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HF Shortwave Radio Propagation Information - A Index and K Index
A Index and K Index
The Earth's magnetic field is continuously monitored by a network of magnetometers. These
readings are converted into the A and K index values.
The K index is computed once every three hours (eight times a day) and the values can range from 0
to 9, with 0 being inactive, and 9 representing an extreme severe storm condition. The values
are quadi-logarithmic.
The A index is linear, and is computed from the eight previous K index values. It ranges from
0 (quiet) to 400 (severe storm).
Generally, propagation conditions are best when the A index is 15 or lower, and the K index is
3 or lower.
Besides causing auroral activity, high geomagnetic field conditions can affect the electrons in
the ionosphere, reducing the maximum usable frequency (MUF).
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