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~
/
H
2
and
F
(m/s
2
)
0.00
0.03
0.06
0.09
500
~
F
v
ni
f
H
2
400
300
200
120
10
25
10
24
10
23
10
22
v
ni
and
f
(s
21
)
Figure 3.7
Graphs of neutral acceleration time constants as well as viscous and pressure
accelerations for midday at sunspot minimum. The upper scale applies to the accelera-
tion due to the pressure gradient,
F
(
−·−·−·
)
, and the normalized kinematic viscosity
H
2
parameter,
. The lower scale refers to the ion drag parameter,
v
ni
(—), and
the Coriolis parameter for a latitude of 45
◦
,
η
˜
u
/ρ
(
−−−
)
f
(----). [After Rishbeth (1972). Reproduced
with permission of Pergamon Press.]
where
u
is the zonal component of the wind. He found an analytic solution to this
equation by generating reasonable models for the functional variations of
p
and
ρ
with
x
and
z
. Even with a low estimate of the pressure term, however, zonal
velocities of about 300m/s were found in this analysis. In later work, Lindzen
(1967) used a numerical solution with a more realistic pressure variation and
found winds as high as 550m/s! We leave this dilemma for the moment to deter-
mine the electrodynamic effect of the observed and predicted postsunset zonally
eastward thermospheric wind. Later we will show that including frictional drag
on the neutrals by the ionospheric plasma explains the lower wind values that
are actually observed.
In the remainder of this chapter and in most of the next, we will slightly
redefine our coordinate system to preserve the conventional notation that the
a
z
axis is upward. At the equator, we thus take
B
a
y
, which is horizon-
tal and northward, and take
a
x
toward the east. The conductivity tensor is
then
=|
B
|
⎛
⎞
σ
P
0
σ
H
⎝
⎠
σ
=
0
σ
0
0
(3.6)
−
σ
H
0
σ
P
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