Geoscience Reference
In-Depth Information
Fig. 2.4
Ratio of gyro to
collision frequencies versus
altitude. The indices i and e
stand for ions and electrons,
respectively. Taken from
Kelley (
1989
)
At the altitudes below 150 km the ion collision frequency,
i
, is no more than
30-40 s
1
. Notice that
e
is much greater than
i
for any altitude of interest. This
implies that in the ULF frequency range .f
D
!=2 <3Hz/ the frequency ! is
negligible compared with both
i
and
e
, so that the components of the plasma
conductivity tensor given by Eqs. (
2.7
)-(
2.9
) can be considered as practically
constant values, that is
k
D
e
2
n
1
;
1
m
i
i
m
e
e
C
(2.13)
P
D
e
2
n
"
#
;
e
m
e
e
C
!
H
C
i
m
i
i
C
2
H
(2.14)
H
D
e
2
n
"
#
:
!
H
m
e
e
C
!
H
H
m
i
i
C
2
H
(2.15)
The typical profiles of the parallel, Pedersen, and Hall conductivities for mid-
latitude ionosphere and for !
D
0 are plotted in Fig.
2.5
. The parallel plasma
conductivity is much more enhanced than
P
and
H
in the altitude range above
90 km both in the nighttime and daytime conditions. This conductivity is so high
that the ratio
k
=
P
is greater than 10
4
above 130 km. This effect follows from
the high electron mobility along the magnetic field lines. In this case the first term
prevails in Eq. (
2.7
), so that the parallel plasma conductivity is equal to e
2
n=.m
e
e
/
as a good approximation.
To a great extent the Pedersen and Hall conductivities define the properties of
the conducting gyrotropic E layer of the ionosphere. In the daytime the Pedersen
conductivity reaches a peak value of about .1:5-3/
10
4
S=m (mho/m) at the
altitudes 130-135 km, while in the nighttime the peak value decreases up to
.0:4-3/
10
5
S=m. As is seen from Fig.
2.5
the peak of the Pedersen conductivity
is smaller than that of the Hall conductivity.
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