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suffice it to say that the geomagnetic activity, solar storms, x-rays, flares, etc., can have an adverse effect on propagation. the planetary a index relates to geomagnetic stability. magnetometers around the world are used to generate a number called the planetary k index. a one-point change in the k index is quite significant. k index readings below 3 generally mean good stable conditions, and above 3 can mean high absorption of radio waves. each point change reflects a significant change in conditions.
classification of k-indices is as follows:
k0 = inactive k5 = minor storm
k1 = very quiet k6 = major storm
k2 = quiet k7 = severe storm
k3 = unsettled k8 = very severe storm
k4 = active k9 = extremely severe storm
as with the k-index, the higher the a-index, the more unstable propagation becomes. classification of a-indices is as follows:
a0 - a7 = quiet
a8 - a15 = unsettled
a16 - a29 = active
a30 - a49 = minor storm
a50 - a99 = major storm
a100 - a400 = severe storm