Geoscience Reference
In-Depth Information
1000
900
10
6
P
3
0
800
700
Day
600
Night
500
400
H
3 10
6
300
200
100
10
22
10
10
2
10
3
0.1
1
Conductivity (mho /m)
Figure 2.6
Typical conductivity values for the midlatitude daytime ionosphere. Notice
the change of scale for
σ
P
and
σ
H
. The dashed curve is a typical nighttime profile of
σ
P
also multiplied by 10
6
.
For
κ
i
1 (above 130 km) this expression becomes even simpler,
ne
2
v
in
/
2
i
B
2
σ
P
=
M
=
nMv
in
/
(2.40b)
σ
H
falls off more rapidly with height than does
P
and is important only in a narrow height range where three conditions are met:
κ
The Hall conductivity
σ
P
is also given.
Finally, we remind the reader that the calculations have thus far been per-
formed in the neutral reference frame, where J
=
σ
·
1,
κ
i
∼
1, and
n
is large. A typical nighttime curve for
e
E
. More usually we measure
the neutral wind
U
and electric field
E
in the earth-fixed frame. However, since
E
J
for nonrelativistic transformations, we have the
important and most usual form of the current equation
=
E
+
U
×
B
and
J
=
J
=
σ
·
(
E
+
U
×
B
)
(2.41)
where all parameters are measured in the earth-fixed coordinates. Earth-fixed
measurements of electric fields, of course, can determine only the
E
in (2.41),
not the entire quantity
E
=
B
.
To summarize, we note that the ionospheric plasma is subject to electromag-
netic forces in addition to those felt by the neutral atmosphere. The dipole nature
of the magnetic field is not greatly affected by ionospheric currents. The result
is that the magnetic field creates geometric constraints on the plasma behavior,
constraints that are quite different at different magnetic latitudes. Electric fields,
E
+
U
×
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