The Ionosphere and Atmosphere - Ultra and Extremely Low Frequency Electromagnetic Fields - page 51

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.