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Although quite a simple expression, this result offers explanations for a num-
ber of properties of CEIS. First, in the initial development of spread F, there is a
strong tendency for a VHF radar to obtain echoes confined to the height range
where the density gradient is upward. In fact, the early Jicamarca study showed
that the onset of nonthermal backscatter usually began at a density level about
1% of the plasma density at the F peak. Several rockets have been flown during
bottomside ESF at times when no radar echoes were obtained above the F peak,
and, indeed, intense irregularities were found below the peak but a smooth pro-
file was found above. These cases are thus in agreement with the linear theory
in that the latter predicts instability only when
g
is antiparallel to
∇
n
. Another
feature predicted by the theory is a height dependency for
due to the colli-
sion frequency term in the growth rate denominator: the higher the layer, the
lower
γ
ν
in
and the larger the instability growth rate. As mentioned earlier, Farley
et al. (1970) noted a strong tendency for irregularities to be generated when the
layer was at a high altitude where the collision term is small. Notice that the
plume structure in Fig. 4.2, the initial plume in Fig. 4.1, and the multiple plumes
in Fig. 4.8 were all generated when the echoing layer was at its peak altitude.
This also suggests that the most spectacular effects occur when the ionosphere
is high.
Indeed, strong evidence for the control of CEIS by the height of the F layer was
presented by Fejer et al. (1999). An example from their study is summarized in
Fig. 4.10, based on Jicamarca radar data. In this plot the vertical lines indicate
the presence of 3m irregularities, which define active conditions. We qualify this
since, as shown in Fig. 4.6, longer wavelength structures can exist long after
the 3m irregularities are gone. The circles indicate the vertical drift velocity and
the x's indicate the altitude of maximum incoherent scatter backscatter power,
which occurs just below the F peak. Magnetic activity (average K
p
)
for these
September 1987 examples were, sequentially, 2.9, 2.9, and 4.5, and the solar
decimetric flux, a measure of solar activity, was near 80 units (solar minimum
conditions). Inspection shows that CEIS begins if and only if the peak reached
400 km at onset. The extended altitude on September 23 occurred on the night
that the highest altitude was reached. Note that high K
p
suppressed the zonal
electric field and CEIS on September 25. This could be due to either a disturbance
dynamo or the penetration electric fields discussed in Chapter 3.
The fundamental destabilizing source is the current density arising due to the
differential drift of ions and electrons, which is dependent on mass. However,
the growth rate of CEIS (4.16) is independent of mass. This is essentially due to
the assumption of a single ion composition in the F region. Molecular ions are
also found to exist in the F region, along with atomic oxygen, during evening
(Sridharan et al., 1997) and nighttime (Narcisi and Szuszczewicz, 1981) in the
equatorial ionosphere. The growth rate expression derived by including the ionic
constituents (Sekar and Kherani, 1999) reveals that the growth rate of CEIS
depends on the mass and number densities of both ions.
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