Electromagnetic Effects Resulted from Explosions - Ultra and Extremely Low Frequency Electromagnetic Fields - page 460

Fig. 11.17 Numerical
simulation of magnetic
perturbations in the polar
magnetosphere. The extrinsic
current at the lower boundary
of the ionosphere is modeled
by the step function of
time. The compo-
nents B z , B r ,andB ' versus

0,50

B , nT

1
2
3

0,25

t , s

l/=V A are
shown with lines 1, 2, and 3,
respectively (Surkov 1996 )

D t . z

0,00

0,0 0,1 0,2

-0,25

-0,50

-0,75

of time in Eqs. ( 11.52 ) and ( 11.53 ) results in smoothing these oscillations though
a few oscillations remain in the initial part of the signal. As we noted above, such
a structure is typical of the nonstationary diffusion process in gyrotropic media. In
the model of the magnetosphere the wave profile is steady in the reference frame
moving at the Alfvén wave velocity. It is not surprising, then, that this structure
of perturbations is saved in the Alfvén and FMS waves and hence it is transferred
upward at the Alfvén velocity.
It should be noted that the solution at z >lis not entirely correct because the
approximation of collisionless plasma is inapplicable to the typical wave frequen-
cies D 0:1-1 Hz at the altitudes of F layer. The decrease of the wave amplitude
due to energy dissipation in the F layer can be roughly estimated by means of the

attenuation factor exp z 2 0 P =4 . Substituting P D 10 5 S=m, z D 200 km
as a mean altitude of the F layer and the above frequencies into this factor gives
a decrease of the amplitude at 1.1-3.5 times. Certainly, a strong fall off of the
spectrum should be expected in the frequency range above 1 Hz.
The effect of opposite polarity can arise approximately 1-2 min the after acoustic
wave arrival at the bottom of the ionosphere when the wave will cross the upper
boundary of the E layer. In such a case the area of uncompensated extrinsic current
appears at this boundary. This current flows oppositely to direction of the extrinsic
current at lower surface of the ionosphere that results in the generation of GMP of
the opposite polarity. The amplitude of these GMPs can significantly exceed the
original perturbations since the velocity amplitude and the length of the acoustic
wave increase with altitude.