Geoscience Reference
In-Depth Information
At higher altitudes one might expect
O
+
+
e
−
→
O
+
photon
to dominate. The former processes are called dissociative recombination, since
the molecule ion breaks apart, while the latter is termed radiative recombination,
since emission of a photon is required to conserve energy and momentum. The
former two processes have a reaction rate much higher than the latter, which
results in a much shorter lifetime for molecular ions than for atomic ions. Since
the molecular ions are much shorter lived, when their production is curtailed
at night, a recombination quickly reduces the plasma concentration. The O
+
plasma at higher altitudes, however, often survives the night at concentrations
between 10
4
and 10
5
cm
−
3
. The O
+
ions actually are lost through two-step
processes (see Chapter 5) below the F peak. First, one of the following charge
exchange reactions occurs:
O
+
+
O
2
O
2
→
O
+
O
+
+
NO
+
+
NO
→
O
and then the dissociative reactions given above complete the recombination pro-
cess. In addition, at night, plasma can flow back into the F region from the high
altitude region called plasmasphere.
The long-lived property of atomic ions also explains the numerous sharp layers
of enhanced plasma density seen at low altitudes in Fig. 1.3. These contain heavy
atomic ions such as Fe
+
and Mg
+
that are deposited by meteors in this height
range. Just why they are gathered into sharp layers is discussed in some detail in
Chapter 6, but once gathered, they last for a long time.
Photoionization by solar radiation is not the only source of plasma in the
ionosphere. Ionization by energetic particle impact on the neutral gas is particu-
larly important at high latitudes. Visible light is also emitted when particles
strike the atmosphere. These light emissions create the visible aurora. A view
of the aurora looking down on the earth is offered in Fig. 1.4. The picture
was taken on the Defense Meteorology Satellite Program (DMSP) satellite from
about 800 km altitude and shows some of the complex structure and intricate
detail that the auroral emission patterns can have. This complexity is mirrored
in the ionization that also results from particle impact. The photograph shows
how much this ionization can vary in space. Variability in time is illustrated
in Fig. 1.5, where consecutive plasma density profiles taken 20 s apart are dis-
played. The data were taken at the Chatanika Radar Observatory in Alaska
(
06
◦
N
39
◦
W
. Within the 40 s period, the E-region plasma density peak
varied by almost one order of magnitude. The particle energies determine their
penetration depth, and the distribution in their energies determines the resulting
65
.
,
147
.
)
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