TC 9-64 _________________________________________________________________________
2-104. Sporadic E. Irregular cloud-like patches of unusually high ionization,
called sporadic E, often form at heights near the normal E layer. Exactly what
causes this phenomenon is not known, nor can its occurrence be predicted. It is
known to vary significantly with latitude, and in the northern latitudes, it
appears to be closely related to the aurora borealis or northern lights.
2-105. At times the sporadic E is so thin that radio waves penetrate it easily
and are returned to earth by the upper layers. At other times, it extends up
to several hundred miles and is heavily ionized.
2-106. These characteristics may be either harmful or helpful to radio wave
propagation. For example, sporadic E may blank out the use of higher, more
favorable ionospheric layers or cause additional absorption of the radio wave
at some frequencies. Also, it can cause additional multipath problems and
delay the arrival times of the rays of RF energy.
2-107. On the other hand, the critical frequency of the sporadic E is very
high and can be greater than double the critical frequency of the normal
ionospheric layers. This condition may permit the long distance transmission
of signals at unusually high frequencies. It may also permit short distance
communications to locations that would normally be in the skip zone.
2-108. The sporadic E can form and disappear in a short time during either
the day or night. However, it usually does not occur at the same time at all
transmitting or receiving stations.
2-109. Sudden ionospheric disturbances. The most startling of the
ionospheric irregularities is known as a sudden ionospheric disturbance
(SID). These disturbances may occur without warning and may prevail for
any length of time, from a few minutes to several hours. When SID occurs,
long distance propagation of HF radio waves is almost totally "blanked out."
The immediate effect is that radio operators listening on normal frequencies
are inclined to believe that their receivers have gone dead.
2-110. When SID has occurred, examination of the sun has revealed a bright
solar eruption. All stations lying wholly, or in part, on the sunward side of
the earth are affected. The solar eruption produces an unusually intense
burst of ultraviolet light, which is not absorbed by the F2, F1, and E layers,
but instead causes a sudden abnormal increase in the ionization density of
the D layer. As a result, frequencies above 1 or 2 megahertz are unable to
penetrate the D layer and are usually completely absorbed by the layer.
2-111. Ionospheric storms. Ionospheric storms are disturbances in the
earth's magnetic field. They are associated, in a manner not fully understood,
with both solar eruptions and the 27-day intervals, thus corresponding to the
rotation of the sun.
2-112. Scientists believe that ionospheric storms result from particle
radiation from the sun. Particles radiated from a solar eruption have a slower
velocity than ultraviolet light waves produced by the eruption. This would
account for the 18-hour or so time difference between a SID and an
ionospheric storm. An ionospheric storm that is associated with sunspot
activity may begin anytime from 2 days before an active sunspot crosses the
central meridian of the sun until four days after it passes the central
meridian. At times, however, active sunspots have crossed the central region
of the sun without any ionospheric storms occurring. Conversely, ionospheric
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