Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 323

(KHI). This causes a violent mixing of the fluid, and, as seen in the 50MHz echoes
presented in Fig. 6.35b, the plasma responds, even at small scales. Evidence for
unstable shears—that is, shears with a Richardson number less than 1/4—were
presented in Section 5.3.2 and seem to be very common in this height range. In
such a process it is also common for periodic billows to form with separation
scales several times the shear scale. In the case of the E region, the result would
be separations in the range of 5-15 km of the radar echoes, which is typical of
Q-P spacings (Larsen, 2000).
Since the Kolmogorov microscale at 105 km is many tens of meters, it is not
at all obvious how 3m scales are created, which is needed to explain the radar
scatter. A simple passive scalar-mixing argument will not produce structures at
such small scales, nor will it produce structures with k strictly perpendicular to B .
Some plasma process is required to allow coupling across the scales from tens
of kilometers to meters. Such a process in polar mesospheric clouds involving
charged ice particles is described in the next chapter. However, that mechanism
is not likely to occur at middle latitudes, and, in any case, the scattering is much
more isotropic in the polar summer mesosphere than was observed in sporadic E
echoes. Several examples from Kane et al. (2001), including Fig. 6.35c, revealed
3m echoes only on the top of the plasma layer. If the wind was strictly westward
across the layer, only the topside would be unstable (see Section 6.7.7). Since
the KHI seems very common in the midaltitude E region, neutral atom billows
have been directly associated with VHF backscatter (see Fig. 6.35) and billow
spacings are the right spacing; it seems the KHI plays a role in at least some Q-P
echoes.
6.7.5 The Role of Horizontal Structure: Amplification by the
Cowling Effect
The occasional observation of Type 1 or two-stream instabilities is difficult to
explain using global scale electric fields and horizontally stratified layers. For
example, as in the equatorial case, a dynamo electric field drives a current across
the magnetic field lines. Since the layers are very sharply bounded, one might
think that they would polarize and the Cowling effect would occur. But since
the magnetic field lines are oriented at an angle, any polarization field in the
meridian plane will be shorted out along the magnetic field lines. Thus, there
is no Cowling effect and no horizontal type 1 waves. Haldoupis et al. (1996,
1997) proposed that sharp plasma boundaries in the zonal direction could lead
to large amplification of an applied meridional electric field. These horizontal
structures have been observed over Arecibo by Smith and Miller (1980) and
clearly are present in Fig. 6.7. In fact, during that E-region plasma and sodium-
atom disturbance, type 1 echoes and large F-region drifts were observed (Swartz
et al., 2002). The geometry for this process is presented in Fig. 6.39 for a south-
ward electric field, the usual polarity for the evening to early morning hours.
This direction electric field drives a westward Hall current through the sporadic