The ''K''-index quantifies disturbances in the horizontal component of

Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...

with an

integer An integer is the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative in ...

in the range 0–9 with 1 being calm and 5 or more indicating a

geomagnetic storm A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere that is driven by interactions between the magnetosphere and large-scale transient Plasma (physics), plasma and magnetic field structur ...

. It is derived from the maximum fluctuations of horizontal components observed on a

magnetometer A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, ...

during a three-hour interval. The label ''K'' comes from the German word meaning ''characteristic digit''. The ''K''-index was introduced by Julius Bartels in 1939. The similar HP30 and HP60 indices were developed in the 2020s, using a shorter interval in order to include shorter but more intense disturbances.

Definition

The ''K''-scale is a quasi-logarithmic scale derived from the maximum fluctuation ''R'' in the horizontal component of

observed on a magnetometer relative to a quiet day during a three-hour interval. The conversion table from maximum fluctuation 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. For example, the corresponding ''R'' value for K = 9 is in Qeqertarsuaq, Greenland; in

Honolulu, Hawaii Honolulu ( ; ) is the List of capitals in the United States, capital and most populous city of the U.S. state of Hawaii, located in the Pacific Ocean. It is the county seat of the Consolidated city-county, consolidated City and County of Honol ...

; and in Kiel, Germany. The real-time ''K''-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.

Derived indices

''K''_p-index

The ''K''_p-index, or the planetary ''K''-index, is derived by calculating a weighted average of ''K''-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. The ''K''_p-index was introduced by Bartels in 1939.

''a''-index

The ''a''-index is the three hourly equivalent amplitude for geomagnetic activity at a specific magnetometer station derived from the station-specific ''K''-index. Because of the quasi-logarithmic relationship of the ''K''-scale to magnetometer fluctuations, it is not meaningful to take the average of a set of ''K''-indices directly. Instead each ''K'' is converted back into a linear scale. The ''A''-index is the daily average of amplitude for geomagnetic activity at a specific magnetometer station, derived from the eight (three hourly) ''a''-indices. The ''A''_p-index is the averaged planetary ''A''-index based on data from a set of specific ''K''_p stations.

Example

If the ''K''-indices for the day were 3, 4, 6, 5, 3, 2, 2 and 1, the daily ''A''-index is the average of the equivalent amplitudes: :

A = (15 + 27 + 80 + 48 + 15 + 7 + 7 + 4)/8 = 25.375

G-scale

The

NOAA The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with forecasting weather, monitoring oceanic and atmospheric conditions, charting the seas, conducting deep-sea exploratio ...

G-scale describes the significance of effects of a

to the public and those affected by the space environment. It is directly derived from the ''K''_p-scale, where G1 is the weakest storm classification (corresponding to a ''K''_p value of 5) and G5 is the strongest (corresponding to a ''K''_p value of 9).

Use in radio propagation studies

The ''K''_p-index is used for the study and prediction of ionospheric propagation of

high frequency High frequency (HF) is the ITU designation for the band of radio waves with frequency between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten decameters (ten to one ...

radio signals. Geomagnetic storms, indicated by a ''K''_p = 5 or higher, have no direct effect on propagation. However they disturb the F-layer of the

ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...

, especially at middle and high geographical latitudes, causing a so-called '' ionospheric storm'' which degrades radio propagation. The degradation mainly consists of a reduction of the maximum usable frequency (MUF) by as much as 50%. Sometimes the E-layer may be affected as well. In contrast with sudden 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.

References

External links

* https://www.swpc.noaa.gov/products/planetary-k-index * https://web.archive.org/web/20061107140346/http://sec.noaa.gov/rt_plots/kp_3d.html * https://www.swpc.noaa.gov/products/solar-cycle-progression * http://www.n3kl.org/sun/noaa.html
Comprehensive radio propagation site
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