The E-region dynamo We consider ,rst the current and electric polarization ,elds that result from the tidal neutral winds in the E-region. At low latitudes a large-scale 24-h variation called the diurnal tide dominates the neutral atmosphere motion. In the E-region the total ion concentration is in photochemical equilibrium and has a local time distribution shown schematically in Fig. 7. The ionospheric conductivity also has this local time distribution and thus the wind driven currents are largely con,ned to the daytime hours. The top panel in
Fig. 8 shows the E-region wind system that has a 24-h period resulting from the absorption of solar euv radiation in the lower mesosphere. Since the Pedersen and Hall conductivities
are both important in the E-region it is instructive to consider their consequences separately. In regions where the Hall conductivity dominates then the neutral wind will move the ions in the direction of the wind. Electric ,elds will result from polarization charges that will accumulate
wherever the wind driven current has a divergence. Near local noon the ion density decreases with latitude but the wind increases rapidly with latitude. The latitude gradient in the current, which is proportional to the product of these two quantities, has a peak near 30◦ where a negative polarization charge will accumulate. An approximately radial electric polarization ,eld will point toward this location. Other gradients in the current will arise primarily from the Considering the current driven by the polarization ,eld it will circulate anti-clockwise as illustrated in the lower panel of Fig. 8. Gradients in the current at low latitudes will produce
small negative and positive polarization charges at the dawn and dusk terminators, respectively. Considering the wind blowing across these gradients where the Hall conductivity dominates we see that negative polarization charges will develop at each terminator. Since the wind is approximately symmetric with respect to local noon, no electric ,eld will result. Finally, in the region where the Pedersen conductivity is important during the daytime the poleward
wind drives a westward current with the accumulation of positive and negative polarization charges at the dawn and dusk terminators, respectively, as shown. The electric ,eld resulting from these charge distributions will map along the magnetic ,eld to the F-region where during the daytime it will produce ion drifts that are upward and to the west. At night, we would expect the ion drifts to be downward and weakly eastward (Heelis et al., 1974).