Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 311

would grow in place and be aligned from NW to SE as observed. This direc-
tionality is a major triumph of the Perkins theory, since the observed wave
fronts do make an angle with the magnetic meridian. However, the growth
rate is very slow and is unlikely to provide significant amplification of thermal
noise.
For the pure electric field case, the real part of ω is negative for the most unsta-

ble wave vector, since k y E e > k x E N and the wave would seem to travel toward

the southwest as observed (e.g., Fig. 6.23f). The problem with this seemingly
positive result is that under typical conditions, both the background stability
and instability are established by the winds, not the electric field. In fact, since
the average electric field is southeastward after sunset,

r is positive and wind-
driven instability structures should drift toward the east, not the west. This is a
major problem for the linear theory.
Figure 6.28b shows that the TEC does vary in these events, so unlike, in the
Perkins model,

ω

0. This is likely a three-dimensional effect. The first
results of a three-dimensional simulation of this phenomenon show promise for
new theoretical insights (Yokoyama et al., 2008). In the next section, a similar
layer instability is discussed for the E region, and we take a new look at coupling
between the layers.

∂ N /∂ t =

6.7 Midlatitude E-Region Instabilities

For many years extrapolation of equatorial instabilities to midlatitudes was
thought to be trivial. Today we know this is far from the truth. One key
difference is that electric fields share importance with neutral winds. Another
is that the plasma gradients are sharper at midlatitudes than elsewhere due to
the layering process discussed above. Even pure neutral atmospheric instabili-
ties, such as the Kelvin-Helmholtz process, seem to play a role in creating plasma
structures.

6.7.1 Radiowave Observations of Nighttime Midlatitude
E-Region Instabilities
Figure 6.30a illustrates a well-documented sporadic E event over Arecibo. As
an intermediate layer descends from around 180 km, it is most likely supported
against diffusion by a meridional wind shear in a tidal wave mode. As NO +
and O 2 decay away, a core of metallics maintains the layer and it steepens even
more if subjected to a zonal wind shear. This mode change may have occurred at
2148 when the layer peak briefly began to rise. Quickly thereafter, as shown in
Fig. 6.30b, the layer goes unstable to long-period variations in the Doppler shift,
similar to equatorial large-scale waves, waves with much greater than the layer

thickness. The mean Doppler shift of 30m/s toward the radar could be due to an

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