Geoscience Reference
In-Depth Information
As one might expect, the front structure of the Alfvén and FMS waves in the
magnetosphere is correlated with the processes in the ionospheric E layer. The front
duration is of the order of t
d
or t
0
while the typical front length is V
A
t
d
or V
A
t
0
,
that is about 15-20 km at the altitudes of a few hundreds km. The total duration of
perturbations is determined by the time of acoustic wave passage through E layer
and thus it can be far beyond the duration of the wave front (Surkov
1992a
,
b
). The
area of perturbations is extended along the geomagnetic field lines. The lateral size
of this area is about 100 km which appear to be much less than the field-aligned
scale. The sharp front and gradual drop of the signals shown in Figs.
11.16
and
11.17
are in qualitative agreement with the onboard observations though the predicted
amplitudes of the signals (
0.1-10 nT) are much smaller than that measured by
the satellite AUREOL-3 (
100 nT). It should be noted that on a basis of Maxwell
equations one can find the following simple estimate of the GMP amplitude (e.g.,
see Danilov and Dovzhenko
1987
)
B
RmB
0
p=p;
(11.63)
where p is excess pressure and Rm
D
0
P
V
a
is magnetic Reynolds number.
Taking the numerical values of the wavelength
D
1 km and p=p
D
0:1 we
obtain the value B
1 nT which is in agreement with the satellite observations
(Pokhotelov et al.
1995
). Certainly these rough estimates essentially depend on the
parameters of the ionosphere, diurnal variations, and so on.
The splitting of the perturbations into two types in the lower ionosphere has
occasionally been observed under nuclear explosions (Daniels et al.
1960
). In the
upper ionosphere these vertically traveling perturbations had different velocities. It
appears that the slow perturbation corresponded to the conventional sound wave,
whereas the velocity of the fast perturbation was increased roughly 2 times. It was
hypothesized by Wickersham (
1970
) that this effect can be due to the excitation of
ion-sound mode at the altitude range of 160-200 km. The velocities of the sound C
a
and ion-sound V
i
waves are given by
k
B
T
m
1=2
k
B
f
Z
e
T
e
C
i
T
i
g
m
i
1=2
V
a
D
; V
i
D
;
(11.64)
where Z denotes the ion charge; m and m
i
are the average masses of neutrals and
ions;
e
,
i
and stand for adiabatic exponents of the electrons, ions, and neutral
gases; and T
e
, T
i
, and T are their temperatures, respectively. In the theory the ion
sound can be generated in a strongly anisothermic plasma .T
e
T
i
/ when the
frequency of inelastic collisions between the charged and neutral particles exceeds
the frequency of their elastic collisions. However in the media of neutral particles
the ion sound mode undergoes a strong attenuation because of the collisions
between ions and neutrals. In order to overcome this difficulty, one should assume
a possibility for some wave-induced exothermic reactions which results in the
enhancement of the particle temperature (Wickersham
1970
).
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