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a period of strong scattering. The mean Doppler shift of the peak in the spectrum
was constant when the zenith angle was between 45
◦
and 70
◦
, both to the east
and to the west (Bowles et al., 1960). This result remained mysterious until
combined rocket and radar data were applied (Kelley et al., 2008). As discussed
in Section 4.8.2, they showed that in these scattering volumes there is some region
where the total vector drift velocity (zero order plus perturbation drift) in the
direction of the radar exceeds the two-stream threshold. Thus, the hypothesis
that
V
ph
=
is the angle between the
total
drift velocity and the
radar beam, explains the observation
V
ph
=
C
s
cos
θ
, where
θ
C
s
at all angles.
The data in Fig. 4.21 were normalized to a peak value, since the echo power
varies over a large range (up to about 40 dB) throughout the day. Thus, the
area below the spectral curves is not proportional to the relative signal strength.
In particular, the scattered power from the oblique type 1 echoes is appreciably
larger than the power from the overhead echoes coming from vertically traveling
“non-two-stream” irregularities. During the daytime the Doppler shift of the
dominant peak is positive when the antenna is directed toward the east and
negative when the antenna is looking to the west. The reverse is true at night.
Therefore the phase velocity of the type 1 irregularities has a component in the
direction of the electron flow, since during the day (night) the electron flow is
westward (eastward). The fact that the Doppler shift is independent of zenith
angle shows that the irregularities are not just advecting with the zero-order
electron flow. Often, the vertical antenna detects echoes with both positive and
negative spectral peaks corresponding to the acoustic speed; that is, propagating
perpendicular to the flow both upward and downward.
The average phase velocity of the type 2 irregularities is smaller than the ion
acoustic velocity and is approximately proportional to the cosine of the radar
elevation angle (Balsley, 1969a). The spectral width is much broader than that
of the type 1 echoes and is often greater than the mean Doppler shift. Figure 4.22
shows the variation of the type 2 spectra and average phase velocity with zenith
angle. The solid curves in the bottom right panel indicate the expected variation
of the average velocity
V
obs
as a function of zenith angle
θ
z
on the basis of the
relation
V
obs
=
θ
z
for three values of the drift velocity
V
D
. The excellent
agreement between the experimental and theoretical variations shows that the
type 2 phase velocity is proportional to the projection of the electrojet drift
velocity on the radar line-of-sight.
Some additional characteristics of the type 1 and type 2 irregularities deduced
from radar data are as follows:
V
D
sin
1.
Threshold
: Type 2 irregularities are observed even for very small values of the west-
ward electron drift velocity during daytime. Ionosonde and radar measurements have
shown that the irregularities are absent during the occasional daytime periods of
westward current flow (counterelectrojet conditions). At nighttime the type 2 irreg-
ularities are almost always observed. An exception occurs when the electric field
reverses sign. An example of this effect can be seen in Fig. 4.1. When the high-altitude
scattering region reached “apogee” at around 2120 LT, the E-region irregularities
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