# K-index

For other uses, see K-index (meteorology).

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 1938.[2]

## Calculation of K-index

The K-scale is quasi-logarithmic. The conversion table from maximum fluctuation R (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. For example, at Godhaven, Greenland, a value of K equal to 9 is derived with R=1500 nT, while in Honolulu, HI, a fluctuation of only 300 nT is recorded as K=9. In Kiel, Germany, K=9 corresponds to R=500 nT or greater.[3] The real-time K-index is determined after the end of prescribed three-hour 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 Kp index and estimated Kp index

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 Kp-index was introduced by Bartels in 1939.[1]

## The relationship between K and A

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 the average 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), according to the following table:[3][4]

 K a K a 0 0+ 1- 1 1+ 2- 2 2+ 3- 3 3+ 4- 4 4+ 0 2 3 4 5 6 7 9 12 15 18 22 27 32 5- 5 5+ 6- 6 6+ 7- 7 7+ 8- 8 8+ 9- 9 39 48 56 67 80 94 111 132 154 179 207 236 300 400

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 and 1, the daily A index is the average of the equivalent amplitudes:

```  A = (15 + 27 + 80 + 48 + 15 + 7 + 7 + 4)/8 = 25.38
```

The Ap index is averaged planetary A-index based on data from a set of specific Kp stations.[4]

## The relationship between the NOAA G-scale and Kp

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[5] was designed to correspond, in a straightforward way, to the significance of effects of geomagnetic storms.