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waves in the electrojet (e.g., see Fig. 10.26). For reference, the sound speed for a
mean ion mass of 31 amu,
T
e
=
500K is 900m/s.
Simultaneous observations of the plasma drift velocity and the wave phase
velocity were made at EISCAT in conjunction with the STARE coherent scatter
radar (Nielsen and Schlegel, 1985). The data are compared in Fig. 10.30 for
(a) the linear “fluid” model and (b) the “acoustic” model. The acoustic model
is clearly in better agreement with the data. The implication is that the waves
travel at the speed of
C
s
cos
1200K, and
T
i
=
, which contradicted the conventional wisdom for
equatorial observations that the waves travel at the speed of
C
s
independent of
θ
θ
. But as discussed in Chapter 4, there is a way to reconcile the results at both
latitudes. Nielsen and Schlegel (1985) also found that the acoustic speed is an
increasing function of plasma drift and thus the applied electric field. This is also
strong evidence for heating.
The
C
s
cos
condition was further supported by simultaneous rocket electric
field and 30MHz radar observatories in Alaska (Bahcivan et al., 2005). The
radar used a sophisticated interferometer that allowed cross field of view as well
as radial velocities to be determined. They used an empirical relationship between
T
e
and
E
developed by Nielsen and Schlegel (1985) to determine
C
s
. This work
also synthesized our understanding of the relationship between type 1, 2, and
3 waves. They found that at high latitudes the narrow type 1 echoes are only
seen when the radar is looking nearly parallel or antiparallel to the electron drift
velocity. The Doppler shift is then
C
s
. When there is a reasonable angle to the
drift velocity, the spectrum widens, but the mean Doppler shift traces
C
s
cos
θ
.
It seems that as far as strong radar echoes are concerned, at high latitudes type
1 echoes are driven by the two-stream instability. Type 4 echoes are simply due
to nonisothermal ion-acoustic waves at elevated electron temperatures.
θ
Stare-Eiscat
Fluid approx.
Ion-Acou. approx.
1500
500
500
1500
500
1500
Eiscat (ms
-1
)
(a)
(b)
Figure 10.30
Magnitude and direction of the electron drift velocity estimated from
the STARE Doppler velocity measurements, applying (a) the cosine relationship and
(b) the ion-acoustic approach, as a function of the electron drift velocity determined by
the incoherent scatter facility EISCAT. [After Nielsen and Schlegel (1985). Reproduced
with permission of the American Geophysical Union.]
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