Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 267

120 km, very intense layers developed on each night and lasted from sunset to
sunrise.
Whenever there are electrons to scatter from, the radar can detect motion and
organization of the plasma. For example, often a piece of the F region seems
to “peel off” the bottomside of the layer at sunset and to propagate downward
into the F-layer valley region. Such a structure has been termed an intermediate
layer by Shen et al. (1976), since it occurs between the F layer and the more
classical sporadic E layers below. Since ionosonde signals are often reflected by
the intense lower-altitude layers, the intermediate structures are not visible with
an ionosonde and can be studied only via rockets or incoherent scatter radars
such as the one at the Arecibo Observatory. The sporadic nature of the various
lower layers, which gives them the name sporadic E, is evident in these profiles.
The strongest intermediate layer lasted almost all night on April 17-18, while it
died out at 0230 LT on April 5-6. Other more sporadic and weaker intermediate
layers came and went on April 17-18.
In the next two sections we discuss the formation and dynamics of these lay-
ers, which are primarily due to oscillatory behavior of the neutral atmosphere.
Such motions are classified as either tides or gravity waves depending on their
frequency. Although, as we shall see, these neutral atmospheric motions are capa-
ble of organizing the plasma into layered structures, this is not the entire story.
The intermediate-layer ionization in particular cannot entirely be explained by
photoionization processes, and we briefly discuss the observation and effect of
ionizing energetic particle fluxes in this regard. F-regionmidlatitude plasma insta-
bilities can amplify (or even create?) mesoscale structures as well.
Finally, once the layers are formed, they may be subject to primary or second-
ary plasma instability processes similar to those discussed for the equatorial
zone, which lead to small scale structures, and we end the chapter with a brief
discussion of such processes.

6.2 Oscillations of the Neutral Atmosphere

From the discussion in Section 5.1, it is clear that when 1, it is

difficult to move plasma across magnetic field lines with a neutral wind. But if
the neutral atmosphere has a velocity component parallel to the magnetic field,
the plasma will be carried along B with the same velocity as the parallel compo-
nent of the neutral atmosphere. Any horizontal neutral velocity with a compo-
nent in the magnetic meridian will therefore create an F-region plasma velocity
projected in the direction of the magnetic field. We previously discussed the
fact that mean meridional neutral wind patterns cause F-region plasma motion
upward or downward along B . Here we expand this discussion to include tides
and gravity waves. Traditionally, upper thermospheric dynamics are discussed
in terms of wind patterns rather than tidal modes, even though they display
clear diurnal variations. On the other hand, long-period lower thermospheric

κ i = i / v i