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the ionosphere simply fell and recombined. On the next night, even though the
layer fell, the density remained high, even at 260 km, where recombination is
very fast. On this night it seems that flux from the plasmasphere maintained the
high F-region density and furthermore that electric field effects were important
(Vlasov et al., 2003). We take up such effects next.
5.2.3 Combined Effects of Electric Fields and Neutral Winds
More generally, to find the equilibrium height at which the vertical velocity
vanishes, we have
g
/
ν
in
sin
2
I
=
(
E
e
/
B
)
cos
I
+
u
s
cos
I
sin
I
ν
in
where
E
e
is the eastward component in the electric field. Solving for
yields
sin
I
g
ν
in
=
(5.22)
E
e
/
B
+
u
s
cos
I
The wind and electric field can be lumped into an effective southward wind,
u
eff
)
−
1
=
(
E
e
/
B
)(
cos
I
+
u
s
. Then the height can be found from (5.21b) since
ν
in
is a unique marker for altitude. Turning this around, if we know the
height of the layer we can deduce the effective wind. Ionosondes can be used
for this application. Equation (5.22) is the Perkins equilibrium condition. We
investigate its stability or lack thereof in the next chapter. Note that (5.22) pre-
dicts the eventual (equilibrium) height of a layer, not its instantaneous value.
A more general set of equations that allows for time-dependent calculations is as
follows:
sin
2
I
V
z
(τ)
=−
(
g
/
ν
in
(τ))
+
u
s
(τ)
cos
I
sin
I
+
(
E
e
(τ)/
B
)
cos
I
(5.23a)
t
h
(
t
)
=
V
z
(τ)
d
τ
+
h
(
0
)
(5.23b)
0
These dynamic equations can account for the motion of the layer on September
16-17 but what about the high plasma density? Why doesn't recombination
destroy the layer below 300 km? As noted previously, the explanation (Vlasov
et al., 2003) seems to be that a high influx of plasmaspheric ions (5
10
8
×
cm
−
2
s
−
1
flooded the tropical ionosphere during this relatively small magnetic
storm. Most likely, this is related to the large eastward electric field measured at
Jicamarca the previous day and well into the evening (Makela et al., 2003), and
a well-established fountain effect occurred. Indeed, a large southward gradient
in plasma density was observed over Arecibo, indicating that it was located on
the poleward edge of the anomaly, which is usually over a thousand kilometers
)
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