Twenty-Seven Day Cycle
The number of sunspots present at any one time
is constantly changing as some disappear and new ones
emerge. As the sun rotates on its own axis, these
sunspots are visible at 27-day intervals, which is the
approximate period for the sun to make one complete
revolution. During this time period, the fluctuations
in ionization are greatest in the F2 layer. For this
reason, calculating critical frequencies for long-distance
communications for the F2 layer is not possible and
allowances for fluctuations must be made.
Sunspots can occur unexpectedly, and the life span
ELEVEN-YEAR SUN SPOT CYCLE is a regular
cycle of sunspot activity that has a minimum and
maximum level of activity that occurs every 11 years.
During periods of maximum activity, the ionization
density of all the layers increases. Because of this,
the absorption in the D layer increases and the critical
frequencies for the E, F1, and F2 layers are higher.
During these times, higher operating frequencies must
be used for long-range communications.
Irregular variations are just that, unpredictable
changes in the ionosphere that can drastically affect
our ability to communicate.
The more common
variations are sporadic E, ionospheric disturbances,
and ionospheric storms.
Irregular cloud-like patches of unusually high
ionization, called the sporadic E, often format heights
near the normal E layer. Their exact cause is not
known and their occurrence cannot be predicted.
However, sporadic E is known to vary significantly
with latitude. In the northern latitudes, it appears to
be closely related to the aurora borealis or northern
The sporadic E layer can be so thin that radio
waves penetrate it easily and are returned to earth by
the upper layers, or it can be heavily ionized and
extend up to several hundred miles into the ionosphere.
This condition may be either harmful or helpful to
On the harmful side, sporadic E may blank out
the use of higher more favorable layers or cause
additional absorption of radio waves at some frequen-
cies. It can also cause additional multipath problems
and delay the arrival times of the rays of RF energy.
On the helpful side, the critical frequency of the
sporadic E can be greater than double the critical
frequency of the normal ionospheric layers. This may
permit long-distance communications with unusually
high frequencies. It may also permit short-distance
communications to locations that would normally be
in the skip zone.
Sporadic E can appear and disappear in a short
time during the day or night and usually does not occur
at same time for all transmitting or receiving stations.
Sudden Ionospheric Disturbances
Commonly known as SID, these disturbances may
occur without warning and may last for a few minutes
to several hours. When SID occurs, long-range hf
communications are almost totally blanked out. The
radio operator listening during this time will believe
his or her receiver has gone dead.
The occurrence of SID is caused by a bright solar
eruption producing an unusually intense burst of
ultraviolet light that is not absorbed by the F1, F2,
or E layers. Instead, it causes the D-layer ionization
density to greatly increase. As a result, frequencies
above 1 or 2 megahertz are unable to penetrate the
D layer and are completely absorbed.
Ionospheric storms are caused by disturbances in
the earths magnetic field. They are associated with
both solar eruptions and the 27-day cycle, meaning
they are related to the rotation of the sun. The effects
of ionospheric storms are a turbulent ionosphere and
very erratic sky-wave propagation. The storms affect
mostly the F2 layer, reducing its ion density and
causing the critical frequencies to be lower than