The K-index is a code that is related to the maximum fluctuations of the horizontal components of the magnetic field measured on a given magnetometer, and relative to a geomagnetically quiet day. Each geomagnetic observing station generates its own K index and the weighted average of all stations is Kp or the Planetary K-index. Because of its method of calculation, the Kp-index is not real time.

At SEC (Space Environment Center, NOAA), the final real-time K-index which is referred to as the "Estimated K_p", (as opposed to the official K_p) appears on the website http://www.sec.noaa.gov/today.html, and is determined after the end of the prescribed three hour intervals (0000-0300,0300-0600, ..., 2100-2400). The maximum negative and positive deviations in the horizontal components during a given 3 hour period are added together to get the total maximum fluctuation.

The Kp-index ranges from 1-9, with 1 being very quiet and 9 indicating major geomagnetic activity. When the Kp approaches 6, Natural Radio activity is usually high. If the Kp is 6 or greater I am almost always able to hear chorus and VLF emissions for an hour or two after sunrise.