K, Kp, and ap Indices
The Kindex is quasilogarithmic local index of the 3hourly range in magnetic activity
relative to an assumed quietday curve for a single geomagnetic observatory site. First
introduced by J. Bartels in 1938, it consists of a singledigit 0 thru 9 for each 3hour
interval of the universal time day (UT).
The planetary 3hourrange index Kp is the mean
standardized Kindex 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 3hourly 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 24hour value obtained by
computing an 8point running average of successive 3hour 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
