Copyright WDC C1 for STP at RAL.
The Ap index is the daily average of the 3-hourly ap index, derived from the 3-hourly Kp values. The Kp index is quasi-logarithmic, whereas the ap scale maps this to a linear scale. The conversion from Kp to ap is given by the following table:
Units are nT, and the ap value can be regarded as approximately half the range of variation of the most disturbed horizontal magnetic component.
The Adolf-Schmidt-Observatory Niemegk of the GeoForschungsZentrum Potsdam took over the responsibility for the calculation and distribution of the Kp and derived activity indices from January 1997