Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 513

Although most theoretical effort has involved current-driven modes, Ganguli
et al. (1985) have pointed out that intense shears can also generate ion cyclotron
waves. Since shears and intense field-aligned currents are often collocated [see
Eq. (10.3)], it is not yet clear which free energy source is most important. Mix-
ing and turbulent cascade will occur as well. Basu et al. (1986) have also shown
a tendency for intense irregularities at large scales to arise in regions of veloc-
ity shear. An ionospheric Kelvin-Helmholtz instability may occur due to these
shears (Keskinen et al. , 1988). Finally, it seems very likely that turbulence in the
auroral acceleration zone will propagate via an Alfvén wave mode to ionospheric
altitudes and create considerable structure in the plasma flow velocity and the
plasma density (Knudsen et al., 1992).

10.4 E-Region Layering at High Latitudes

Plasma layers are also common at high latitudes and are reasonably common in
the central polar cap. The upper panel of Fig. 10.23 shows a very nice example
detected with the Sondre Stromfjord radar in Greenland. The dark altitude-
extended regions are due to auroral particle impact. The layer stands out very

Electron density

110 2.0

105
100
95
90

1.5

1.0

85
80
75 0

0.5

0

1 2 3 4 5 6

(a)

Sodium density

110
105
100
95
90
85
80
75

8

6

4

2

0

0 1 2 3 4 5 6

Time (hours UT)

(b)

Figure 10.23 Electron and neutral sodium number densities (m − 3 ) as functions of alti-
tude and time on December 11, 1997. [After Heinselman et al. (1998). Reproduced with
permission of the American Geophysical Union.]