Technical information

During periods of geomagnetic disturbances, the window of usable
frequencies is likely to shift rapidly.
As the K index goes up and geomagnetic disturbance increases, HF
propagation is deteriorating because the density of electrically
charged particles in the ionosphere is being reduced. The fewer of
these particles there are, the less effective the ionosphere becomes
as a mirror reflecting HF radio waves back toward earth. The lower
frequency radio waves start to be absorbed in the ionosphere and the
higher frequency ones start to shoot through to outer space.
Another index reported in the geophysical alert announcements is the
A index. As mentioned earlier, the K index is measured every 3
hours. Eight successive K values over a 24 hour period are used to
derive the A index. The K values are equated to a quasi-logarithmic
scale and averaged.
The A index is a measure of how disturbed the geomagnetic field has
been over a 24 hour period. A index values vary with each observa-
tion station. At Boulder, Colorado, for example, the range of values is
0 to 400. Values from 7 to 15 indicate an unsettled condition. From
15 to 30 indicates an active condition. From 30 to 50 indicates a
minor geomagnetic storm. Above 50 indicates a major geomagnetic
storm (a condition which occurs very seldom but which can make HF
communications impossible.)
The final index included in these geophysical alert broadcasts which
you might find useful is the 10 centimeter solar flux. Solar flux is a
measure of the level of ultraviolet solar radiation. Solar radiation
makes the ionosphere a better mirror for reflecting HF radio waves
because it causes ionization or the formation of electrically charged
particles which serve to bend HF radio waves back toward earth. So
it stands to reason that the more solar radiation there is (that is the
higher the value reported for solar flux), the better HF propagation will
be. Solar flux levels are also a measure of how well the ionosphere
may stand up to geomagnetic disturbances or storms. A high solar
flux indicates the ionosphere is hard and will withstand more distur-
bances than if solar flux is low and the ionosphere is relatively soft.
Solar flux values can run from 65 to over 400. Normal daily levels will
be within this range. Generally speaking, the higher this number is,
the better HF propagation will be.
So if you listen to the geophysical alert announcements on WWV or
call up the commercial number, and you determine the A and K
indices and the 10 centimeter solar flux, you will be in a position to
know whether you can expect successful HF communications or con-
Services
8-4
KHF 950/990 Pilots Guide
Rev. 0
Dec/96