What does this data represent?

The Estimated Planetary K index (Kp) chart is updated every 3 hours and represents the maximum observed Kp Index within the previous 3 hour sampling period. The Kp index is one of the most common indices used to indicate the severity of the global magnetic disturbances in near-Earth space. A Kp index of 5 or higher indicates a geomagnetic storm is in progress. For more information on each Geomagnetic Storm level visit the following link. http://www.swpc.noaa.gov/NOAAscales/index.html#GeomagneticStorms.

Example of a Kp plot representing an extreme geomagnetic storm

What is the K-Index?

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.

What is the Estimated Planetary K-Index (Kp) ?

The Kp index is an average of several K-indices. 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; Jeju, Korea. 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, the Institut de Physique du Globe de Paris, and the Korean Space Weather Center.

More detailed information regarding the K and Kp index at the following link http://www.swpc.noaa.gov/info/Kindex.html.