(2) Irregular variations in the ionosphere can adversely affect communications without any
advance warning. Common irregular variations include sporadic E, sudden ionospheric disturbances,
and ionospheric storms.
(a) Sporadic E variations occur when the excessively ionized E layer often blanks out the
reflections back from the higher layers. It may also cause unexpected propagation of signals hundreds
of miles beyond the normal range.
(b) Sudden ionospheric disturbance (SID) is attributed to a bright solar eruption and
results in abnormal ionization of the D layer. SID causes total absorption of all frequencies above about
1 MHz. It occurs without warning and can last from a few minutes to several hours. The immediate
effect is that radio receivers seem to "go dead."
(c) Ionospheric storms are caused by disturbances in the Earth's magnetic field as a result
of solar eruptions. During ionospheric storms, sky wave reception above about 1 MHz shows low
intensity and is subject to a type of rapid blasting and fading called "flutter fading." These storms may
last from several hours to days, and usually extend over the entire Earth.
3. Summary. In this lesson, you learned about sky wave propagation and the effects of atmospheric
a. The ionosphere has three regions (D, E, and F). The F region is divided into two layers (F1
and F2). At night, these layers combine and are useful for HF long-distance radio communications.
c. Each ionospheric layer has a maximum frequency (called the critical frequency) at which
radio waves can be transmitted vertically and refracted back to Earth.
d. The rate at which a wave of a given frequency is reacted by an ionized layer depends on the
angle at which the wave enters the layer.
e. Additional signal losses are due to ground-reflection loss and free space loss.
f. EMI is derived from two sources--manmade and natural. Examples of manmade EMI
generate interfering signals. Natural EMI sources include thunderstorms, cosmic sources, and the sun.
g. Variations in the ionosphere are caused by the Earth's position in relation to the sun, and by
the sun's activity.
(1) Regular variations that affect the degree of ionization in the ionosphere are divided into
four classes. These classes are daily, seasonal, 27-day sunspot cycle, and 11-year sunspot cycle.