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^[1] meaning "characteristic digit". The K-index was introduced by Julius Bartels in 1939.^[2]^[1]

^ ^a ^b Bartels, J.; Heck, N. H.; Johnston, H. F. (1939). "The three-hour-range index measuring geomagnetic activity". Journal of Geophysical Research. 44 (4): 411–454. Bibcode:1939TeMAE..44..411B. doi:10.1029/TE044i004p00411.
^ Fleming, J. A.; Harradon, H. D.; Joyce, J. W. (1939). "Seventh General Assembly of the Association of Terrestrial Magnetism and Electricity at Washington, D.C., September 4–15, 1939". Terrestrial Magnetism and Atmospheric Electricity. 44 (4). pp. 477–478, Resolution 2. Bibcode:1939TeMAE..44..471F. doi:10.1029/TE044i004p00471.

[Bartels1939-1] Bartels, J.; Heck, N. H.; Johnston, H. F. (1939). "The three-hour-range index measuring geomagnetic activity". Journal of Geophysical Research. 44 (4): 411–454. Bibcode:1939TeMAE..44..411B. doi:10.1029/TE044i004p00411.

[resolution2-2] Fleming, J. A.; Harradon, H. D.; Joyce, J. W. (1939). "Seventh General Assembly of the Association of Terrestrial Magnetism and Electricity at Washington, D.C., September 4–15, 1939". Terrestrial Magnetism and Atmospheric Electricity. 44 (4). pp. 477–478, Resolution 2. Bibcode:1939TeMAE..44..471F. doi:10.1029/TE044i004p00471.

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