultraviolet ight from the sun is cut off 
by the earth. Heights display some variation with geo- 
graphic location and vary diurnally, seasonally, and 
with the sunspot cycle. Lowest E-region heights are 
near 90 km, with maxima near 130 km. Electron distri- 
bution within the E-region is not known accurately but 
probably follows either a Chapman distribution or a 
parabolic law. The lower boundary is usually well- 
defined. The half-thickness is of the order of 5 to 20 
km. It is m this region that the transition from pre- 
dominantly molecular oxygen to predominantly atomic 
oxygen occurs. It is thought that electrons are freed 
in the E-region by photo-ionization of oxygen molecules. 
337 
Sporadic E-Region 
In addition to the normal E-region, there occurs an 
H-region ionization sporadic in occurrence and density. 
Some of this sporadic ionization is brought about by 
meteors encountering the earth’s atmosphere. However, 
at present it is believed that meteors are not the only 
source, but that some other mechanism is present. There 
is a semblance of system in occurrences of sporadic 
H-region ionization such that the phenomenon appears 
most frequently in the arctic regions at night, particu- 
larly near zones of maximum auroral activity. 
Maximum electron concentrations reach values for 
the sporadic E-region of over 3 X 10° electrons per 
cc. Heights appear to be about the same as those of the 
normal H-region. Duration of sporadic E-region ioniza- 
tion may be from a few seconds to several hours. There 
is some indication that the sporadic H-region may be 
of patchy form and at times may move or propagate 
rapidly. The recombination coefficient between elec- 
trons and positive ions is of the order of 1 X 10™ ce 
sec! for the H-region generally. 
The E,-Region 
Until recent times it was thought that E- and F,- 
regions were virtually separate entities with essentially 
no intermediate free-electron density. However, enough 
experimental evidence is now available to mdicate the 
probability of an E, distribution centering near 150 
km in height. This distribution is not always seen be- 
cause the maximum concentration is normally less than 
that of the H-region. Little is known of the H»-region, 
but it may be existent by virtue of photo-ionization of 
atomic oxygen. It appears to occur most frequently in 
the daytime. 
The F,-Region 
The F\-region exists in identifiable form in the day- 
time with large solar altitudes. In general behavior the 
F,-region resembles the E-region. Maximum densities 
occur a little after local noon and attain values of the 
order of 4 X 10° electrons per ce. With low solar altitudes 
F,- and F.-regions merge to form the general F-region 
which persists throughout the night at all latitudes 
where measurements have been made. Maximum elec- 
tron density varies diurnally, seasonally, and with sun- 
spot activity. There is at present no conclusive evidence 
that ionization is caused by other than ultraviolet 
light. Wulf and Deming favor one of the nitrogen- 
molecule absorptions as responsible for formation of 
the F,-region. Mohler has pointed out that, because 
the recombination coefficient between electrons and posi- 
tive ions is a function of pressure, the height at which 
maximum production of ionization takes place is prob- 
ably below the height at which maximum concentra- 
tion occurs. Heights of the Fi-region vary over w ider 
limits than do those of the E-region. The range in height 
is roughly from 160 to 280 km. There is a diurnal v arla- 
tion with lowest heights around midday. This region 
rises slightly to merge with the F.-region near sunrise 
and sunset. Seasonal and longer-period variations in 
height occur and there is a variation with latitude. 
