The K-index is a code that is related to the maximum fluctuations of horizontal components observed on a magnetometer relative to a quiet day, during a three-hour interval http://www.swpc.noaa.gov/info/Kindex.html
The Estimated 3-hour Planetary Kp-index is derived at the NOAA Space Weather Prediction Center using data from the following ground-based magnetometers: Boulder, Colorado; Chambon la Foret, France; Fredericksburg, Virginia; Fresno, California; Hartland, UK; Newport, Washington; Sitka, Alaska. These data are made available thanks to the cooperative efforts between SWPC and data providers around the world, which currently includes the U.S. Geological Survey, the British Geological Survey, and the Institut de Physique du Globe de Paris.
Kp-indices of 5 or greater indicate storm-level geomagnetic activity. Geomagnetic storms have been associated with satellite surface charging and increased atmospheric drag.
This page updates dynamically every 5 minutes: http://www.swpc.noaa.gov/rt_plots/kp_3d.html