Quick Guide to HF Propagation Using Solar Indices

SOLAR FLUX INDEX (SFI)

While increasing SFI may be good for HF propagation, it also tends to correspond with high Ap and K indices, which cause D-Layer absorption and noisy band condition.

PLANETARY Ap INDEX

The subscript "p" means planetary and designates a global magnetic activity index. The following 13 observatories, which lie between 46 and 63 degrees north and south geomagnetic latitude, now contribute to the planetary indices: Lerwick (UK), Eskdalemuir (UK), Hartland (UK), Ottawa (Canada), Fredericksburg (USA), Meannook (Canada), Sitka (USA), Eyrewell (New Zealand), Canberra (Australia), Lovo (Sweden), Brorfelde (Denmark), Wingst (Germany), and Niemegk (Germany).

THREE-HOUR-RANGE INDEX K

K indices isolate solar particle effects on the earth's magnetic field; over a 3-hour period, they classify into disturbance levels the range of variation of the more unsettled horizontal field component. Each activity level relates almost logarithmically to its corresponding disturbance amplitude. Three-hour indices discriminate conservatively between true magnetic field perturbations and the quiet-day variations produced by ionospheric currents. K indices range in 28 steps from 0 (quiet) to 9 (greatly disturbed) with fractional parts expressed in thirds of a unit. A K-value equal to 27, for example, means 2 and 2/3 or 3-; a K-value equal to 30 means 3 and 0/3 or 3 exactly; and a K-value equal to 33 means 3 and 1/3 or 3+. The arithmetic mean of the K values scaled at the 13 observatories listed above gives Kp.

EQUIVALENT AMPLITUDE

The a-index ranges from 0 to 400 and represents a K-value converted to a linear scale in gammas (nanoTeslas, nT) – a scale that measures equivalent disturbance amplitude of a station at which K=9 has a lower limit of 400 gammas.