Kp index

Kp is a common index used to indicate the severity of the global magnetic disturbances in near-Earth space. Kp is an index based on the average of weighted K indices at 13 ground magnetic field observatories. It is based on the range of the magnetic field variation within 3 hour intervals that is caused by phenomena other than the diurnal variation and the long-term components of the storm time variations. The values of the Kp range from 0 (very quiet) to 9 (very disturbed) in 28 discrete steps, resulting in values of 0, 0+, 1-, 1, 1+, 2-, 2, 2+,...9.
An Estimated Kp is calculated by NOAA in near-real-time.
The official, after-the-fact, 3-hourly planetary index of geomagnetic activity is calculated twice per month by the the German GeoForschungsZentrum (GFZ) (Research Center for Geosciences), from the K indexes observed at 13 stations primarily in the Northern Hemisphere. The Kp indices, which date from 1932, are used to determine the ap indices.