|Kp, Sum_Kp, (3-hour) ap, and (Daily) Ap geomagnetic indices.
Kp and ap are 3-hour planetary indices and Sum_Kp and Ap are
daily summed and averaged (respectively) indices created from Kp.
These indices were created at the Geophysical Institute of Gottingen
University through 1996, and at GeoForschungsZentrum (GFZ, Potsdam)
GFZ, url given below, is the primary distribution site.
The web site of WDC for Geomagnetism at Kyoto U., url given below,
offers plots and numeric downloads for Kp and related indices.
The data are currently (as of 12/07/2011) "final" from January, 1957,
through December, 2008, and are "real time" thereafter, through the
Alternative sources of many of the Kp and related
indices include the hourly OMNI data set at NASA/GSFC/SPDF.
The subscript p means planetary and designates a global magnetic activity
index. The following 13 observatories, which lie between 46 and 63 degrees
north and south geomagnetic latitude, now contribute to the planetary
indices: Lerwick (UK), Eskdalemuir (UK), Hartland (UK), Ottawa (Canada),
Fredericksburg (USA), Meanook (Canada), Sitka (USA), Eyrewell (New Zealand),
Canberra (Australia), Lovo (Sweden), Brorfelde (Denmark), Wingst and Niemegk
Three-hour-range Index Kp
Kp index derives from Ks indices that isolate solar particle effects on
the earth's magnetic field; over a 3-hour period, they classify into
disturbance levels the range of variation of the more unsettled horizontal
field component. Each activity level relates almost logarithmically to its
corresponding disturbance amplitude. Three-hour indices discriminate
conservatively between true magnetic field perturbations and the quiet-day
variations produced by ionospheric currents.
Ks indices range in 28 steps from 0 (quiet) to 9 (greatly disturbed) with
fractional parts expressed in thirds of a unit. A Ks-value equal to 27, for
example, means 2 and 2/3 or 3-; a Ks-value equal to 30 means 3 and 0/3 or 3
exactly; and a Ks-value equal to 33 means 3 and 1/3 or 3+. The arithmetic
mean of the Ks values scaled at the 13 observatories listed above gives Kp.
Equivalent Amplitude ap
The ap-index ranges from 0 to 400 and represents a Kp-value converted to a
linear scale in nT (nanoTesla) - a scale that measures equivalent
disturbance amplitude of a station at which K=9 has a lower limit of 400 nT.