Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 312

22 : 00

2 1 : 0 0

2 0 : 0 0 n e

( c m 2 3 )

5 3 1 0 1 6

1 9 : 0 0

1 3 1 0 1 6

5 3 1 0 1 5

1 3 1 0 1 5

1 8 : 0 0

5 3 1 0 1 4

1 3 1 0 1 4

5 3 1 0 1 3

9 0 1 6 0 2 3 0 3 0 0

H e i g h t ( k m )

Figure 6.30a Consecutive electron density profiles measured over Arecibo on May 7,
1983. Notice the intense sporadic E layer, which existed even prior to sunset and the
intermediate layer which descended through the F-layer valley. [After Riggin et al. (1986).
Reproduced with permission of the American Geophysical Union.]

E B drift or a mean wind toward the radar. The Doppler oscillations, however,

must be due to meridional perturbation electric fields since the radar was located
in St. Croix, which is east of Puerto Rico. As we shall see below, equatorial-
like geometries may indeed occur at midlatitudes and lead to two-stream
conditions.
Figure 6.30c shows a more spectacular event detected in the same geometry
in which the spectra seem to saturate at close to the speed of sound. Schlegel
and Haldoupis (1994) presented several examples of midlatitude Type 1 waves
just like this case, one of which is reproduced in Fig. 6.31. They concluded
that a modified two-stream condition existed. By “modified” they mean that
the narrow two-stream spectra saturate at a relatively low velocity compared to
the equatorial case. They concluded that this is most likely due to metallic ions
and to the correspondingly lower acoustic speed. It is not at all obvious how
such large drifts could occur, since the equatorial case has a unique geometry

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