Geoscience Reference
In-Depth Information
9.2.3 Velocity-Dependent Recombination
In addition to frictional heating, the relative energy with which the ion and
neutral particles collide can affect the chemical reaction rates of the ions. This
is particularly true in the ionospheric F region, where the rates for the charge
exchange reactions are extremely sensitive to the relative energy of the reacting
particles. If these rates are enhanced, then the plasma density will decrease since
NO
+
and O
2
recombine quite rapidly because of dissociative recombination.
The plasma ion composition will also change if O
+
is converted to molecular
species via the reactions
O
+
+
NO
+
+
N
2
→
N
(
with rate
k
1
)
(9.13a)
O
+
+
O
2
O
2
→
+
O
(
with rate
k
2
)
(9.13b)
Laboratory studies of these reactions express their rates in terms of an effective
temperature. The experiments are performed by passing one gas through the
other in a drift tube in a manner quite similar to the drift of ions through the
neutrals in the ionosphere. The most important reaction rate in the F region is
the ion-atom exchange between O
+
and N
2
. The rate coefficient of this reaction
can be written in the form
1
10
−
12
10
−
13
k
1
=
.
53
×
−
5
.
92
×
(
T
eff
/
300
)
2
cm
3
10
−
14
+
8
.
6
×
(
T
eff
/
300
)
/
s
,
T
eff
=
0
.
636
T
i
+
0
.
364
T
n
.
To determine the effective temperature for the ionosphere, we must take into
account the relative velocity of the neutral gas with respect to the ions. If we
then express the relative ion-neutral velocity in terms of the electric field in the
neutral frame such that
E
⊥
/
|
V
i
⊥
−
U
⊥
|=
B
,
then the effective temperature is given by
33
E
2
⊥
T
eff
=
T
i
+
0
.
(9.14)
where
E
⊥
is expressed in millivolts per meter (Schunk et al., 1975). We see,
therefore, that the rate coefficient,
k
1
, increases strongly with the ion-neutral
velocity difference. This highly sensitive variation in the charge exchange rate
can be reflected quite dramatically, both in the absolute ion concentration and in
the relative ion composition at high latitudes. Figure 9.12 shows an example of
this effect in the high-latitude F region. Near 250 km, O
+
is the dominant ion by
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