Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 296

uplifted plasma wedge near Ascension Island in the Appleton Anomaly. Data
from five consecutive traversals are shown in Fig. 6.16b. The line-of-sight total
electron content (TEC) and the scintillation level at 249MHz are depicted for
each pass. The westward edge is clearly more structured than the eastward edge,
which is consistent with the preceding discussion and the fact that the wind
is usually eastward at this local time and latitude. Structures on the top and
eastward regions are probably due to instabilities caused by large-scale secondary
perturbation electric fields

δ E , which can also create δ E × B instabilities on the

gradients.
Many “active experiments” have been conducted at midlatitudes to model this
process by creating artificial plasma density gradients perpendicular to B and
observing the resulting structure. The active technique is to inject large amounts
of barium gas into the ionosphere from a rocket, where the barium is vaporized
from the metal state by the intensely energetic thermite chemical reaction. The
barium is ionized by sunlight and, if released at sunset or sunrise, results in a
visible long-lived plasma made usable by resonant scattering of sunlight (see
Appendix A for more details). A photograph of such a release made over the
Gulf of Mexico is given in Fig. 6.17. Notice that striated regions form in the
plasma and that they occur in only a portion of the cloud.
The ultimate active experiment was conducted in the mid-1960s when a
nuclear explosion was carried out in the ionosphere. The right-hand side of
Fig. 6.18 shows the result, which has a similar appearance to Fig. 6.17 and to
another barium cloud photograph to the left. Remarkably, even the spacing of the
striations are comparable, on the order of 500-1000m. The physics here is that
of the deceleration of the blast as it expands outward, carrying a plasma with it.

Figure 6.17 A photograph of a striated barium cloud taken at right angles to the mag-
netic field. (Courtesy of W. Boquist.)