The Planetary K-Index, or Kp Index, is a measure of geomagnetic activity affecting the Earth. It is a derived measurement comprised of readings from various locations around the world. According to the Space Weather Prediction Center (SWPC), there are eight locations utilized by SWPC to compute the Kp Index: Sitka, Alaska; Meanook, Canada; Ottawa, Canada; Fredericksburg, Virginia; Hartland, United Kingdom; Wingst, Germany; Niemegk, Germany; and Canberra, Australia. These eight locations collect magnetometer data which is then analyzed to calculate the Kp Index value.
A magnetometer is a device which measures the strength and direction of magnetic fields. All of the observations from those eight locations are used to make a determination of the status of Earth’s magnetic field. Because of this, the Kp Index usually lags behind actual aurora activity; sometimes, up to three hours. For this reason, it is not conducive to using only the Kp Index to know when the time is right to chase the aurora. If you only rely on the Kp Index, you could miss some of the best activity since it could be over before you are ever alerted. In the app, the observed Kp values are represented by the bar graph and the forecast Kp values are represented by the line graph. A simple way to keep them separate but displayed easily together!
Kp Index values range from zero to nine with a Kp Index value of five (Kp 5) indicating the onset of geomagnetic storming. Values from zero to two (Kp 0 to Kp 2) indicate quiet levels of geomagnetic activity and a value of three (Kp 3) indicates unsettled activity. Once we reach a value of four (Kp 4) activity can start to increase due to active levels being reached. Above five (Kp 5), geomagnetic storming begins with storms being classified G1 (Kp 5) to G5 (Kp 9). Generally speaking, aurora can be visible from 66 degrees latitude and higher at Kp 0 and each level of Kp lowers that by approximately two degrees of latitude. So, Kp 9 means aurora activity can be seen as low as 48 degrees latitude or lower. See the images below for approximations of aurora visibility at associated Kp values.
Free Versus Subscription
The free version of the app gives you approximately two hours and 30 minutes of Kp data: the previous 90 minutes plus the next hour. The subscription version lets you go back and look at Kp data for the past 36 hours in 6-hour increments.
Global Kp Approximations
Tap or click on the images below to see a graphical representation of the approximate locations where aurora activity can be seen under the various Kp Index values. Keep in mind that, even though the line is over certain regions, aurora activity can bee seen from hundreds of miles away on the horizon in areas of low light pollution and pristine (clear) skies.