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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 conversion table from maximum fluctuation (nT) 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.

The real-time K-index is determined after the end of prescribed three hourly intervals (0000-0300, 0300-0600, ..., 2100-2400). 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 Kp index is derived by calculating a weighted average of K-indices from a network of geomagnetic observatories. 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 A-index provides a daily average level for geomagnetic activity. Because of the non-linear relationship of the K-scale to magnetometer fluctuations, it is not meaningful to take averages of a set of K indices. What is done instead is to convert each K back into a linear scale called the "equivalent three hourly range" a-index (note the lower case). The daily A index is merely the average of eight "a" indices.

Thus, for example, if the K indices for the day were 3 4 6 5 3 2 2 1, the daily A index is the average of the equivalent amplitudes:

  A = (15 + 27 + 80 + 48 + 15 + 7 + 7 + 3)/8 = 25.25

The Kp scale is a reasonable way to summarize the global level of geomagnetic activity, but it has not always been easy for those affected by the space environment to understand its significance. The NOAA G-scale was designed to correspond, in a straightforward way, to the significance of effects of geomagnetic storms.

• http://www.sec.noaa.gov/info/Kindex.html - primary source for this article
• http://www.sec.noaa.gov/rt_plots/kp_3d.html
• http://www.n3kl.org/sun/noaa.html
• Comprehensive radio propagation site

This article incorporates text from http://www.sec.noaa.gov/info/Kindex.html, a public domain work of the United States Government.