Dynamics and Electrodynamics of the Equatorial Zone - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 89

interferometer data. This shear most likely has an important role in the plasma
physics described in Chapter 4.

3.3 E-Region Dynamo Theory and the Daytime
Equatorial Electrojet

To this point, we have discussed only the nighttime vertical equatorial electric
field component associated with the F-region dynamo. During the daytime, the
vertical field is controlled by the meridional electric field, which is generated by
the global E-region dynamo we discuss next. The zonal electric field is generated
in both daytime and nighttime by the E-region dynamo. Although the zonal com-
ponent is small, it is very important because it causes the plasma to move verti-
cally. This motion greatly affects the plasma density, since it causes the F-region
plasma to interact with quite different neutral densities as it changes altitude,
strongly affecting the recombination rate and, in turn, the plasma content.
The zonal component of the electric field and the daytime vertical component
are due primarily to winds in the E region. This E-region dynamo is driven
by tidal oscillations of the atmosphere. An excellent topic on this topic has
been written by Chapman and Lindzen (1970), and we only touch on some
aspects of tidal theory here. The largest atmospheric tides are the diurnal and
semidiurnal tides driven by solar heating. The semidiurnal lunar gravitational
tide is next in strength in the upper atmosphere. It is interesting to note that the
lunar semidiurnal tide is the strongest in the case of ocean tides.
One may legitimately question a terminology in which we discuss diurnal
“tides” in the lower thermosphere (E region) but refer to a diurnal “wind” in the
upper thermosphere (F region). The difference is that the tidal modes propagate
into the lower thermosphere frombelow, whereas the upper thermospheric winds
are driven by absorption of energy in the thermosphere itself. We could thus refer
to the thermospheric response as being due to an in situ diurnal tide as opposed
to a propagating tide, but we will stick with the traditional usage here.
Tidal theory is quite complex. The equations of the neutral atmosphere must be
solved on a rotating spherical shell subject to the earth's gravitational field. Con-
siderable insight is obtained by studying the free oscillations of the atmosphere—
that is, the normal modes of the system. This is accomplished by reducing the
set of equations to one second-order partial differential equation that is often
written in terms of the divergence of the wind field. The resulting equation is sep-
arable in terms of functions of latitude (θ) , longitude (φ) , altitude ( z ) , and time

( t ) . The longitude/time dependence is of the form exp[ i ( s φ + ft ) ] , where s must be

an integer. For s = 0, the temporal behavior does not propagate with respect to

the earth. For s 1, the disturbance has one oscillation in longitude and propa-

gates westward following the sun; this is the diurnal tide. The

=

dependence can
be expressed in terms of so-called Hough functions, which may be related to
spherical harmonic functions.

θ