Copyright WDC C1 for STP at RAL.
This data is derived (along with Cp) from observatory K readings from a global network of observatories, and provides an indicator of global geomagnetic disturbance. The poorest quality of data is the early hand-calculated values from 1932 through 1936.
Estimated values are available daily in the 'Kp and Ap indices (estimated)' database. These are extracted from the daily SGAS report issued by the Space Enviroment Center in the USA (available elsewhere on this server), and collated here for your convenience. It should be noted that these estimated values are not derived using precisely the same network of stations as the final values. Final values are in the 'Solar Magnetic Activity' (SMA) database.