Information about the aa-indices
The aa-index is a simple global geomagnetic activity index. It is derived from the K indices from two approximately antipodal observatories and has units of 1 nT. Current observatories used are Hartland in the UK, operated by BGS and Canberra in Australia, operated by Geoscience Australia. The observatories used have changed over the time span of the series - click for more details 115kB pdf.
Since it is based on data from only two observatories, it is the simplest of all the 3 hourly planetary indices. However, provided averages over 12 hours or longer are used, the index strongly correlates with the ap and am indices, which are derived using data from more extensive observatory networks.
The main advantages of using aa indices are that the time series spans further back (to 1868) than any of the other planetary index time series; and also up-to-date values are produced and made available on a weekly basis. In between the weekly updates, BGS calculates estimated aa indices, which are updated on an hourly basis providing speedy availability of real time “nowcasts". These estimates are clearly marked with the letters "Est".
Although calculated by the same method, the aa indices available on this service are not the definitive values (see note on compilation and changes) . Definitive aa are published by the International Service for Geomagnetic Indices (ISGI). Operated by LATMOS, France, ISGI, has an advisory board that is appointed by the Executive Committee of the International Association of Geomagnetism and Aeronomy (IAGA) and operates as part of the French BCMT (Bureau Central du Magnétisme Terrestre).