Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 315

SESCAT Doppler spectra Y : 1992 D : 214
Start (LT) : 0 40 24.83 End (LT) : 0 43 25.06
Power spectra Ave. time (s) : 8.19

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

2 400 2 200 0 200 400 15 30

Rel. power

45 60

Vr, m/s

Figure 6.31 A detailed sequence of self-normalized Doppler power spectra prior, dur-
ing, and after the occurrence of typical type 1 echoes (numbers 7-11), characterized by
an invariant peak at velocities about 300m/s. The 22 boxes correspond to 22 ranges.

These are generated by the Farley-Buneman instability during a brief period of an impul-
sive electric field that exceeds the instability threshold near the ion acoustic speed. Also
seen are typical type 2 echoes (numbers 1-4 and 15-22), characterized by a broad and
variable spectrum centered at low Doppler shifts. Type 2 irregularities are believed to be
secondaries generated from longer wavelength waves during conditions of strong plasma
turbulence. [After Schlegel and Haldoupis (1994). Reproduced with permission of the
American Geophysical Union.]

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Most convincing are the results obtained by Goodwin and Summers (1970),
who used a network (near Brisbane, Australia) of two ionosondes, and four
receivers that were geographically separated. With this network, 10 reflection
points in the ionosphere could be monitored. They categorized E s into two types: