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Poker Flat, Alaska
130
125
T
e
5 500°
120
115
110
105
100
95
T
e
5 250°
90
85
80
0
50
100
1
)
1/2
(
0
/
1
/
2
in the auroral zone for constant tempera-
Figure 10.34
Curves representing (
σ
0
/σ
P
)
ture profiles corresponding to
T
e
=
500K and
T
e
=
250K.
where
σ
0
and
σ
P
are the parallel and Pedersen conductivities. In Fig. 10.34
2
is plotted as a function of altitude using a model neutral atmo-
sphere and the geomagnetic field at Poker Flat, Alaska. The computation of
this ratio was very sensitive to the particular expression used for the collision
frequencies and may vary quite a bit at these low altitudes. In addition, since
these collision frequency values depend critically on the electron temperature,
which also varies rapidly here, we have plotted two curves corresponding to
T
e
1
/
(σ
0
σ
P
)
250K. Notice that the conductivity ratio changes con-
siderably in the region we are considering. For the sake of discussion, consider
the value of this ratio at 96 km to be approximately 35. Thus, using (10.22),
a 300m wave would map through a distance of 10.5 km as it decays along
the field lines. The calculated distance is somewhat larger than that given by
the observations, but the basic idea seems correct. In any event, this calcula-
tion is consistent with the hypothesis that the source region of the waves was
very narrow in height. In turn, this supports the claim that the waves were
produced by a locally unstable electron density gradient and that the observed
dominant wavelength may have corresponded to the peak in the linear growth
rate for the gradient drift instability. The perturbation electric field mapped
to a region where the waves were stable. In Chapter 5 we showed an exam-
ple in which waves were detected on only one side of a midlatitude layer for
which this condition held (Kelley and Gelinas, 2000). Seyler et al. (2004) devel-
oped a nonlocal theory for the midlatitude case, which can deal with such
conditions.
=
500K and
T
e
=
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