Correlation of wind and electric field in the nocturnal F-region

The incoherent scatter radar at the Arecibo Observatory measures the ion drift velocity vector in the E- and F-regions of the ionosphere. This vector can be resolved into components parallel and perpendicular to the geomagnetic field. Nocturnal F-region data reveal a particularly marked negative correlation between the parallel component, which represents the combined effects of diffusion and a meridional wind, and the perpendicular component, which is caused by a zonal electric field. Various explanations of this correlation have been offered. One is that ion drag causes the F-region wind to respond to changes in the ion drift velocity. This explanation is not adequate, however, because nocturnal ion densities are too low for ion drag to affect the wind on the observed time scales of an hour or less. Another mechanism suggests that F-region winds lead to the generation of F-region polarization electrostatic fields, causing the perpendicular drift to respond to changes in the wind. Analysis of the data suggests that this mechanism does not account for the most striking examples of the correlation (those with short period), although it may operate weakly. The mechanism that appears to be most successful in accounting for the phenomenon attributes changes in the parallel component of the ion drift velocity to changes in the diffusion velocity resulting from changes in the F-layer height which result, in turn, from changes in the zonal electric field. Why the electrostatic field in the nocturnal ionosphere (and thus the perpendicular component of the ion drift velocity) should fluctuate as widely as the Arecibo data reveal is not known.