IAGA
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FAGS
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IAGA |
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FAGS |
| aa INDICES |
aa
indices :The aa indices are derived using data from two nearly antipodal observatories, where magnetograms were available since 1868. For each three hour interval, K indices are measured at the two stations and converted back into amplitude; an individual aa index is the average of the northern and southern values, weighted to account for the small difference in latitude of the two stations, or for the slight changes in the very place of the observatory.
The observatories used to derive aa are given in with weighting coefficients given in brackets ;
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Northern Hemisphere |
Southern Hemisphere |
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Observatory |
Corr. Geom. Lat |
Observatory |
Corr. Geom. Lat |
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1868-1925 |
Greenwich (1.007) |
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1868-1919 |
Melbourne(0.967) |
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1926-1956 |
Abinger (0.934) |
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1920-1979 |
Toolangui (1.033) |
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1957- ... |
Hartland (1.059) |
50.0° |
1980- ... |
Canberra (1.084) |
45.2° |
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The aa index is in nanotesla (nT) and it represents the activity level at an invariant magnetic latitude of about 50°. The half-daily and daily mean values of aa are very close to the corresponding values of the am indices. The Monthly tables of aa indices are given as follows :
| Date | Q.D. | aa_3h_ind | Aa | Kpa_3h_ind | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Date : | Day/Month |
|---|---|
| Q.D. : |
The letters C and K indicate the quiet 24-hour and 48-hour intervals as obtained from aa : C = really quiet, K = quiet but with one or a few slightly disturbed three-hourly intervals . The letter on the left column refers to the 24-hour Greenwich day, that on the right one refers to a period of 48 hours centered on the Greenwich noon. The asterisks mark the five international quietest days of the month (as deduced from Kp). |
| aa_3h_ind : | 8 three hourly values of aa, in nT (-1 correspond to missing data) |
| Aa : | daily mean value of aa, in nT |
| Kpa_3h_ind: | 8 three hourly values of Kpa which is the code corresponding to aa expressed in the same scale than Kp ( - correspond to missing data) |
A musical diagram of aa indices (logarithmic scale) is drawn since 1868.