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south of the observatory. Note that a poleward displacement of the anomaly
accompanies an enhanced fountain effect.
5.2.4 Complexities of the Real Nighttime Tropical Ionosphere
The detailed dynamics of the earth's ionosphere is governed by a complex inter-
play between motions of the neutral atmosphere, gravity, electromagnetic forces,
pressure gradients, and plasma production and loss. In the nocturnal low- tomid-
latitude F layer, the latter two processes are relatively unimportant and can be
ignored for our purposes once the molecular ions, which have a high recombina-
tion coefficient compared to O
+
, are removed from the system. Since molecular
ions are found at low altitudes, the highly conducting daytime E region virtually
disappears, which allows the F-layer plasma greater control of the electrody-
namics of the region. This can be seen in Fig. 5.12, where the ratios of F region
to E-region height-integrated nighttime conductivities over Arecibo are plotted
for several months (Burnside, 1984). Each plot corresponds to an average of
5 days. The ratio
PF
/
PE
considerably exceeds unity for all but a few data
points. The 1981-1982 data (near solar maximum) show evening values as high
as 20, whereas in the 1983 data, the largest values approach 10. Earlier, Harper
and Walker (1977) found the ratio to be closer to unity at solar minimum. This
result most likely reflects the lower F-layer plasma densities that occur at solar
minimum.
The Arecibo Observatory is particularly well suited to the study of interre-
lationships between these forces, and much of our knowledge concerning the
tropical off-equatorial ionosphere comes from Arecibo data. The near-45
◦
dip
angle allows each of the important forces an “equal vote” in the control of
the F-region plasma dynamics. This is not entirely a geometric effect but stems
also from the near equality of the following four characteristic “velocities” at
altitudes near the F-layer peak:
/ν
in
;
lV
th
E
/
B
;
U
;
g
/
L
(5.24)
i
where
V
t
i
is the ion thermal speed,
l
is the ion mean free path, and
L
is the plasma
pressure gradient scale length. The last term is a measure of the term
p
i
/ρ
i
ν
in
in the ion equation of motion, and is the velocity at which a pressure gradient
would drive the ion flow velocity against friction with the neutral gas (ignoring
ambipolar effects). Representative values and their sum are plotted in Fig. 5.13
for a particular (observed) electron density profile over Arecibo. If any of these
terms dominated the dynamics, the F-layer as we know it could not be limited
to the relatively modest attitude excursions found experimentally. Turning this
argument around, we have argued above that the F layer seeks out an attitude
where a balance between these factors is reached. But does this balance occur?
Numerous measurements of vector plasma velocities have been conducted at
Arecibo. The “natural” coordinate system for display of ionospheric F-region
∇
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