Dynamics and Electrodynamics of the Equatorial Zone - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 100

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Figure 3.18 Calculated vertical ion drift velocities for several driving wind components.
The solid line includes only the tidal-driven E-region dynamo, while the dashed line
includes the F-region dynamo as well. Typical measured vertical plasma drifts are indi-
cated by the dotted line. [After Heelis et al. (1974). Reproduced with permission of Per-
gamon Press.]

preclude the PRE development by short-circuiting the F-region dynamo. The
E-region semidiurnal 2,2 tidal wave largely determines the magnitude and phase
of the daytime F region drifts in this model (see Fig. 3.19).
A simple sketch that may explain the prereversal enhancement is given in
Fig. 3.20. The equatorial plane is shown here as a vertical plane, and its projection
onto the Southern Hemisphere along B is also shown. The wind blows across
the terminator, generating a vertical electric field, E z , which is downward on
both sides of the terminator. E z is much smaller on the dayside than on the
nightside due to the shorting effect, but it is never zero. Thus, E z maps along B
to an equatorward electric field component off the equator. This field drives a
westward Hall current J

on both sides of the terminator. However, even though
E z is smaller on the dayside by up to 90%, the Hall conductivity is more than
10 times the nighttime value. The result is that a negative charge density builds
up near the terminator, creating the localized zonal E

θφ

perturbation (the PRE)
shown in the figure. Using the notation of Farley et al. (1986), the current J

φ

φφ

cancels J in a steady state. The absence of a sunrise enhancement may be due

to the lower value of E z , which occurs on the nightside of the dawn terminator
relative to the nightside of the dusk terminator as well as the lower conductivities
that occur in the predawn F region. Another idea is that the equatorial electrojet
partially closes with a current through the postsunset F-layer valley. With the low
conductivity in this region, both an eastward and an upward E field would result.
This could explain both the prereversal enhancement and westward flows seen

θφ