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DX Toolbox gives you real time information on current HF radio conditions, alerts you to flares and geomagnetic storms, and helps you determine the best bands for any path under various conditions.

HF Shortwave Radio Propagation Information - A Index and K Index

A Index and K Index

The Earth's magnetic field is continuously monitored by a network of magnetometers. These readings are converted into the A and K index values.

The K index is computed once every three hours (eight times a day) and the values can range from 0 to 9, with 0 being inactive, and 9 representing an extreme severe storm condition. The values are quadi-logarithmic.

  • K = 0 Inactive
  • K = 1 Very quiet
  • K = 2 Quiet
  • K = 3 Unsettled
  • K = 4 Active
  • K = 5 Minor storm
  • K = 6 Major storm
  • K = 7 Severe storm
  • K = 8 Very severe storm
  • K = 9 Extremely severe storm

The A index is linear, and is computed from the eight previous K index values. It ranges from 0 (quiet) to 400 (severe storm).

  • A = 0 - 7 Quiet
  • A = 8 - 15 Unsettled
  • A= 8 - 15 Unsettled
  • A = 16 - 29 Active
  • A = 30 - 49 Minor storm
  • A = 50 - 99 Major storm
  • A = 100 - 400 Severe storm

Generally, propagation conditions are best when the A index is 15 or lower, and the K index is 3 or lower.

Besides causing auroral activity, high geomagnetic field conditions can affect the electrons in the ionosphere, reducing the maximum usable frequency (MUF).

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Last modified July 27, 2004