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 Kp“, (as opposed to the official Kp) 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.
Sunspots are areas seen as dark spots on the photosphere of the sun. Sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups. They appear dark because they are cooler than the surrounding photosphere. Sunspots vary on an eleven year cycle. Geomagnetic activity on Earth roughly coordinates with the sunspot cycle. The period of minimum sunspots usually means low geomagnetic activity on Earth.
The outer solar atmosphere, the corona, is structured by strong magnetic fields. Where these fields are closed, often above sunspot groups, the confined solar atmosphere can suddenly and violently release bubbles or tongues of gas and magnetic fields called coronal mass ejections. A large CME can contain 10.0E16 grams (a billion tons) of matter that can be accelerated to several million miles per hour in a spectacular explosion. Solar material streaks out through the interplanetary medium, impacting any planets or spacecraft in its path. CMEs are often associated with flares but can also occur independently. If a CME hits earth, it can cause a geomagnetic storm. The intensity of the storm will be affected by the orientation of the CME’s magnetic field, its speed and density. A strong CME hit often causes Aurora displays at low latitudes.
