Preferred term

aa index

Description

aa (type of index: K-derived planetary; time resolution: 3-hour UT interval; unit: nT; availability: from 1868 onwards) is derived from the K indices calculated at two nearly antipodal observatories 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. aa and related deived geomagnetic indices such as: Aa, Kpa, CK24 and CK48) are provided by the Ecole et Observatoire des Sciences de la Terre, Strasbourg, France. aa time series is formally recognised and endorsed since 1975 by International Association of Geomagnetism and Aeronomy (IAGA).

https://service.poleterresolide.fr/voc/variable/V0010100