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Auroral Electrojet (AE, AL, AO, AU) - A Global Measure of Auroral Zone Magnetic Activity
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The AE index is derived from geomagnetic variations in the horizontal component observed
at selected (10-13) observatories along the auroral zone in the northern hemisphere.
To normalize the data a base value for each station is first calculated for each month
by averaging all the data from the station on the five international quietest days.
This base value is subtracted from each value of one-minute data obtained at the station
during that month. Then among
the data from all the stations at each given time (UT), the largest and smallest values
are selected. The AU and AL indices are respectively defined by the largest and the
smallest values so selected. The symbols, AU and AL, derive from the fact that these
values form the upper and lower envelopes of the superposed plots of all the data
from these stations as functions of UT. The difference, AU minus AL, defines the AE
index, and the mean value of the AU and AL, i.e.
(AU+AL)/2, defines the AO index. The term "AE indices" is usually used to represent
these four indices (AU, AL, AE and AO). The AU and AL indices are intended to express
the strongest current intensity of the eastward and westward auroral electrojets,
respectively. The AE index represents the overall activity of the electrojets, and
the AO index provides a measure of the equivalent zonal current.
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