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(2000) have reported the common occurrence of low altitude echoes (
95 km)
that have low Doppler shifts and low Doppler widths. It seems quite likely that
fluctuations in electron density associated with the electric fields presented in
Fig. 6.42 are responsible for these VHF echoes and that a wind-driven instability
could be the source. More importantly, perhaps, is the fact that the free energy
release associated with the wavelength-limited process is manifested in fluctua-
tions with an outer scale only an order of magnitude larger than the typical 3m
observing scale.
Given the primary perturbation structures associated with any of the processes
described above (e.g., KHI, E s layer, and drift instabilities), it is highly likely that
patches with kilometer scales are present multiple places in a typical radar range
gate/beam width area. Then fluctuation spectra similar to that in Fig. 6.31 will
develop as secondary instabilities due to the mean wind or zero-order electric
field on the unstable gradient side.
The theory of the nonlocal gradient-drift instability, which is required when
the density gradient cannot be treated as constant (e.g., E s layers), has been
worked out by Rosado-Roman et al. (2004) and Seyler et al. (2004). The theory
involves using a realistic vertical density profile and looking for solutions of
the linearized equations of motion as sums of plane waves with wavevectors
ranging over a two-dimensional plane. The resulting solutions are localized in
the magnetic field direction and reside mainly on the side that would be unstable
under ordinary local gradient-drift theory. This theory provides a formulation
of the gradient drift instability that can be applied to E s layers. Such a theory is
needed to explain the cascade of irregularity scales from the km-scale E s patches
down to the few meter-scale irregularities that are detected by coherent scatter
radars.
Another instability, applicable to the midlatitude region, was discovered by
Hysell (2002) while simulating polarization and the resulting evolution of E s
patches coupled to the F region in a background electric field. The instability,
which he refers to as a collisional drift instability, derives free energy from field-
aligned currents through gradients in the parallel conductivity. The growth rate
for the instability peaks for about one km wavelength waves, and thus may also
be involved in the cascade of irregularity scales.
6.7.9 Wind-Driven Thermal Instabilities
Finally we turn to a new idea proposed to explain what seem to be 3m waves
where no layer exists. Consider Fig. 6.43, which has echoes from a huge altitude
range. Figure 6.44 illustrates the proposed thermal effect. A wind is assumed as
well as an initial perturbation. In low-density regions the perturbation electric
field causes Joule heating, since J and
E are in the same direction. In regions
of plasma enhancement the opposite effect occurs and no heating occurs. If
the heating is great enough, the pressure force drives out more plasma and the
depletion deepens—a plasma instability with no gradient needed (Kagan and
δ
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