Index quantifying disturbances in the horizontal component of the Earth's magnetic field with a number in the range 0–9, with 1 being calm and ≥5 indicating a geomagnetic storm. The K index is derived from the maximum fluctuations of horizontal components observed on a magnetometer during a 3-h interval. The official planetary Kp index is derived by calculating a weighted average of K indices from a network of geomagnetic observatories. (ICRP Publication 132, 2013)
ICRP Glossary entry - May 2019
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