ap-index
The 3-hourly ap and the daily Ap indices are planetary magnetic activity indices, with units of 2nT. Related to the Kp index, they are average values of the irregular disturbance levels in the horizontal field components, observed at selected magnetic observatories worldwide.
Definitive values of Ap are provided by GeoForschungsZentrum (GFZ) Potsdam, Adolf-Schmidt-Observatorium für Geomagnetismus, Niemegk, Germany on behalf of the International Service of Geomagnetic Indices (ISGI) of the International Association of Geomagnetism and Aeronomy (IAGA). These are available from GFZ twice per month and so in between the updates, BGS calculates estimated values, apest and Apest, which are available in near real-time to meet the needs of the space weather community. These estimates are clearly marked with the letters "Est".
