Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 490

5

Plumex upleg
100 km cutoff

0

5

250 300 350 400

Range — km

450 500 550

Figure 10.14 A vertical plasma density profile during equatorial spread F “tipped on its
side” to compare with Fig. 10.13. [After Kelley et al. (1982a). Reproducedwith permission
of the American Geophysical Union.]

currents. As noted earlier, when ( E × B ) has a component parallel to ∇ n —that

is, when

E ×

( B ) ·∇ n > 0 (10.4)

the system is unstable. Referring back to Fig. 4.9a, we see that in this case per-
turbation electric fields develop when a Pedersen current flows perpendicular to
the zero-order density gradient in the presence of a small disturbance. In effect,
a high-density region polarizes in such a way that it has a slower drift velocity
than the background plasma. A high-density region therefore lags behind and
seems to grow with respect to the background density as it drifts “down” the
gradient. Low-density regions move in the opposite direction and seem to grow
with respect to the background. Indeed, based on the AUREOL-3 satellite mea-
surements of the electric field, the event in the upper panel of Fig. 10.13 was such
that the instability condition in (10.4) was satisfied, since a 12mV/m eastward
zonal electric field component was measured and the density gradient was almost
certainly poleward. We say “almost certainly” because only one component of
∇

n is measurable from a polar-orbiting satellite. Cerisier et al. (1985) have stud-
ied six events of this type and found four of them to have (10.4), to the satisfied,
to the best of their knowledge, given the ambiguity in ∇ n . It seems clear that the

E B process can and does occur in the high-latitude sector, and most of the

discussion of the generalized Rayleigh-Taylor instability in Chapter 4 is directly
applicable at high latitudes. However, there is one important caveat to raise. The
satellite measures only E , not E =

×

E + U × B , and the role of the neutral wind

is just not known in most experiments.
Usually one argues that because B 2 at high latitudes, the neu-

tral wind plays only a minor role. However, at the boundaries of the auroral
oval, where the plasma flow changes direction, electrodynamically driven neu-
tral winds will not change direction as quickly as the plasma flow and could
act as a source for instability. In the midnight sector, for example, we expect a

| E × B / | > | U |