Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 492

The shorting process discussed previously is accomplished by field-aligned
currents due to the E region, which shorts out both the driving fields, E , and

the ambipolar electric fields (see Section 10.2.3) that limit diffusion across B .
In addition to these structure-related currents, there are also large-scale field-
aligned currents that link the magnetospheric and solar wind generators to the
ionospheric load and that were discussed in some detail in Chapter 8. Here we
investigate the role that such applied currents play with regard to the E

δ

B
instability. When field-aligned currents are included, the process is referred to
in the literature as the current convective instability (Ossakow and Chaturvedi,
1979).
In brief, when the ion and electron species have different drift velocities along
the magnetic field line (and thus a net parallel current is carried by the ther-
mal plasma), the E

×

B instability is modified in such a way that even plasma
gradients stable to the E

×

B instability may in principle be unstable to the cur-
rent convective process. In the case of the pure E

×

× B process the most unstable

wave has k

|| =

0. For the current convective instability to operate, a finite k is

||

required.
To understand why this instability occurs we isolate the effect of an upward
(northern hemisphere) field-aligned current acting alone as shown in Fig. 10.15.
The finite k is exaggerated to make the effect easier to illustrate. We assume a

density perturbation of the form

||

n e i ( ks − ω t )

n ˜ ( s , t ) = δ

k

J 0II , E 0II

E 3 B

E

E B

E 3 B

E 3 B

E

â y , = n 0

â x

â z

Figure 10.15 Electrostatic wave with finite (exaggerated) k || propagating in the presence
of a field-aligned current. If a zero-order density gradient exists pointed into the page the
wave is unstable.