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).