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In-Depth Information
V
e
) in the lower auroral
ionosphere were pursued by Providakes et al. (1985) and Satyanarayana and
Chaturvedi (1985). These authors have shown that the destabilizing effect of
electron collisions is necessary for the excitation of EIC waves in the upper E
region, where ion-neutral collisions are stabilizing.
Satyanarayana and Chaturvedi (1985) showed that the maximumgrowth rates
for collisional EIC waves in the bottomside ionosphere are comparable to those
of collisionless EIC waves above the F region, where ion-neutral collisions are
negligible and ion-ion collisions become an important damping mechanism. The
critical electron drift velocity to excite EICwaves depends on electron density, on
T
e
Kinetic studies of collisional EIC waves (
ν
e
>
k
||
T
i
and on the ion mass. Figure 10.36 shows three typical ionospheric electron
density profiles and the corresponding critical drift velocities for generation of
EIC waves. The critical drift velocity has a minimum between 150 and 200 km.
This height range is consistent with VHF coherent radar observations. At 150 km
the electron thermal speed (
V
e
) is about 100 km/s, which gives a threshold parallel
drift velocity
V
D
=
/
20 km/s.
The theories discussed previously indicate that field-aligned drifts of about 30
km/s can excite collisional EIC waves at wavelengths of about 20 m in the upper
E region. For an electron density of 5
10
4
cm
−
3
, this corresponds to a field-
×
A/m
2
. As noted previously, field-aligned
aligned current density of about 100
μ
A/m
2
and higher were observed during disturbed condi-
tions in highly localized regions near the cusp on the Orsted satellite (Neubert
and Christiansen, 2003). Since the currents along the geomagnetic field are alter-
natively going up and down, the field-aligned current density averaged over
larger scales (as, for example, the spacial resolution of the EISCAT radar) are
close to that observed with radar techniques and is about 10
currents up to 1000
μ
A/m
2
. These
results provide considerable evidence that auroral field-aligned currents are
μ
600
NO
1
ion
k
II
/k
5
0.06
⊥
ρ
5
√
2
500
k
400
300
A
B
C
A
B
C
200
100
10
3
10
4
10
5
10
6
0.02
0.10
0.18
0.26
N
e
(cm
-3
)
V
c
/
V
e
Figure 10.36
Three typical electron density profiles and the corresponding plot of the
critical drift velocity for the generation of EIC.
V
e
is the electron thermal speed. (Figure
courtesy of J. Providakes.)
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