Report on the Derivation of am, an, as and aa
Planetary Magnetic Indices
presented by Michel Menvielle during the 8th IAGA Scientific Assembly
On 1997, January 1st, the am and aa serviices moved from the Université Paris Sud (Orsay, France)
to the Centre d'étude des Environnements Terrestre et Planétaires (C.E.T.P., Saint Maur, France).
Provisional and definitive values are computed with the K values provided by the observatories.
Provisional values are circulated on a monthly basis with a few weeks delay for am, an, and as, and
on a weekly basis with a few days delay for aa. Reducing these delays down to a few days implied to
derive provisional K values from digital minute values.
Algorithm for computer derivation of SR, leading to automatic calculation of K indices have then
been set up and tested by different research groups. Four of them have been acknowledged by I.A.G.A.
and are now available through I.S.G.I. [IAGA News N°32, p. 27, December 1993]. A software which aims
at calculating in quasi real time these indices was then developed at the I.S.G.I. Publication
Office. It gets automatically on a daily basis digital minute values from geomagnetic observatories
through electronic data transfer procedures, and computes K indices using the FMI method.
The quick-look values of planetary indices are then computed and circulated once data
availability makes it possible. Since the end of 1996, quick-look values of K-derived planetary
indices are then made available on the server IMAGINE, with a nominal time delay of a few days.
The am and Km quick-look indices are derived on an 'available data' basis, and a due caution is
required in using them.
Uppsala, August 1997
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