TheK-index quantifies disturbances in the horizontal component ofEarth's magnetic field with aninteger in the range 0–9 with 1 being calm and 5 or more indicating ageomagnetic storm. It is derived from the maximum fluctuations of horizontal components observed on amagnetometer during a three-hour interval. The labelK comes from the German wordkennziffer[1] meaningcharacteristic digit. TheK-index was introduced byJulius Bartels in 1939.[2][1]
The similar HP30 and HP60 indices were developed in the 2020s, using a shorter interval in order to include shorter but more intense disturbances.[3]
TheK-scale is a quasi-logarithmic scale derived from the maximum fluctuationR in the horizontal component ofEarth's magnetic field observed on a magnetometer relative to a quiet day during a three-hour interval. The conversion table from maximum fluctuation toK-index varies from observatory to observatory in such a way that the historical rate of occurrence of certain levels ofK are about the same at all observatories. In practice this means that observatories at higher geomagnetic latitude require higher levels of fluctuation for a givenK-index. For example, the correspondingR value for K = 9 is1500 nT inQeqertarsuaq, Greenland;300 nT inHonolulu, Hawaii; and500 nT inKiel, Germany.[4]
The real-timeK-index is determined after the end of prescribed intervals of 3 hours each: 00:00–03:00, 03:00–06:00, ..., 21:00–24:00. 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 any time during the 3-hour period.
TheKp-index, or theplanetaryK-index, is derived by calculating a weighted average ofK-indices from a network of 13 geomagnetic observatories at mid-latitude locations. Since these observatories do not report their data in real-time, various operations centers around the globe estimate the index based on data available from their local network of observatories. TheKp-index was introduced by Bartels in 1939.[2]
Thea-index is the three hourly equivalent amplitude for geomagnetic activity at a specific magnetometer station derived from the station-specificK-index. Because of the quasi-logarithmic relationship of theK-scale to magnetometer fluctuations, it is not meaningful to take the average of a set ofK-indices directly. Instead eachK is converted back into a linear scale.[5][4]
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TheA-index is the daily average of amplitude for geomagnetic activity at a specific magnetometer station, derived from the eight (three hourly)a-indices.
TheAp-index is the averaged planetaryA-index based on data from a set of specificKp stations.[5]
If theK-indices for the day were 3, 4, 6, 5, 3, 2, 2 and 1, the dailyA-index is the average of the equivalent amplitudes:
TheNOAAG-scale describes the significance of effects of ageomagnetic storm to the public and those affected by the space environment. It is directly derived from theKp-scale, where G1 is the weakest storm classification (corresponding to aKp value of 5) and G5 is the strongest (corresponding to aKp value of 9).[7]
| Scale | Level | Effect | Kp equivalent | Average frequency (1 cycle = 11 years) | Days duringsolar cycle 24[8] | ||
|---|---|---|---|---|---|---|---|
| Power system | Spacecraft operations | Other systems | |||||
| G1 | Minor | Weak power grid fluctuations can occur. | Minor impact on satellite operations possible. | Migratory animals are affected at this and higher levels; aurora is commonly visible at high latitudes (northern Michigan and Maine). | 5 | 1700 per cycle (900 days per cycle) | 256 |
| G2 | Moderate | High-latitude power systems may experience voltage alarms, long-duration storms may cause transformer damage. | Corrective actions to orientation may be required by ground control; possible changes in drag affect orbit predictions. | HF radio propagation can fade at higher latitudes, and aurora has been seen as low as New York and Idaho (typically 55° geomagnetic lat.). | 6 | 600 per cycle (360 days per cycle) | 86 |
| G3 | Strong | Voltage corrections may be required, false alarms triggered on some protection devices. | Surface charging may occur on satellite components, drag may increase on low-Earth-orbit satellites, and corrections may be needed for orientation problems. | Intermittent satellite navigation and low-frequency radio navigation problems may occur, HF radio may be intermittent, and aurora has been seen as low as Illinois and Oregon (typically 50° geomagnetic lat.). | 7 | 200 per cycle (130 days per cycle) | 18 |
| G4 | Severe | Possible widespread voltage control problems and some protective systems will mistakenly trip out key assets from the grid. | May experience surface charging and tracking problems, corrections may be needed for orientation problems. | Induced pipeline currents affect preventive measures, HF radio propagation sporadic, satellite navigation degraded for hours, low-frequency radio navigation disrupted, and aurora has been seen as low as Alabama and northern California (typically 45° geomagnetic lat.). | 8-9 | 100 per cycle (60 days per cycle) | 9 |
| G5 | Extreme | Widespread voltage control problems and protective system problems can occur, some grid systems may experience complete collapse or blackouts. Transformers may experience damage. | May experience extensive surface charging, problems with orientation, uplink/downlink and tracking satellites. | Pipeline currents can reach hundreds of amperes, HF (high frequency) radio propagation may be impossible in many areas for one to two days, satellite navigation may be degraded for days, low-frequency radio navigation can be out for hours, and aurora has been seen as low as Florida and southern Texas (typically 40° geomagnetic lat.). | 9 | 4 per cycle (4 days per cycle) | 1 |
TheKp-index is used for the study and prediction of ionospheric propagation ofhigh frequency radio signals. Geomagnetic storms, indicated by aKp = 5 or higher, have no direct effect on propagation. However they disturb theF-layer of theionosphere, especially at middle and high geographical latitudes, causing a so-calledionospheric storm which degrades radio propagation. The degradation mainly consists of a reduction of themaximum usable frequency (MUF) by as much as 50%.[9] Sometimes theE-layer may be affected as well. In contrast withsudden ionospheric disturbances (SID), which affect high frequency radio paths mostly at mid and low latitudes, the effects of ionospheric storms are more intense at high latitudes and the polar regions.
This article incorporatespublic domain material from the United States government
This article incorporatespublic domain material from the United States government