Study of the Return Current of the Equatorial Electrojet.

A review of indications of the return currents of the equatorial electrojet (EEJ) and associated phenomena has been presented. The need for good fitting of the observed dip equatorial magnetic variation profiles had compelled the inclusion of westward currents on the flanks of the dip equator in practically all the cases. The three sources of the return currents: the curvature of the field lines through the ionospheric dynamo region, the divergence of east-west currents close to the magnetic dip equator, and the local neutral winds varying with height combine constructively, each peaking around 5° dip latitude, to provide for the return currents of practically all the eastward electrojet current. The return currents flow on the flanks of the dip equator from about 3° and terminate at a dip distance L1 in the interval 7°≤L1≤17° with a mean of L1=10±3°, sometimes extending further but not beyond the Sq focus. At a location about 5.2±0.8° dip latitude, they reach a peak of about -27±3 per cent of the peak of the eastward equatorial electrojet. The negative correlation between the width and the intensity of the equatorial electrojet has been confirmed with data derived from physical model, indicating its origin in the westward return currents. The ionospheric current system so far detected by rockets is essentially in two layers. It is suggested that the intense lower layer including the return currents around 5° dip latitude, be associated with the equatorial electrojet; and that the weak upper layer that maintains more steady altitude extent everywhere be associated with the worldwide part of the Sq currents.