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 NOAA > NESDIS > NCEI (formerly NGDC) > Geomagnetism

Models are joint research with:
The Cooperative Institute for Research in Environmental Sciences (CIRES).

Geomagnetic kp and ap Indices

graph of kp ap Indices

Click on image to see larger version of graph

Daily regular magnetic field variation arise from current systems caused by regular solar radiation changes. Other irregular current systems produce magnetic field changes caused by the interaction of the solar wind with the magnetosphere, by the magnetosphere itself, by the interactions between the magnetosphere and ionosphere, and by the ionosphere itself.

Magnetic activity indices were designed to describe variation in the geomagnetic field caused by these irregular current systems.

K, Kp, and ap Indices

The K-index is quasi-logarithmic local index of the 3-hourly range in magnetic activity relative to an assumed quiet-day curve for a single geomagnetic observatory site. First introduced by J. Bartels in 1938, it consists of a single-digit 0 thru 9 for each 3-hour interval of the universal time day (UT).

The planetary 3-hour-range index Kp is the mean standardized K-index from 13 geomagnetic observatories between 44 degrees and 60 degrees northern or southern geomagnetic latitude. The scale is O to 9 expressed in thirds of a unit, e.g. 5- is 4 2/3, 5 is 5 and 5+ is 5 1/3. This planetary index is designed to measure solar particle radiation by its magnetic effects. The 3-hourly ap (equivalent range) index is derived from the Kp index as follows:

	Kp = 0o   0+   1-   1o   1+   2-   2o   2+   3-   3o   3+   4-   4o   4+

	ap =  0    2    3    4    5    6    7    9   12   15   18   22   27   32

	Kp = 5-   5o   5+   6-   6o   6+   7-   7o   7+   8-   8o   8+   9-   9o

	ap = 39   48   56   67   80   94  111  132  154  179  207  236  300  400


Other planetary indices include the Ap* and AA*. The Ap* index is defined as the earliest occurring maximum 24-hour value obtained by computing an 8-point running average of successive 3-hour ap indices during a geomagnetic storm event and is uniquely associated with the storm event. The AA* index is similar to the Ap* index, but has a longer history and is based on reports from only two stations.

You can download the Kp, Ap, and other indices via FTP