Three major indexes are used by HF Ham operators to interpret probable HF band propagation at any given time – K index, A index and SFI.
The Sun is constantly shooting electro-magnetic particles into the Earth’s ionosphere. These particles along with seasonal weather, solar flares and Sunspot activity, combine to create ever-changing, low to severe geomagnetic and ionospheric fluctuations that hourly and daily affect the DX capability of HF band conditions.
K INDEX – Planetary Average
Observatories around the globe, at specified three-hour intervals throughout the day, measure current disturbances and activity in the Earth’s magnetic field. The K index is a 3-hour average of All of the locally observed K measurements from around the globe.
K index values are between 0 – 9. 0 to 1 is excellent. 2 – 9 moves up the scale from fair to total blackout.
A INDEX – Planetary Value
Because of the mathematical method by which the K index is measured it cannot be used as a long-term geomagnetic indicator. The A index provides the answer.
As stated above, because the K index is measured globally at the same three hour (UTC) segments we end up with 8 daily K indexes (24 hours divided by 3 hours = 8). The average of these 8 K indexes provides the daily planetary value – the A index.
The range is between 0 – 400. 0 – 15 is considered the most favorable propagation window. 16 – 400 things become considerably worse and unpredictable.
SFI – SOLAR FLUX INDICATOR
Describes the general level of solar activity and radiation being received from the Sun. The measure is produced every day at Penticton Radio Observatory in British Columbia, Canada. The way this works is that the concentration of charged particles in the highest level of the ionosphere – the F2 Region – sends out radio noise and fluctuation. The Penticton Observatory constantly measures the F2 emissions at a standardized frequency of 2800 MHz (10.7 cm).
The measurement of the reception of the signal provides a generally recognized “baseline” for determining the positive or negative value of Maximum Usable Frequency (MUF).
The SFI range 65 – 300. The higher to 300 the better. The lower the range indicates the reduction of ionization in the upper atmosphere to support long-distance communications (Maximum Usable Frequency) with higher frequencies.
The Propagation Combination
To summarize: The K index is “3 – Hour Average” of all the global measurements – changing every three hours. 1-3 Good
The A index is the daily average of the 8 K indexes within the last 24 hours. 0-15 Good
The SFI is the frequency emissions from the F2 layer that are compiled to determine the Maximum Usable Frequency for a given period of time. 65 Bad, 100 Good, 200 – 300 Excellent