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V
P
I
V
P
~
C
s
(Ion acoustic speed)
Width ,
V
P
II
V
P
,
C
s
Width .
V
P
V
P
III
V
P
,
C
s
Width ,
V
P
V
P
IV
V
P
~
C
s
(Enhanced
T
e
)
Width
,
V
P
V
P
-1000
Doppler velocity (m /s)
0
Figure 10.24
The four types of radar spectra that are observed in the auroral electrojet.
(Figure courtesy of J. Providakes.)
An example showing the complexity of the radar aurora is given in Fig. 10.25.
Consecutive spectra separated by about 6 s are shown from four different ranges.
Many examples of broad type 2 Doppler shifts were obtained during this time
period, but in addition very strong sporadic echoes were seen which have very
large Doppler shifts. The spectra look like type 1 events but have shifts much
larger than expected for model values of the temperature and thus the acoustic
speed at E region altitudes. These are type 4 echoes.
Since the coherent scatter radar data are complicated to interpret, it is impor-
tant to determine the zero-order plasma properties and the
k
spectrum of the
waves independently in order to distinguish and understand the various possible
plasma instability mechanisms. As in the equatorial case, rocket measurements
have been used to add to our knowledge of auroral electrojet instabilities. These
data are discussed next.
10.5.2 Rocket Observations of Auroral Electrojet Instabilities
The plot in Fig. 10.26 is a composite from six different rocket wave electric
field measurements in very different background dc electric field conditions
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