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Auroral Electroject Indices


The auroral electrojets indices (AU, AL, AO and AE) are designed to provide a global, quantitative measure of auroral zone magnetic activity produced by enhanced ionospheric currents flowing below and within the auroral oval. These indices are derived from geomagnetic variations in the horizontal component observed at selected (10-13) observatories along the auroral zone in the northern hemisphere.
In detail, for each given time (UT), at one minute intervals:

  • AU (Auroral Upper) is the uppermost envelope of the superposed H-component perturbations at auroral latitudes;
  • AL (Auroral Lower) is the lowest envelope of the superposed H-component perturbations at auroral latitudes;
  • AO= (AU + AL )/ 2;
  • AE= AU – AL.
Geomagnetic observatories used for the estimation of the auroral electrojet indices.

From a physical point of view:

  • AU is a measure of the eastward auroral electrojet (EEJ), flowing in the dayside auroral oval and coupled to magnetopause currents. AU is a measure of the directly driven response of the magnetosphere to the changes of the interplanetary medium (solar-wind) conditions.
  • AL is a measure of the westward auroral electroject (WEJ) that intensifies during substorms. Thus AL provides a good tool to follow the evolution and the different phases of a substorm.
  • AO provides a measure of the equivalent zonal current.
  • AE index does reflect the integrated effect of different current systems and not necessarily quantify specific physical processes occurring in the magnetosphere-ionosphere coupling system.
East (EEJ) and West (WEJ) auroral electrojet.

Auroral electrojets indices (AU, AL, AO and AE) minute values are estimated by “Data Analysis Center for Geomagnetism and Spacemagnetism” WDC-C2, while detailed graphs and tables are available on a series of books published by “World Data Center C2 for Geomagnetism”.


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