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Current Grayline Map of MUFs
The map above can be used by Topband operators to determine the current
location of the grayline (sunrise/sunset terminator), and other useful
information regarding the state of the Earth's ionosphere and auroral zones.
The sunrise/sunset terminator is the thick solid gray line that is located
nearest the Sun symbol (the symbol showing where the Sun is positioned
directly overhead). The other thick gray line represents regions where the
Sun has set on the ionosphere.
The green lines show the locations of the auroral ovals. Topband radio
communicators should interpret these green lines as follows: The regions
between the green lines show areas where auroral activity is most likely to
be observed. They do NOT necessarily represent the location or current
latitudinal thickness of the auroral ovals! But the auroral oval should
be located somewhere between the green lines. For the precise location of
the auroral oval, consult the previous ultraviolet aurora maps shown
For convenience only, the great-circle path from the eastern U.S. to Japan is
shown on the plot, with the distance between these two points displayed at
the mid-point of the path, in kilometers.
The cyan-colored contours represent the estimated maximum usable frequencies
for paths of 3,000 kilometers. Consult the Realtime
MUF Map section for more details regarding the use of these contours.