The Kp-index is the global geomagnetic storm index and is based on 3 hour measurements of the K-indices, for a given value, for each of the past days. The K-index itself is a three hour long quasi-logarithmic local index of the geomagnetic activity, relative to a calm day curve for the given location. The Kp-index measures the deviation of the most disturbed horizontal component of the magnetic field on fixed stations worldwide with their own local K-index. The global Kp-index is then determined with an algorithm that puts the averages of every station together. The result is the global Kp-index. The Kp-index ranges from 0 to 9 where a value of 0 means that there is very little geomagnetic activity and a value of 9 means extreme geomagnetic storming.
The Kp plot on this website shows you the Kp-values of the past 24 hours and a forecast for the coming hour. This plot gives you an immediate idea what the most recently measured geomagnetic conditions are, what the conditions of the past 24 hours were and a forecast for the coming hour.
The preliminary Kp-index is the Kp-index from the NOAA SWPC that updates every 3 hours with an estimate of the measured Kp of the past 3 hours. These periods are: 0000-0300 UTC, 0300-0600 UTC, etc. The preliminary Kp-index consists of 10 values and ranges from 0 to 9 and are an estimate of the observed Kp-value over a certain 3-hour period. It is thus not a forecast or an indicator of the current conditions, it always shows the Kp-value that was observed during a certain period. The image below shows a plot of the preliminary Kp-index from October 2003 with 3 days of intense geomagnetic storming.
The table below shows you the preliminary Kp-index with it's 10 values, which G-scale a certain Kp-value represents, the boundary of the auroral oval at local midnight for a specific Kp-value, a description of the auroral activity for a specific Kp-value and a rogue estimate on how often a certain Kp-value occurs during one solar cycle.
|Kp||G-scale||Geomagnetic latitude||Auroral activity||Average frequency|
|0||G0||66.5° or higher||Very low|
|5||G1||56.3°||Minor storm||1700 per cycle (900 days per cycle)|
|6||G2||54.2°||Moderate storm||600 per cycle (360 days per cycle)|
|7||G3||52.2°||Strong storm||200 per cycle (130 days per cycle)|
|8||G4||50.1°||Severe storm||100 per cycle (60 days per cycle)|
|9||G5||48.1° or lower||Extreme storm||4 per cycle (4 days per cycle)|
The finalized Kp-index comes from the GFZ in Potsdam, Germany and is updated twice a month. These are the official, finalized Kp-values for scientific research and archive purposes. The finalized Kp-index is slightly different then the preliminary Kp-index. Unlike the preliminary Kp-index, the finalized Kp data is expressed in a scale of thirds and has 28 values were the preliminary Kp-index only has 10 values.
The U.S. Air Force Weather Agency Wing Kp model is also expressed in a scale of thirds and has 28 values and consists of preliminary values. This model shows the observed Kp and provides a forecast for the coming hour and the coming 4 hours. It uses data from the Advanced Composition Explorer (ACE) for it's forecast. The Wing Kp-index is the index that we use on the website to get the latest observed Kp and the Kp values that are being forecast. We only use the forecast for the coming hour however because the 4-hour forecast is often unreliable. The image below shows an example of the Kp-index graph as you can find on our website. The orange line shows the predicted Kp-index and the bars are the observed Kp-index.
The table below shows the values that both the finalized Kp-index and the Wing Kp-index work with. This Kp-index works with 28 values instead of 10 values that the preliminary Kp-index works with.
|Kp||Kp in decimals||G-scale||Auroral activity|
NOAA uses a five-level system called the G-scale, to indicate the severity of both observed and predicted geomagnetic activity. This scale is used to give a quick indication of the severity of a geomagnetic storm. This scale ranges from G1 to G5, with G1 being the lowest level and G5 being the highest level. Conditions below storm level are labelled as G0 but this value is not commonly used. Every G-level has a certain Kp-value associated with it. This ranges from G1 for a Kp-value of 5 to G5 for a Kp-value of 9. We often use this G-scale on the website so it’s wise to familiarize yourself with it. The table below will help you with that.
|G-scale||Kp||Auroral activity||Average frequency|
|G0||4 and lower||Below storm|
|G1||5||Minor storm||1700 per cycle (900 days per cycle)|
|G2||6||Moderate storm||600 per cycle (360 days per cycle)|
|G3||7||Strong storm||200 per cycle (130 days per cycle)|
|G4||8||Severe storm||100 per cycle (60 days per cycle)|
|G5||9||Extreme storm||4 per cycle (4 days per cycle)|
Any location on the high latitudes will be able to see auroras with a Kp of 4. For any location on the middle latitudes a Kp-value of 7 is needed. The low latitudes need Kp-values of 8 or 9. The Kp-value that you need of course depends on where you are located on Earth. We made a handy list which is a good guide for what Kp-value you need for any given location within the reach of the auroral ovals.
Important! Note that the locations below give you a reasonable chance to see auroras for the given Kp-index provided local viewing conditions are good. This includes but is not limited to: a clear sight towards the northern or southern horizon, no clouds, no light pollution and complete darkness.
|Predicted Kp max||3|
|M-class solar flare||1%|
|X-class solar flare||1%|
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|Last geomagnetic storm:||2016/08/24||Kp5 (G1)|
|Number of spotless days in 2016:||21|
|Last spotless day:||2016/08/04|