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.
The conversion table from maximum fluctuation (nT) to K-index, varies from observatory
to observatory in such a way that the historical rate of occurrence of certain levels
of K are about the same at all observatories. In practice this means that observatories
at higher geomagnetic latitude require higher levels of fluctuation for a given K-index.
The conversion table for the Boulder magnetometer is shown below:
At SWPC, we monitor the preliminary values of the K-index, minute by minute, and
we notify our rapid alert customers when we exceed critical thresholds of 6, 7, and
8. The final real-time K-index is determined after the end of prescribed three hourly
intervals (0000-0300, 0300-0600, ..., 2100-2400) and is used on our announcements
and appears on our web site. The maximum positive and negative deviations during
the 3-hour period are added together to determine the total maximum fluctuation.
These maximum deviations may occur anytime during the 3-hour period.
The relationship between K, Kp, and estimated Kp
The official planetary Kp index is derived by calculating a weighted average of K-indices
from a network of geomagnetic observatories. Since these observatories do not report
their data in real-time, it is necessary for an operations center such as ourselves
to make the best estimate we can of this index based on available data. Space weather
operations uses near real-time estimates of the Kp index which are derived by the
U.S. Air Force 55th Space Weather Squadron. These estimated of Kp are based on a
network of observatories reporting in near real-time. Most of the observatories are
in North America, although there is one European station also contributing at this
time (Hartland, UK).
Source: Source: NOAA/Space Weather Prediction Centre
