K-index
Geomagnetic storm indicator, 0–9 / From Wikipedia, the free encyclopedia
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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 1939.[2][1]