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)

Citation

Mayaud, P.-N., Menvielle, M., & Chambodut, A. (2023). aa geomagnetic index. EOST. (Dataset). doi:10.25577/9z05-v751

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	Toolangi	1.033
1957-...	Hartland	1.059	1980-...	Canberra	1.084

ISGI Collaborating Institute

EOST, Strasbourg, France

IAGA endorsed

IAGA Bulletin 37, 1975, p. 128, resolution 3, doi:10.25577/g2bk-d694

Related geomagnetic indices
(enclosed in main index downloaded file)

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Historical references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. doi:10.1029/JA077i034p06870.

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