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6 Waves and Instabilities at
Midlatitudes
In this chapter we discuss analogues to the plasma processes presented in Chapter 4.
At midlatitudes the length of magnetic field lines containing high-density plasma
is much shorter, gravity acts parallel to the magnetic field, and wind effects are
at least as important as electric fields. We finally discuss internal gravity waves
in some detail and explore their importance as well as that of energetic particle
precipitation. Familiar plasma instabilities are extended to include finite dip angle
and wind effects and some new processes, including the Perkins (F-layer) instability,
the sporadic E-layer instability, neutral Kelvin-Helmholtz instabilities, and thermal
effects, are addressed.
6.1 Mesoscale Vertical Organization of Ionospheric
Plasma: General Considerations
Dynamical considerations, of course, do affect the dayside midlatitude iono-
spheric content and altitude. However, production and recombination are suffi-
ciently rapid to mask much of the structure in the medium other than large-scale
vertical and horizontal gradients. At night, however, dynamics rules, and many
interesting dynamic and electrodynamic processes contribute to the formation
of mesoscale structures (50
.
50 km vertically).
Two graphic examples of consecutive plasma density profiles detected in the
nighttime Arecibo ionosphere were presented in Figs. 5.21 and 5.22. The April
16-17, 1974, night shown in Fig. 5.21 was very quiet magnetically, while the
other night was moderately disturbed. The solar influence can be clearly seen
at sunrise, when the deeply depressed plasma in the F-layer “valley” between
160 and 240 km altitude fills in and causes even the E-region structure to merge
into the fairly featureless daytime ionospheric profiles at the top of each figure.
The magnetically active night had a much higher valley plasma density. Some
features are common to these and most other nights over Arecibo. The high-
density F layer displays undulations with a typical period of 2 h in which the
layer rose and fell by many tens of kilometers. In the E region between 90 and
1000 km horizontally and 0
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