Solar Flux, A index and K index

A- and K-index: Geomagnetic activity indices, high indices (K:>5 or A:>20) means stormy conditions with 
an active geomagnetic field. The more active, the more unstable propagation with possible periods of total propagation fade-out. Especially around the higher latitudes and especially at the polar regions, where the geomagnetic field is weak, propagation may disappear completely. Extreme high indices may result in aurora propagation, with strongly degraded long distance propagation at all latitudes. Sporadic-E is strongest during low indices. Low indices result in relative good propagation, especially noticeable around the higher latitudes, when transpolar paths may open up. Maximum K-index is 9, and the A-index can exceed well over 100 during very severe storm conditions, with no maximum. The ARRL often reports the K-index from the Alaskan station where this index is known as the College K-index. Other stations reporting K-indices include Planetary and Boulder. In contrast, the A-indices are usually reported for the Planetary station only.

The higher the K-index, the more unstable propagation becomes, the effect is stronger at high latitudes, but weaker near low latitudes. When storm level is reached, propagation strongly degrades, possibly fade out at high latitudes.

Classification of K-indices are as follows:

K0=Inactive
K1=Very quiet
K2=Quiet
K3=Unsettled
K4=Active
K5=Minor storm
K6=Major storm
K7=Severe storm
K8=Very severe storm
K9=Extremely severe storm

As with the K-index, the higher the A-index, the more unstable propagation becomes.
Classification of A-indices are as follows:

A0 - A7 = quiet
A8 - A15 = unsettled
A16 - A29 = active
A30 - A49 = minor storm
A50 - A99 = major storm 
A100 - A400 = severe storm

Background X-ray level: This may vary from B (very low), C (low to moderate), M (moderate to high) to X 
(high to extremely high), the higher the number after the letter, the stronger the X-ray radiation.   So an X0.1
is stronger than an M9.9. High amounts of X-ray radiation causes intense ionization of the D-layer, resulting
in strong absorption of HF-signals. Solar flares are commonly measured in the amount of X-ray radiation. 

Solar Flux: This flux number is measured from the amount of radiation on the 10.7cm band (2800MHz). It is closely related to the amount of ultraviolet radiation, which is needed to create an ionosphere. The lowest possible number for this solar flux is 63.75. Single hop propagation already starts at 70 in lower latitude areas. Worldwide long distance propagation (DX) may turn up already with a solar flux at 120. From experience, an average solar flux of 180 seems to be ideal for 10m-20m bands QRP DX with good possibilities during these conditions to reach every possible part of the globe with a simple dipole running as low as 5 Watts!

T-index: This index is used and developed by IPS Radio & Space Services in Australia. In this index, all other indices are used, so it's a very general index for forecasting propagation.

This is an excerpt from the article "The Basics of Radio Wave Propagation" by AE4TM

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