K-index

The K-index quantifies disturbances in the horizontal component of Earth's magnetic field with an integer in the range 0–9 with 1 being calm and 5 or more indicating a geomagnetic storm.

[3] The K-scale is a quasi-logarithmic scale derived from the maximum fluctuation R (in units of nanoteslas, nT) in the horizontal component of Earth's magnetic field observed on a magnetometer relative to a quiet day during a three-hour interval.

The official planetary Kp-index is derived by calculating a weighted average of K-indices from a network of 13 geomagnetic observatories at mid-latitude locations.

[2] The a-index is the three hourly equivalent amplitude for geomagnetic activity at a specific magnetometer station derived from the station-specific K-index.

[5] If the K-indices for the day were 3, 4, 6, 5, 3, 2, 2 and 1, the daily A-index is the average of the equivalent amplitudes: The NOAA G-scale describes the significance of effects of a geomagnetic storm to the public and those affected by the space environment.