Equatorial Plasma Instabilities and Mesospheric Turbulence - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 176

atmospheric turbulence is isotropic at the 3m scale. Both electron density and
electric field but not magnetic field fluctuations have been detected, indicating
an electrostatic wave process. Radar spectral studies have shown the existence
of two classes of irregularities, called type 1 and type 2, associated with the
electrojet. The characteristics of the type 1 or two-stream (the reason for this
name will be clear later) irregularities were determined in the early measurements
in Peru. The type 2 irregularities have been studied in detail only with the advent
of improved sensitivity at Jicamarca. The line of echoes centered near 100 km in
Figs. 4.1 and 4.2 is due to the electrojet waves.
The type 1 irregularities have a narrow spectrum with a Doppler shift (120

±
20Hz for a 50MHz radar) that corresponds approximately to the ion acoustic
velocity (about 360m

s) in the electrojet region. These echoes appear nearly
simultaneously over a large range of zenith angles when the electron drift velocity
is larger than a certain value. Figure 4.21 shows a series of spectra taken during

/

f 5 49.92 MHz

Po w er Po w er

1 1

708 West
11 h 40 m

708 East
12 h 15 m

2200 2100 0 100 200 2200 2100 0 100 200

Doppler shift (Hz) Doppler shift (Hz)

Po w er

1

Vertical
11 h 58 m

2 200 2 100 0 100 200

Doppler shift (Hz)

Po w er
1

Po w er
1

45 West

11 h 48 m

8 45 East

12 h 08 m

8

2 200 2 100 0 100 200 2 200 2 100 0 100 200

Doppler shift (Hz) Doppler shift (Hz)

Figure 4.21 Series of Doppler spectra from the equatorial electrojet irregularities at
different elevation angles obtained at Jicamarca during a period of relatively strong scat-
tering. The spectra are normalized to the peak value. [After Cohen and Bowles (1967).
Reproduced with permission of the American Geophysical Union.]