Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 516

is followed by a theoretical discussion in which additional data are introduced
where appropriate.
Since auroral currents can be quite large, the term auroral electrojet is often
used. The “jet” in this case refers to the relatively localized height range of the
current rather than a narrow latitudinal range which characterizes the equatorial
jet as well.

10.5.1 Radar Observations
In the equatorial electrojet most radar observations resolve into two classes: type
1 irregularities, which display a narrow Doppler spike that is offset from zero
by the acoustic frequency

kC s ; and type 2 irregularities, which display a
broad Doppler spread centered on the frequency corresponding to the line-of-
sight electron drift velocity. Both of these echo types have their counterparts in
the auroral case. However, as summarized in a review by Fejer and Kelley (1980),
there are various Doppler signatures associated with the “radar aurora.” Since
that review was written, considerable progress has been made in theoretical and
experimental studies aimed at sorting out the origins of the various spectral
signatures.
The four Doppler spectral types are shown in Fig. 10.24. The spectrum in the
upper panel is very much like a narrow type 1 echo at the magnetic equator that
locks onto the acoustic frequency. The second spectrum is very broad with a
Doppler shift of less than C s and is not unlike the equatorial type 2 case.
In the third panel a very narrow spectrum is shown that has a Doppler velocity
that is much less than the acoustic speed and which corresponds to a Doppler
shift of about 70Hz. The signals in this mode are very strong and thus when
the spectra are normalized the narrow spike becomes a dominant feature. These
waves were first reported by Fejer et al. (1984b) and have inspired considerable
theoretical interest. The implication of the narrow spike is that a very coherent
wave is present. The 70Hz Doppler shift is about 40% above the gyrofrequency
of O + , which has led to a number of studies dealing with the excitation of oxygen
cyclotron waves in the upper E region or lower F region. We return to this topic
in the discussion section following.
We turn now to the fourth type of Doppler signature. High-latitude Doppler
spectra often display such large “type 1” Doppler shifts that it was originally
thought that in the high-latitude case, 3m waves did not reach a limiting phase
velocity at C s . The Doppler drift was therefore assumed to be equal to the
line-of-sight electron drift speed, a result in agreement with the linear theory
of two-stream irregularities. This interpretation is very much in doubt now,
and a considerable literature exists on anomalous electron heating due to elec-
trojet turbulence. This topic is taken up separately in some detail in Section
10.5.2. As we shall see, the result is that the type 4 waves might still saturate
at a phase velocity equal to C s but that the sound velocity is much higher than
expected.

ω A =