Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 518

Ithaca, NY September 6, 1982

Range 5 472.5 km 480.0 km 487.5 km 495.0 km

04:00:15

04:01:00

04:02:05

04:03:03

04:04:01
(EST)

-1000 0 -1000 0 -1000 0 -1000 0

Phase velocity (m/s)

Figure 10.25 Examples of the temporal and spatial variations of backscattered power
and Doppler velocity at 50 MHz in the auroral oval. The spectra were normalized to
the same value and the integration time was about 6 s. [After Providakes et al. (1985).
Reproduced with permission of the American Geophysical Union.]

(Pfaff et al., 1984; Pfaff, 1985). The frequency-altitude-intensity format used
here shows where the waves occur and their relative intensity. In the bottom
right-hand panel ( E dc = 10mV/m) there is no detectable signal. As the dc electric

field strength increases the layer thickness and the intensity both increase dramat-
ically. Near the top of the layer a discrete narrow band emission is observed in
some cases (e.g., panels a through d). This feature is also apparent in the sequence
of discrete spectra in Fig. 10.27, which correspond to the experiment in panel
b of Fig. 10.26. At a height of 120 km a very narrow band signal was detected
which, except for the exact value of the center frequency, is nearly indistinguish-
able from the topside equatorial electrojet spectrum plotted in Fig. 4.30. The
value of the center frequency in this coherent feature can vary considerably from
flight to flight. This has been explained through the dependence of the Doppler
shift of the signal on the rocket velocity vector, the plasma velocity vector, and
the acoustic speed in a given flight (Pfaff, 1985). The data seem to indicate a very
turbulent “heart” to the electrojet with coherent structures in the upper regions.
This description is very similar to that of the daytime equatorial electrojet rocket
data discussed in Chapter 4.