Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 327

fields through essentially the mechanism of Haldoupis et al. (1996), with current
closure occurring entirely within the E s layer. Given the large midlatitude wind
shears found by Larsen (2002), relatively smooth altitude modulations of an E s
layer can result in substantial polarization electric fields through this mechanism
with more or less arbitrary orientations.
Finally, although we have concentrated on the two-stream case in this section,
the sizable polarization electric field and the existence of the zonally structured
sporadic E patches suggest that the gradient drift process will occur even more
often. Indeed, in Fig. 6.31, even during times when two-stream is not occurring,
type-II echoes are evident. The spectra are a bit narrower than the equatorial
case, which suggests that Sudan-like strong turbulence may be less common at
midlatitudes (Haldoupis et al., 2003).
6.7.6 Spontaneous Structuring by the E s -Layer Instability
As previously described, Perkins (1973) studied altitude modulations of the F
layer and thereby discovered the Perkins instability. Armed with the theory of
polarization of E s layers in a wind shear brought about by altitude (or field
line-integrated (FLI) conductivity) modulation, Cosgrove and Tsunoda (2002b)
undertook a similar study of the stability of E s layers. They found that the equilib-
rium configuration of an E s layer at a zonal wind shear node is unstable at night
to altitude and FLI density modulations. (During the day, polarization fields are
heavily loaded by the highly conducting E region.) The unstable modulations are
horizontally distributed as plane waves, and the growth rate maximizes when
the plane wave phase fronts are aligned northwest to southeast (southwest to
northeast) in the Northern (Southern) Hemisphere. Hence, this E s -layer insta-
bility ( E s LI) is in the same class as the Perkins instability, since it shares a similar
geometry. However, it differs from the Perkins instability in that it involves Hall
currents and derives its free energy from a wind shear instead of gravitation. In
addition, the compressibility of the E region leads to the coupling of FLI den-
sity modulations to altitude modulations, which does not occur for the Perkins
instability.
Tsunoda et al. (2004) have summarized the experimental evidence for the E s LI,
which can explain wavelengths from about 500 meters up to hundreds of kilo-
meters. Hence, it applies to the longer wavelengths for which the gradient drift
instability does not apply (Kelley and Gelinas, 2000; see Section 6.7.6). It also
provides an alternative to the Kelvin-Helmholtz instability (see Section 6.7.2) as
a way to create horizontal structuring of E s layers, which applies when the wind
shear is hydrodynamically stable. Therefore, through the Haldoupis et al. (1996)
polarization mechanism, it can also lead to large polarization electric fields,
which are needed to explain the observations of type 1 and large-spread type
2 Doppler spectra. Simulations by Cosgrove and Tsunoda (2003) have shown
the nonlinear evolution of the E s LI and demonstrated the generation of large