Geoscience Reference
In-Depth Information
Fig. 15.6.
Results of magnetic pulse measurements
variation are a sharp surge or pulse in the magnetic field lasting
t<
0
.
08 s, hav-
ing components in the satellite's coordinate system:
b
xs
=
−
25,
b
ys
=
−
114,
b
zs
=0
= 117 nT, with vector
b
directed almost exactly west.
This pulse is accompanied by a pulse of the electric field having component:
∆E
xs
=97
.
7,
∆E
ys
=26
.
4,
∆E
zs
=0
±
5, and
|
b
|
= 101
.
2 mV/m, with
vector
∆
E
directed almost exactly north. It is evident that within the lim-
its of measurements accuracy,
∆
B
±
5and
|
∆
E
|
⊥
∆
E
. Hence, the phase velocity of the
given electromagnetic wave is 860
±
140 km/s. The Poynting vector is directed
upward along the magnetic field.
According to measurements, at this point the local frequency of the upper
hybrid resonance
f
BHy
=2
.
49 MHz and the magnetic field is
|
B
0
|
=0
.
342 G,
from which
N
e
=6
.
56
10
4
cm
−
3
. From data of the mass spectrometer thermal
ions, one can assume that most of the ions at this point (at an altitude
of about 800 km) are 0
+
ions. Hence, the magnitude of the Alfven velocity
c
A
= 1000 km/s (for 50% 0
+
and 50% H
+
) or 745 km/s (for 100% 0
+
. Within
the limits of accuracy, this concurs with the measured value
v
of an elec-
tromagnetic wave. Therefore, these data are direct evidence in favor of the
movement of the electromagnetic pulse in the magnetosphere in the form of
an Alfven wave.
An important point is that the Alfven pulse has been recorded within 300 s
after the blast and at a distance of
×
700 km from the explosion's field tube.
An acoustic wave could not reach the
E
-layer for this time and traveled only to
100 km altitude. The early arrival the Alfven pulse can be indirect indication
that the strong acoustic wave changes the low ionosphere. The results of the
observations of electromagnetic noise in the wide frequency range 10-1000 Hz
made by the same satellite during a series of subsequent active experiments
≈
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