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hydrogen atom density is not shown in this early plot but exceeds helium above
about 600 km. The hydrogen ions shown in Fig. 1.2 are produced by charge
exchange with O
+
,
O
+
+
H
+
+
O.
Turning to the nighttime profile in Fig. 1.1, the plasma density near the F
peak is reduced in magnitude but not nearly so drastically as is the density
at the lower altitudes. With reference to Fig. 1.2, this difference mirrors the
change in composition. That is, in lower altitude regions where molecular ions
dominate, the density is sharply curtailed at night. The O
+
plasma density, on the
other hand, is sustained through the night. This trend is dramatically illustrated
in Fig. 1.3 by the series of plasma frequency
H
→
(
)
profiles that were obtained by
the incoherent scatter radar facility in Arecibo, Puerto Rico
f
p
5
◦
N
8
◦
W
(
18
.
,
66
.
)
.
cm
−
3
The electron plasma freq
u
ency (Hz) is related to the plasma density
(
)
by
8980
√
n
. The sunrise and sunset effects are very strong at the
the equation
f
p
=
lower altitudes.
This substantial difference in ion behavior is due to the fact that molecular
ions have a much higher recombination rate with electrons than do atomic ions.
The two fast reactions that occur in E-region recombination are of the type
NO
+
+
e
−
→
N
+
O
and
O
2
e
−
→
+
O
+
O
September 16-17, 1999
500
500
450
450
400
400
10
8
350
350
F
6
4
300
300
250
250
2
0
200
200
150
150
E
100
100
D
Noon
6:00 pm
Midnight
6:00 am
Noon
Local time-Atlantic standard
Figure 1.3
Plasma frequency contours during a typical night over Arecibo, Puerto Rico.
(Figure courtesy of S. Collins.)
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