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Main Page   >   Data   >   Geomagnetic Variations   >   Indices of geomagnetic activity   >   aa and Aa indices

aa and Aa Indices of Geomagnetic Activity

aa index is a planetary three-hour geomagnetic activity index calculated from the values of K index measured in two almost diametrically opposite magnetic observatories in the northern and southern hemispheres - now it is Hartland in the UK and Canberra in Australia.
The values of the K index, determined by two observatories, are converted back to amplitudes. The index aa is defined as the average value between these two amplitudes, taking into account the correction for a small difference in the latitudes of two observatories or small changes in their arrangement. The index aa is measured in nanoTeslas (nT).

Aa index is the average value of the index aa per day.

The final values of the aa index is calculated and made available by International Service of Geomagnetic Indices (ISGI) from data collected at magnetic observatories.

Download Aa and aa indices since 1868 in IAGA2002 format.

WDC for STP, Moscow is grateful to the ISGI Collaborating Institute EOST, Strasbourg, France, INTERMAGNET network and International Service of Geomagnetic Indices – ISGI (

Using the data, please be sure to indicate the above organizations according to the following template.

aa index on the site of the BGS - British Geological Survey.

aa index on the site of the NCEI - National Centers for Environmental Information (formerly NGDS), NOAA, USA.

Как цитировать наборы данных: Ball, A. and M. Duke [2011], How to cite datasets and link to publications, DCC How-to Guides, Edinburgh: Digital Curation Centre.

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