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. 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 meaning characteristic digit. The K-index was introduced by Julius Bartels in 1939.

The similar HP30 and HP60 indices were developed in the 2020s, using a shorter interval in order to include shorter but more intense disturbances.

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