|Available||from 1868 onwards|
|Type of index||K-derived planetary|
|Purpose||To measure the amplitude of global geomagnetic activity during 3-hour intervals normalized to geomagnetic latitude ±50°. aa was introduced to monitor geomagnetic activity over the longest possible time period.|
|Network||Made of 2 antipodal magnetic observatories.
(see list of actual and previous aa magnetic observatories)
|Derivation||aa is derived from the K indices measured at two antipodal observatories. The K indices
are converted into amplitudes using mid-class amplitudes (K2aK) then averaged with weighting factors that account for
slight changes in geomagnetic disturbance intensities between successive Northern and Southern aa-observatories.
|ISGI Collaborating Institute||
EOST, Strasbourg, France
(previously LATMOS, Guyancourt, France)
|IAGA endorsed||IAGA Bulletin 37, 1975, p. 128, resolution 3|
|Related geomagnetic indices
(enclosed in main index downloaded file)
• Mayaud, P. N. (1971) - Une mesure planétaire d'activité magnétique basée sur deux observatoires antipodaux, Ann. Geophys., 27, 67-70.
• Mayaud, P. N. (1972) - The aa indices: A 100-year series characterizing the magnetic activity. J. Geophys. Res., 77(34), 6870-6874. Get PDF (555K)