Service International des Indices Géomagnétiques
International Service of Geomagnetic Indices

am index

am time resolution: 3-hour (UT) interval
unit: nT
Available from 1959 onwards
Type of index K-derived planetary (and hemispheric)
Purpose To provide a characterization of global geomagnetic activity using a large set of stations representing all longitudes and possible hemispheric discrepancies.
Network The stations are situated close to 50° corrected geomagnetic latitude (in the subauroral zones), and are gathered into groups, each corresponding to a longitude sector. There are 5 such groups in the Northern hemisphere, and 4 in the Southern one.
(see list of actual and previous am magnetic observatories)
Derivation In each longitude sector, the K scaled for each station, are averaged, and the result is converted into amplitude using mid-class amplitudes for L9 = 500 nT (L9 being the K=9 lower limit; conversion table K2aK). The weighted average of the so-obtained amplitudes give rise to the hemispheric indices an (North) and as (South), the weighting factors accounting for the differences in the longitude sector extents in each hemisphere. The planetary index am is equal to (an + as) / 2.

An example of "Longitude Weighting Factors' computation principles" may be found here.
ISGI Collaborating Institute EOST, Strasbourg, France
(previously LATMOS, Guyancourt, France)
IAGA endorsed IAGA Bulletin 27, 1969, p. 123, resolution 2
Related geomagnetic indices
(enclosed in main index downloaded file)
an and as time resolution: 3-hour (UT) interval
unit: linear scale in unit nT, obtained from K values through a conversion table (K2aK)
am = (an + as) / 2
Kpm time resolution: 3-hour (UT) interval
unit: quasi-logarithmic scale as a third of am units (28 values) through a conversion table (am2Kpm)
Am, An and As time resolution: 1 day (UT) interval
unit: linear scale in unit nT, obtained from daily average of eight am (respectively an and as) values
Historical references • Mayaud, P. N. (1968) - Indices Kn, Ks et Km, 1964-1967, Ed. C.N.R.S., Paris, 156 p. Get PDF (3370K)
• Mayaud, P. N. (1980) - Derivation, meaning, and use of geomagnetic indices, Geophys. Monogr. Ser., 22. AGU, Washington, DC. Get PDF (28369K)