Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 526

4.0

H 5 105 km

210 km

3.0

2 30 km

2.0

L N 5 `

1.0

30 km

10 km

0 20 40 60 80 100

Wavelength (m)

Figure 10.31 Variation of the normalized threshold drift velocity with wavelength and
electron density gradient length for waves perpendicular to the magnetic field. The solid
(dashed) curves correspond to destabilizing (stabilizing) electron density gradients. [After
Fejer et al. (1984a). Reproduced with permission of the American Geophysical Union.]

E South

E North

B

B

Electron density

(a) (b)

Figure 10.32 Sketch showing (a) unstable positive gradient in the vertical electron den-
sity profile in the presence of a poleward (northward in the northern hemisphere) electric
field and (b) unstable negative gradient in the presence of an equatorial (southward in the
northern hemisphere) electric field.

10.6.1 The Gradient Drift Instability
There are several ways to obtain density gradients at high latitudes. Figure 10.32
illustrates the case when a vertical density gradient exists in the presence of a dip
angle which is large but is not equal to 90 ◦ . For example, this situation applies
when solar illumination produces the E region plasma or when diffuse auroral
precipitation is the source. Then a vertically upward gradient is unstable for a