Geomagnetic Activity Indices

Indices are useful for empirical modeling as well as for forecasting because they provide a convenient parameter set that may be used for driving the model. We have seen that sunspot number Rl2 or the flux index <¡>¡2 are convenient, if not totally representative, of solar activity. The magnetic activity also lends itself to the development of a wide range of index representations. Moreover, the magnetic activity indices are organized and smoothed in a variety of ways that may have the potential for confusing the user who is not an ionospheric specialist. Mayaud [1980] has discussed the array of indices in his book, Derivation, Meaning, and Vse of Geomagnetic Indices. He traces the history of magnetic index development from the earliest forms to those of the present. Table 2-8 is a list of the current indices sanctioned by the International Association of Geomagnetism and Aeronomy (IAGA).

Table 2-8: Indices of Magnetic Activity


Three-hourly quasi-logarithmic index. It is a measure of the irregular variations of the horizontal field component at a specified station or group of stations. For example KFR corresponds to the Fredericksburg site, and KP is associated with the planetary "average" value. The values of K run from 0-9 with "9" representing the most disturbed condition. KP is derived from 12 stations between geomagnetic latitudes 48-63 degrees.


Hourly Index associated with low latitude magnetic activity. It is designed to be a measure of the ring current in the magnetosphere (i.e., that region above the geomagnetic equator at ~ 5.6 earth radii). Dst stands for "disturbance amplitude storm time", and its units are nT. Four midlatitude sites are used in the construction of Dst.


Auroral electrojet activity index. This is an hourly index derived from a number of auroral stations.


High latitude index with a 15-minute time resolution. It is related to the auroral oval position, and is employed in several propagation prediction algorithms (viz., ICEPAC; see Chapter 3). Q ~ 2Kp -3.5 (The relationship used by AFWA Space Weather Operations Center [ref: NWRA website])


Daily index of magnetic activity at an individual station or a global array of stations. The A-index ranges between 0-400. It is the linear equivalent to K. (See Table 2-9). The three-hourly K-indices may be converted to a set of eight three-hourly A-indices that are averaged to yield a single daily A-index. The U.S. Air Force has developed an operational (planetary) Ap index corresponding to shorter time frames. Such indices can be employed in foF2 correction models such as STORM (see Chapters 3 and 4).


Three-hourly indices computed from K-indices of two nearly antipodal magnetic observatories with an invariant latitude of- 50 degrees. This index is designed to provide an index of global activity.

The most widely used index is KP. It is used for ionospheric predictions. However, if we want a simple daily average for the magnetic activity, the fact that KP is quasi-logarithmic makes it a mathematically poor choice. Even so, a number of studies have used the sum of the eight 3-hourly values of KP to represent the smoothed daily behavior. The A-index is a better choice for use in averaging. Table 2-9 gives a transformation between Aindex and K-index.

Magnetic field data may be obtained from publications and bulletins issued by the International Service of Geomagnetic Indices (ISGI) or the International Association of Geomagnetism and Aeronomy (IAGA) by writing the publications office of the International Union of Geophysics and

Geodesy (IUGG) located in Paris, France. The World Data Centers (WDC) also maintain archives of geomagnetic data. Subcenters of WDC-A (USA) are located in Boulder, Colorado (NGDC) and Greenbelt, Maryland (NSSDC). Bulletins issued by NOAA/NGDC are also mailed to interested users, and NOAA-SEC and Regional Warning Centers of ISES publish reports of the various indices. Generally speaking the K-index (or its A equivalent) is available from the same sources that issue sunspot number reports and advisories.

Table 2-9: Transformation from K-index to A-index























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