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. It is derived from the maximum fluctuations of horizontal components observed on a magnetometer during a three-hour interval. The label K comes from the German word Kennziffer^[1] meaning "characteristic digit". The K-index was introduced by Julius Bartels in 1939.^[2][1]

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 conversion table from maximum fluctuation to K-index varies from observatory to observatory in such a way that the historical rate of occurrence of certain levels of K are about the same at all observatories. In practice this means that observatories at higher geomagnetic latitude require higher levels of fluctuation for a given K-index. For example, the corresponding R value for K = 9 is 1500 nT in Godhavn, Greenland, 300 nT in Honolulu, Hawaii and 500 nT in Kiel, Germany.^[3]

The real-time K-index is determined after the end of prescribed intervals of 3 hours each: 00:00–03:00, 03:00–06:00, ..., 21:00–24:00. The maximum positive and negative deviations during the 3 hour period are added together to determine the total maximum fluctuation. These maximum deviations may occur any time during the 3 hour period.

The official planetary K_p-index is derived by calculating a weighted average of K-indices from a network of 13 geomagnetic observatories at mid-latitude locations. Since these observatories do not report their data in real-time, various operations centers around the globe estimate the index based on data available from their local network of observatories. The K_p-index was introduced by Bartels in 1939.^[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. Because of the quasi-logarithmic relationship of the K-scale to magnetometer fluctuations, it is not meaningful to take the average of a set of K-indices directly. Instead each K is converted back into a linear scale. ^{[4] [3]}

The A-index is the daily average of amplitude for geomagnetic activity at a specific magnetometer station, derived from the eight (three hourly) a-indices.

The A_p-index is the averaged planetary A-index based on data from a set of specific K_p stations.^[4]

The NOAA G-scale describes the significance of effects of a geomagnetic storm to the public and those affected by the space environment. It is directly derived from the K_p-scale, where G1 is the weakest storm classification (corresponding to a K_p value of 5) and G5 is the strongest (corresponding to a K_p value of 9).^[6]

The K_p-index is used for the study and prediction of ionospheric propagation of high frequency radio signals. Geomagnetic storms, indicated by a K_p = 5 or higher, have no direct effect on propagation. However they disturb the F-layer of the ionosphere, especially at middle and high geographical latitudes, causing a so-called ionospheric storm which degrades radio propagation. The degradation mainly consists of a reduction of the maximum usable frequency (MUF) by as much as 50%.^[8] Sometimes the E-layer may be affected as well. In contrast with sudden ionospheric disturbances (SID), which affect high frequency radio paths mostly at mid and low latitudes, the effects of ionospheric storms are more intense at high latitudes and the polar regions.

• Disturbance storm time index