Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 299

E × cylindrically symmetric plasma cloud where ( B ) ·∇ n > 0. If we work in

0, then E the neutral frame of reference where U = is the electric field and

E × B 2 is the background plasma flow velocity in that reference frame. A

region of enhanced plasma density will have an elevated field line-integrated
Pedersen conductivity and, due to the shorting effect, will drift slower than the
adjacent depleted region for which

B /

P is lower. Now since ( E × B ) is parallel

to n , the lower-density region will move up the gradient to regions of higher

surrounding density, while the high-density region will move down the gradient.
Thus, relative to the surroundings, both enhancements and depletions seem to
grow in intensity, and we say the configuration is unstable. This argument is
equivalent to that associated with Fig. 4.9.
Midlatitude barium cloud striations have been penetrated by instrument-laden
sounding-rocket payloads. Data from one such experiment are reproduced in
Fig. 6.20. The smooth, enhanced-density profile due to the plasma cloud is inter-
rupted on one edge by fingers of alternating high and low plasma density. These
are the unstable electrostatic perturbations just discussed. Such a process is com-
pared to the naturally occurring phenomenon of bottomside equatorial spread
F in Fig. 6.21 by comparing the power spectrum of the barium fluctuations

∇

260

240

ST707.51-5
Electron density

220

200

Descent

180

160

140

120

Ascent

100

80 10 3

10 4 10 5 10 6 10 7

Electron density (cm 3 )

Figure 6.20 Electron density profiles from a probe rocket flight through a barium cloud.
The dashed curves are the measurements of the undisturbed F-region profile on rocket
descent. [After Kelley et al. (1979). Reproduced with permission of the American Geo-
physical Union.]