UK Solar System Data Centre
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:
| Kp | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ap | 0 | 2 | 3 | 4 | 5 | 6 | 7 | 9 | 12 | 15 | 18 | 22 | 27 | 32 | 39 | 48 | 56 | 67 | 80 | 94 | 111 | 132 | 154 | 179 | 207 | 236 | 300 | 400 |
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