|Auroral Electrojet (AE, AL, AO, AU) - A Global Measure of Auroral Zone Magnetic Activity
|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.