Dynamics and Electrodynamics of the Equatorial Zone - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 115

and the two expressions for F E and F D are seen to agree with each other. In the
preceding last term, we have simply noted that n i = n , the plasma density.

Referring back to Fig. 3.7, we may now interpret the parameters

ν ni and f . The

solid curve is ν ni , the daytime neutral-ion collision frequency, and the vertical

dotted line is the Coriolis parameter, f , evaluated for 45 ◦ latitude. (Here = sin θ

10 − 4 rad/s.) The latter plays no
role at the equator but the drag term does. For example, consider the steady-state
solution to

is the rotation frequency of the earth, 7 . 35 ×

/ =− ( /ρ) ∇ − = −

d U dt 1 p v ni U F v ni U

which is given by

U =− F / v ni

where typical values for F are also plotted in Fig. 3.7. Near the F-layer plasma
density peak at 250 km the equation yields a magnitude for U of only about
70m/s. At 300 km the velocity rises to about 130m/s. These values are much
more reasonable than those found by Lindzen (1966) without ion drag and show
that the ionosphere has great control over the neutral atmosphere. In summary,
the J B force on the plasma is transferred to the neutral gas by collisions.

As discussed earlier, vertical wind variations are suppressed by viscosity in the
middle and upper thermosphere.
There is an interesting and important day-night asymmetry to the drag term
due to the nighttime F-region dynamo. During the day, the F-region dynamo is
shorted out by the E layer and the plasma cannot move along with the wind.
Thus, the thermospheric wind blows through ions that are tied to the magnetic
field lines. Collisions between neutrals and the “immovable” ions create the drag.
But at night, the plasma tends to catch up to the neutrals, and the drag becomes
small. Alternatively, we could argue that J

×

= E + U × B becomes small and, in

any case, the wind can accelerate.
Given the control exhibited by the ionospheric plasma on the thermosphere,
it is not surprising that the winds are so variable. In addition to the F-layer
dynamo and the effect of the vertical electric fields, another factor is the altitude
of the nighttime ionospheric plasma layer, which is determined by the magnitude
of the zonal component of the electric field. Anderson and Roble (1974) have
calculated zonal neutral wind contours in conjunction with an empirical model
of the electric field pattern for the two cases illustrated in Fig. 3.29. In the upper
plot a diurnally varying electric field pattern is used that is typical of sunspot
minimum conditions, while the lower plot includes the prereversal enhancement
of the electric field, which is largest during sunspot maximum. In the latter case
the ionospheric plasma is driven to very high altitudes in the 2000-2400 LT
sector. With the plasma out of the way, the ion drag is greatly reduced and the
thermospheric wind speed nearly doubles. Since changes in the zonal electric field