Service International des Indices Géomagnétiques
International Service of Geomagnetic Indices

aa index

Index
aa time resolution: 3-hour (UT) interval
unit: nT
 
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.

Northern hemisphere observatory Southern hemisphere observatory
period observatory weighting factor period observatory weighting factor
1868-1925 Greenwich 1.007 1868-1919 Melbourne 0.967
1926-1956 Abinger 0.934 1920-1979 Toolangui 1.033
1957-... Hartland 1.059 1980-... Canberra 1.084
 
ISGI Collaborating Institute EOST, Strasbourg, France
(previously LATMOS, Guyancourt, France)
 
IAGA endorsed IAGA Bulletin 37, 195, p. 128, resolution 3
 
Related geomagnetic indices
(enclosed in main index downloaded file)
Aa time resolution: 1 day (UT) interval
unit: linear scale in unit nT, obtained from daily average of eight aa values
Kpa time resolution: 3-hour (UT) interval
unit: quasi-logarithmic scale as a third of K units (28 values) obtained from aa through a conversion table (aa2Kpa)
CK24 time resolution: 1 day (UT) interval
event: quietest days over 24-hours with mean(aa) = Aa < 13 nT;
"C" indicates a really quiet C-day with ∑p < 4 or
"K" indicates a quiet K-day with ∑p ≥ 4
CK48 time resolution: 1 day (UT) interval
event: quietest days over 48-hours with mean(aa) < 13 nT;
"C" indicates a really quiet C-day with ∑p < 6 or
"K" indicates a quiet K-day with ∑p ≥ 6
Each individual aa value being represented by a weight p:

Ranges (nT) p
aa ≤ 17 0
17 < aa ≤ 21 1
21 < aa ≤ 28 2
28 < aa ≤ 32 4
32 < aa 6
 
Historical references • Mayaud, P. N. (1971) - Une mesure planétaire d'activité magnétique basée sur deux observatoires antipodaux, Ann.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)
 
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