Near-Real-Time E-Layer Critical Frequency Map
The following image is a recent high-resolution global
map of E-layer critical frequencies. This corresponds to the maximum radio
frequency that can be reflected by E-region of the ionosphere at vertical
incidence (that is, when the signal is transmitted straight up into the
ionosphere). It is also a map showing the current location of the auroral
ovals, the sunrise/sunset terminator and the regions of the world where
the sun is 12 degrees below the horizon (which estimates the gray-line
corridor where HF propagation is usually enhanced). This is one of a plethora
of constructable maps that is produced by PROPLAB-PRO
Version 2.0 (formerly known as SKYCOM PRO), a very powerful
radio propagation laboratory software package for IBM or compatible computers,
ideal for amateur or professional radio communicators. Instructions on
how to use this map follow below.
(This map is updated every 5 minutes.)
Click on PROPLAB-PRO
Version 2.0 for additional map samples.
Using this Map
The E-region is the second-lowest primary ionospheric
region and is the location of the ionosphere where the greatest signal
attenuation can occur. Radio communicators most often do not want
their signals to spend very much time in this region of the ionosphere.
Signals which are reflected within the E-region spend the greatest time
in the E-region and are accordingly attenuated the most.
To help avoid the effects of the E-region, transmission
frequencies are increased to levels high enough so that transmitted signals
quickly penetrate through the E-region and travel on toward the F-regions.
The E-region critical frequency corresponds to
the highest frequency of a signal is transmitted straight upward
(vertically incident) that can be returned back to the Earth by the
E-region alone. Signals that exceed the E-layer critical frequency
(and are vertically incident) will penetrate the E-region and travel toward
the F-regions. Signals that are below the critical E-layer frequency will
always be reflected back to the Earth.
These maps are therefore useful to help determine
the regions of the ionosphere where the E-region is strong enough to influence
communications. Unless you want to use the E-region as the reflecting medium,
the transmission frequency must exceed the greatest contoured value along
the desired path.
As can be seen, the E-layer disappears during
the evening.
The map shows the radio auroral zones as green bands
near the northern and southern poles. The area within the green bands is
known as the auroral zone. Radio signals passing through these auroral
zones will experience increased signal degradation in the form of fading,
multipathing and absorption.
The radio auroral zones are typically displaced
equatorward from the optical auroral zones (or the regions where
visible auroral activity can be seen with the eye).
The great-circle signal path from the Eastern
United States to Tokyo is shown along with the distance of the path (in
km) and the bearing from the U.S. to Tokyo (in degrees from north).
If this signal path crosses through the
green lines indicating the position and width of the radio auroral zones,
propagation will be less stable and degraded compared to if the signal
never crossed through the auroral zones. Using your mouse, PROPLAB-PRO
will let you plot the great-circle paths and azimuths between any two points
while this display is continually updated.
The yellow Sun symbol near the equator indicates the
location where the Sun is directly overhead.
The regions of the world where the Sun is exactly rising or setting
is known as the Grayline and is shown as the solid gray-colored
line that is closest to the Sun symbol.
The second solid gray-colored line defines the regions of the world
where the Sun is exactly 12 degrees below the horizon. This line defines
the end of evening twilight. Everything inside of this second
line is experiencing night-time conditions.
The area between the two lines (shaded a lighter shade than the night-time
sector) is known as the grayline and has special significance
to radio communicators. Signals which travel inside the grayline
region often experience significant improvements in propagation because
of the loss of ionization in the D-region as the Sun sets. However, because
the higher F-regions of the ionosphere remain strongly ionized for longer
periods of time, signals with higher frequencies are able to travel to
greater distances with less attenuation when they are within the grayline.
The great-circle path from the eastern U.S. to Japan
is also shown with the accompanying distance (in kilometers)
and bearing (clockwise from north). Notice how this path
may occassionally pass into the influential auroral zones if geomagnetic
activity increases or during the night-times.
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