Geoscience Reference
In-Depth Information
1000 km
Time
5000 km
Time
10,000 km
Time
Fig. 8.12.
Sketches show the waveforms of the
b
(
g
x
-component at 1000, 5000, and
10,000km from a beam with the step-function time dependency
θ
(
t
) in the source
literature on the theory of wave propagation from localized sources, there
are numerous references to such effects. For instance, Tamir and Oliner [10]
proved an example of long-wave emission from a local source located on a
plasma layer that waves can appear with the phase velocity aimed towards the
source.
At distances larger than the skin depth
d
g
, where the
b
(
g
x
(
x
) is given
by (8.64), the wave moves over the surface with Alfven velocity. At larger
distances the TEM-mode magnetic component becomes noticeable and the
main. The wave phase velocity then becomes comparable to the speed of light.
The Alfven wave incident from the magnetosphere onto the ionosphere
produces the atmospheric TEM-mode characterized by a finite vertical electric
field
E
z
at the ground.
E
z
for variations with
T
= 100 s is about 0
.
3mV/m
at magnetic amplitude of the incident wave 1 nT. Account taken of sphericity
increases
E
z
by two orders [2]. A peculiarity of the spatial distribution
E
z
caused by a TEM-mode is the independence of its amplitude of distance.
For non-monochromatic sources the temporal pattern depends signifi-
cantly on the distance from the source. Figure 8.12 sketches the temporal
behavior of the
b
x
-component at 1000
,
5000
,
10
,
000 km from a beam with
the step-function type in the source. At large distances where the TEM-mode
becomes noticeable, an inverse pulse appears which could explain the pulse
Sc
∗
[11]. The propagation velocity of this pulse is of the order of light velocity,
whereas the basic magnetic signal propagates with Alfven velocity.
Appendix
M agnetic mode. σ
g
→∞
It follows from (8.38) that
k
2
=
ik
coth
k
+
τ
K
2
k
A
−
(8.A.1)
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