Geoscience Reference
In-Depth Information
The collision frequencies
v
jk
play a crucial role in a partially ionized plasma.
Texts by Rishbeth and Garriott (1969), Banks and Kockarts (1973), and Schunk
and Nagy (2000) discuss the physics in some detail. Here it suffices to plot and
discuss representative collision profiles such as those given in Fig. 2.3, which are
valid for high sunspot conditions (e.g., see Johnson, 1961). The numerical values
of various ionospheric and atmospheric parameters used in making this plot are
given in Appendix B. A useful approximate formula for the ion-neutral collision
frequency is
10
−
9
A
−
1
/
2
v
in
=
.
×
(
+
n
i
)
2
6
n
n
(2.29a)
where
A
denotes the mean neutral molecular mass in atomic mass units and the
electron neutral plus the electron ion collision frequency is
10
−
10
n
n
T
1
/
2
v
e
≡
v
en
+
v
ei
=
5
.
4
×
e
34
18 ln
T
e
/
n
e
n
e
T
−
3
/
2
+
+
4
.
(2.29b)
e
where
T
e
is measured in Kelvin and all the densities are expressed per cubic
centimeter. The ion-electron collision frequency is not included because it is
negligible over the altitude range of interest. At night the electron-neutral col-
lision frequency equals the electron-ion collision frequency near about 280 km.
As we shall see, the equatorial F-region plasma is often driven to very high
altitudes in the evening hours, and in such a case the transition height can
be much higher. During the daytime, transition between
v
en
and
v
ei
occurs at
about 200 km. For the moment we consider the case that electron-neutral colli-
sions are more common than electron-ion collisions, which simplifies the algebra
considerably.
800
Ions
Electrons (noon)
600
Electrons (post midnight)
400
200
10
2
10
3
10
4
10
5
0.01
0.1
1
10
Collision frequencies (s
21
)
Figure 2.3
Typical electron neutral plus electron ion collision frequency along with the
ion-neutral collision frequency at a high sunspot number.
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