K-index
| K-index | a | Boulder, CO observatory measurement (nT) |
NOAA G-scale |
|---|---|---|---|
| 0 | 0 | 0 - 5 | G0 |
| 1 | 3 | 5 - 10 | G0 |
| 2 | 7 | 10 - 20 | G0 |
| 3 | 15 | 20 - 40 | G0 |
| 4 | 27 | 40 - 70 | G0 |
| 5 | 48 | 70 - 120 | G1 |
| 6 | 80 | 120 - 200 | G2 |
| 7 | 140 | 200 - 330 | G3 |
| 8 | 240 | 330 - 500 | G4 |
| 9 | 400 | >500 | G5 |
The K-index quantifies disturbances in the horizontal component of earth's magnetic field with an integer in the range 0-9 with 1 being calm and 5 or more indicating a geomagnetic storm. It is derived from the maximum fluctuations of horizontal components observed on a magnetometer during a three-hour interval. The label 'K' origins from German 'Kennziffer'[1] meaning 'characteristic digit'.
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. The real-time K-index is determined after the end of prescribed three-hour intervals (0000-0300, 0300-0600, ..., 2100-2400). 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.
The K-index was introduced by Julius Bartels in 1938.[2]
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[edit] The relationship between K, Kp, and estimated Kp
The official planetary Kp index is derived by calculating a weighted average of K-indices from a network of geomagnetic observatories. 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.
The Kp-index was introduced by Bartels in 1939.[1]
[edit] The relationship between K and A
The A-index provides a daily average level for geomagnetic activity. Because of the non-linear relationship of the K-scale to magnetometer fluctuations, it is not meaningful to take averages of a set of K indices. What is done instead is to convert each K back into a linear scale called the "equivalent three hourly range" a-index (note the lower case). The daily A index is merely the average of eight "a" indices.
Thus, for example, if the K indices for the day were 3 4 6 5 3 2 2 1, the daily A index is the average of the equivalent amplitudes:
A = (15 + 27 + 80 + 48 + 15 + 7 + 7 + 3)/8 = 25.25
The Ap index is averaged planetary A-index based on data from a set of specific Kp stations[3].
[edit] The relationship between the NOAA G-scale and Kp
The Kp scale is a reasonable way to summarize the global level of geomagnetic activity, but it has not always been easy for those affected by the space environment to understand its significance. The NOAA G-scale[4] was designed to correspond, in a straightforward way, to the significance of effects of geomagnetic storms.
[edit] References
- ^ a b Bartels, J., Heck, N.H. & Johnston, HF., 1939. The three-hour range index measuring geomagnetic activity, Geophys. Res., 44, 411-454 (p 411)
- ^ Bartels, J., Heck, N.H. & Johnston, HF., 1939. The three-hour range index measuring geomagnetic activity, Geophys. Res., 44, 411-454 (c.f. ref 4 on p 417)
- ^ http://spidr.ngdc.noaa.gov/spidr/help.do?group=geomInd#ap - Equivalent Amplitudes
- ^ [1] NOAA/SWPC Space Weather Scales used to communicate to the public current and future space weather conditions and their possible effects
[edit] External links
- http://www.sec.noaa.gov/info/Kindex.html
- http://www.sec.noaa.gov/rt_plots/kp_3d.html
- http://www.swpc.noaa.gov/SolarCycle/
- http://www.n3kl.org/sun/noaa.html
- Comprehensive radio propagation site
This article incorporates public domain material from the United States Government document "http://www.sec.noaa.gov/info/Kindex.html".
