Dynamics and Electrodynamics of the Equatorial Zone - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 72

of the Doppler shifts detected by the two beams yields the zonal eastward drift
speed of the ion gas, and their average yields the vertical drift speed of the ions.
In the F region,

κ i is very large, and the ion velocity perpendicular to B is given
by (2.36d)—that is,

B

B 2 V i ) ⊥ = E − k B T i / q i ∇ + M q i g × ( n / n / / (3.1)

where all quantities are measured in the earth-fixed frame. Notice that the neutral
wind velocity does not appear in this equation, so the radar measurements cannot
be used directly to measure any F-region wind components perpendicular to B .
At typical measurement heights, we can estimate the drift velocities due to the
pressure and gravity terms. The former is the order of k B T i / q i LB , where L is

the density gradient scale length. Taking L 1000K yields a

drift velocity of roughly 0.4m/s. The gravitational drift is even smaller, less than
one-tenth of this value. Since the observed drift velocities are much greater than
these values, they must be due to the electric field.
These results hold for both daytime and nighttime conditions; that is, inco-
herent scatter radar measurements in the F region can, in principle and often
in practice, yield the ion drift and thus the electric field at all local times
(Fejer, 1991). There are some limitations of the technique, however. A plasma
instability termed equatorial spread F often occurs in the evening and, when
present, precludes incoherent scatter measurements, even in the upper F region.
Also, the incoherent scatter method requires a minimum plasma density in the
scattering volume determined by the system noise, antenna size, transmitter
power, integration time, and so on. For the Jicamarca Radar Observatory, this
minimum is about 10 4 cm − 3 , which often precludes measurements at night in
the altitude range below the F peak. Barium ion cloud releases, rocket probes,
and radar interferometric methods have been used successfully in this height
range.
A compilation of the Jicamarca data set spanning nearly an entire solar cycle is
presented in Figs. 3.1 and 3.2. In Fig. 3.1 the zonal eastward drift measured near
the F peak is plotted, whereas in Fig. 3.2 the vertical drift component is shown.
Both solar and seasonal dependencies are illustrated, and both components have
a strong diurnal modulation. To first order, the drifts are up and to the west
during the day and down and to the east at night.
Some aspects we wish to emphasize are as follows:

= 10 km and T i =

1. The peak eastward drift at night is twice as great as the peak westward drift during
the day.
2. The zonal drifts are much larger than the vertical velocities.
3. The vertical drift is often strongly enhanced just after sunset but shows no comparable
feature near sunrise. This is termed the prereversal enhancement of the vertical drift
or, equivalently, of the eastward electric field component.
4. There are strong solar cycle effects in the vertical drifts and moderate seasonal effects
in both data sets.