Equatorial electrojet and regular daily variation Ssub(R)

Latitudinal profiles of magnetic variations across the magnetic equator in Chad, are compared with a physical model of the equatorial electrojet which includes the effects of ionospheric winds and plasma instabilities. According to the model, east-west winds can have two types of influence on the ionospheric currents, both of which are clearly reflected in the observed magnetic profiles. First, the winds can create the appearance of a secondary current ribbon, opposed to and wider than the primary electrojet ribbon due to an east-west electric field. Secondly, winds can augment (or diminish) the level of the 'planetary' current component in the low-latitude region, in comparison to that due to a pure electric field. Arguments are presented strongly supporting the existence of mean westward winds at high altitudes (125 to 200 km) in the daytime equatorial ionosphere. The data also suggest the possible presence of plasma instability effects, which the model indicates should tend to inhibit the electrojet enhancement current and widen the primary current ribbon. The influence of the two-stream (Type I) instability, which the model takes into account, is not entirely obvious. However, it is suggested that the gradient-drift (Type II) instability, which the model does not take into account, may have an important influence on the electrojet currents. (author)