Dynamics and Electrodynamics of the Equatorial Zone - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 99

The excellent review by Forbes (1981) details these and other electrojet studies,
including day-to-day variability, neutral wind, and lunar tidal effects that are
beyond the scope of the present text.

3.4 Further Complexities of Equatorial Electrodynamics

The simple E- and F-region dynamos just described explain some of the observa-
tions presented earlier in Fig. 3.1. Since the E region controls the physics during
the day, the small daytime vertical electric fields in the F region presumably mir-
ror the meridional E-region polarization fields in the latitude ranges just north
and south of the equator. The zonal field, both daytime and nighttime, is global
in nature and is driven by the large-scale tides and winds in the sunlit hemisphere.
In the postsunset period, the F-layer vertical field is enhanced due to the local
F-layer dynamo previously discussed. In the next paragraphs we point out some
of the features not explained by these simple models.

3.4.1 The Prereversal Enhancement
The postsunset enhancement or prereversal enhancement of the zonal field occurs
during all epochs and seasons studied except for the solar minimum solstices. The
effect of this brief duration, large eastward electric field can be quite significant
since the F-layer plasma often is driven to very high altitudes where recombina-
tion is slight and collisions are rare. Heelis et al. (1974) successfully predicted
the postsunset effect in their model, which included horizontal conductivity gra-
dients near sunset in the F-layer dynamo mechanism (in addition to the vertical
gradients we have studied). In effect, near such a sharp east-west gradient, an
enhanced zonal electric field is established to keep 0. Experimental data

support this explanation since the enhancement begins when the sun sets on
either of the E layers in contact along B with the equatorial F region.
The evidence from their numerical simulation is presented in Fig. 3.18. The
solid line shows the calculated vertical ion velocity in the F region (equivalent
to a zonal electric field) when only a diurnal E-region tidal mode is considered.
The basic day-night features are reproduced, but no prereversal enhancement
occurs. The dashed line shows a calculation including the F-region dynamo in
the physics. The latter agrees quite well with the dotted line, which is a typical
vertical drift profile for the plasma over Jicamarca.
The prereversal enhancement has been successfully simulated for the first time
by a general circulation model, the National Center for Atmospheric Research
thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM;
Fesen et al., 2000). The TIEGCM reproduces the zonal and vertical plasma drifts
for equinox, June, and December for low, medium, and high solar activity. The
crucial parameter in the model needed to produce the prereversal enhancement
(PRE) is the nighttime E-region electron densities; densities greater than 10 4 cm − 3

∇· J =