A Index and K Index Explained
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 = 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).