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where V t e is the electron thermal velocity, the plasma is unstable to generation
of Langmuir waves. This is the instability threshold for T e =
T i , but for T e
T i ,
the plasma is unstable for
k
(
T e +
T i )
V d
C s
>
=
(9.16)
M
where C s is the ion acoustic speed. This threshold is much lower than (9.15). In
this case, sound waves are generated and could lead to an anomalous resistivity.
However, it turns out that many other exotic plasma processes occur around
and even below this threshold and it seems very likely that other nonlinear phe-
nomena dominate the physics before anomalous resistivity comes into play. We
cannot hope to do this nonlinear physics justice but end this section with data
from the FAST satellite, illustrating the complexity of the parallel electric field
acceleration zone (Frey et al., 2001). Figure 9.21 shows typical examples of
electron acceleration by both upward (left-hand panel) and downward parallel
The Symmetric Aurora
Aurora
Inverse Aurora
Downgoing electrons
Upgoing electrons
10 4
10 4
10 3
10 3
10 2
10 2
10 1
10 1
E
E
E
E
Converging
shocks
Diverging
shocks
J
J
Upgoing current
Downward accelerated
electrons
Downgoing current
Upward accelerated
electrons
Figure 9.21 Energetic electrons and ions for opposite types of accelerating structures
that have scales of about 3 km. (Figure courtesy of C. Carlson.)
 
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