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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.
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.
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.
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.
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.
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 their receivers have
gone dead.
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.
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.
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 storms have occurred when there were no visible
spots on the sun and no preceding sid. As you can see, some correlation between ionospheric storms, sid,
and sunspot activity is possible, but there are no hard and fast rules. Ionospheric storms can occur
suddenly without warning.
The most prominent effects of ionospheric storms are a turbulent ionosphere and very erratic sky
wave propagation. Critical frequencies are lower than normal, particularly for the F2 layer. Ionospheric
storms affect the higher F2 layer first, reducing its ion density. Lower layers are not appreciably affected
by the storms unless the disturbance is great. The practical effect of ionospheric storms is that the range of
