Vector measurements ofFregion ion transport at Arecibo

Vector measurements of the ion transport velocity in the F region above Arecibo (18°N, 50° dip angle) are reported for two daytime and five nighttime periods. From these vector measurements both the electric fields and neutral winds in the F region can be found. Daytime results indicate eastward electric fields during the morning, changing to westward near 1400. The most outstanding feature of the nighttime results is a large (∼3 mv/m) westward electric field at 0400. The overall diurnal electric field pattern suggests that an 8-hour tidal mode dominates the wind fields in the dynamo region. Daytime neutral wind measurements indicate the importance of ion drag in determining the neutral winds; the poleward winds are observed to be strongest at ∼1000, though, according to satellite drag data and evidence presented here, the N-S neutral pressure gradient maximizes in the afternoon. At night the neutral winds are first seen to blow strongly equatorward but then abate and often blow poleward by 0200. Application of the neutral wind and electric field measurements to F region morphology indicates that (1) during sunset strong downward ion transport is the result of poleward neutral winds; (2) the generally occurring (at Arecibo) sudden drop in the height of the F layer near midnight is the result of a reversing wind from equatorward to poleward; and (3) the rise of the F layer often seen at ∼0400 at Arecibo is the result of E × B drifts. Finally, there is a strong tendency for the ion motion to be horizontal. Possible reasons for this behavior are discussed.