Click on PROPLAB-PRO Version 2.0 for additional map samples.
It is important to remember that these contours refer to the transmitted signals that are vertically incident on the ionosphere. All long-distance communications use signals that are obliquely incident on the ionosphere (that is, the radio signals are passing through the ionosphere at an angle instead of head-on).
The purpose of this map is to help illustrate
regions of the ionosphere that are weak and strong. Critical F2 layer frequencies
in excess of about 8 MHz correspond to regions of the ionosphere that are
relatively strong and capable of reflecting high-frequency signals over
longer distances. Critical frequencies below about 4 MHz are weaker and
will result in greater signal loss to space, lower MUFs and greater signal
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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