Dynamics and Electrodynamics of the Equatorial Zone - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 86

Flux-tube integrated Pedersen conductivity (mhos)
5

700 0

1 2 3 4 6 7 8 9 10

(17.0)

600 1900 LT

(21.2)

500

(27.7)

(48.2)

400

(95.9)

300

Downward electric field
Flux-tube-integrated
Pedersen conductivity
Percent of Pedersen
conductivity lying
below 300 km

200

( )

100

21.0 0 1.0 2.0 3.0 4.0

Vertical electric field (mV/m)

Figure 3.11 Equatorial F-region electric field and conductivity calculations using realistic
ionospheric parameters, a 160 m/s zonally eastward neutral wind in the F layer, and a
diurnal tidal mode in the off-equatorial E region. [After Anderson and Mendillo (1983).
Reproduced with permission of the American Geophysical Union.]

and it decreases rapidly. A considerable shear thus occurs in the plasma flow due
to the rapid change in the driving electric field, even though the thermospheric
wind was taken to be uniform with height. This detailed calculation is totally
consistent with the simple model presented above. (Note that in Fig. 3.11 the
electric field reverses sign at low altitude, since a tidal wind field was included in
the E region.)
Historically such a shear has been difficult to measure with an incoherent scat-
ter radar, since it occurs where the plasma density decreases to values below the
signal-to-noise threshold for plasma velocity measurements. The barium cloud
technique can be used, however, and a number of experiments have been per-
formed using sounding rockets. The results are summarized in Fig. 3.12a as a
function of altitude and local time. The dashed line roughly indicates the base
of the F layer. Below the F layer the plasma velocity is strongly westward. Thus,
the downward electric field is not only reduced but also reversed in sign. Simul-
taneous neutral wind measurements displayed velocities that were very different
from the plasma velocities. These surprising results were verified by measuring
the velocity of plasma irregularities, using the Jicamarca radar system in an inter-
ferometer mode. In this technique coherent scatter echoes are used to trace the
position of localized nonthermal scatterers in the radar beam. Four consecutive