Near-Real-Time Map of Solar Zenith Angles
The following image is a recent high-resolution global
map of Solar Zenith Angles, or angles indicating the elevation of the Sun
above the horizon (in degrees). 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, a very powerful radio propagation 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
This map shows you precisely how high in the sky
the Sun is at any location around the world. The contours of this map are
given in degrees and refer to the distance the Sun is away from the zenith
(or that point directly overhead). A contour labelled 10 degrees would
define the regions of the world where the Sun is exactly 10 degrees away
from the zenith (or in other words, the Sun would be 10 degrees [or about
an outstretched hand-span] away from the point directly above your head).
A contour labelled 45 degrees would define regions of the world where the
Sun is half-way between the horizon and the zenith. A contour labelled
90-degrees would define all of those regions around the world where the
Sun is exactly on the horizon (rising or setting). Contours greater than
90 degrees indicate regions of the world where the Sun is below the horizon.
These maps are very useful for radio communicators
for several reasons. First, they can be used to help determine where the
sun is rising or setting (notice on this map that the 90-degree contour
line is obscured by the thicker gray-colored sunrise/sunset terminator
line). They can also be used to help determine the regions of the world
where solar-flare related short-wave fadeouts are strongest.
Solar flares result in the attentuation of radio
signals if the signals are passing through regions of the ionosphere that
are sunlit (indicated by contours that are less than or equal to 90 degrees).
The intensity of the attenuation is approximately proportional to the solar
zenith angle. A signal that passes through the ionosphere where the solar
zenith angle is low (corresponding to the region where the Sun is the highest
in the sky) will experience the greatest signal loss. Signals that pass
through regions where the solar zenith angle is higher will experience
less signal loss (and hence greater signal strength at the receiver). These
maps are therefore useful to help diagnose the potential impacts of solar
flares on communications.
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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