|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
|IAGA endorsed||IAGA Bulletin 27, 1969, p. 123, resolution 2|
|Related geomagnetic indices
(enclosed in main index downloaded file)
• Mayaud, P. N. (1968) - Indices Kn, Ks et Km, 1964-1967, Ed. C.N.R.S., Paris, 156 p.
Get PDF (3.3MB)
• Mayaud, P. N. (1980) - Derivation, meaning, and use of geomagnetic indices, Geophys. Monogr. Ser., 22. AGU, Washington, DC. Get PDF (27.7MB)