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The numerical calculations have shown that this estimate is consistent in
magnitude with the IAR observations if the distance r is not far as 1,000 km from
the thunderstorm center (Surkov et al.
2005a
). This implies that this mechanism
can explain, in principle, the IAR observations at low latitudes. On the other hand,
this estimate disagrees sharply with the IAR spectrum magnitude recorded far from
the thunderstorm center. For example, substituting the parameter r
D
10
4
km into
Eqs. (
5.66
) and (
5.67
) gives the value 0:01-0:02 pT/Hz
1=2
, that is one or two order
of magnitude smaller than the power spectrum observed at middle latitudes. For
example, one may compare this value with data gathered at Karimshino station in
Kamchatka peninsula (Molchanov et al.
2004
), also see Figs.
5.6
,
5.7
,
5.8
,
5.9
.
Finally we arrive at the following conclusions. The stochastic model of lightning
activity predicts that the global thunderstorm can make a main contribution to SRS
of IAR at low latitudes whereas the nearby thunderstorms are the most appropriate
candidate to explain the IAR observations at middle latitudes.
5.3.6
IAR Excitation Due to Ionospheric Neutral Wind
In the remainder of this section we focus our attention on the neutral winds in the
bottom ionosphere as a possible origin of the IAR excitation in the mid- and high-
latitudes. In this important altitude range, the electric fields are basically generated
by neutral winds in the E-layer of the ionosphere (Kelley
1989
). As for the
acoustic energy transfer from the neutral gas flow into the energy of electromagnetic
vibrations, we note that this mechanism is similar to the acoustic autovibration in
such a system as “a police whistle” (Surkov et al.
2005b
). Indeed, let us imagine
a vertical cylindrical case/shell bounded from one end and opened from another.
It is known that an aerial flux externally tangent to the open end of the shell
results in excitation of the vertical gas vibrations inside the shell that in turn gives
rise to amplification of aerial column eigenmodes. In such a case the energy flux
coming from the external source is governed solely by the aerial column itself.
The fluctuations of the tangent aerial flux whose frequencies are close to the aerial
column eigenfrequencies in the shell can give rise to enhancement of the eigenmode
magnitudes.
Our analysis shows that the similar scenario may operate in the ionospheric
resonance cavity. In this case the neutral wind in the lower ionosphere can serve as
an energy source for excitation of the shear Alfvén and FMS waves in the IAR. First,
a part of the gas kinetic energy is transferred into the energy of the electric current
in the conductive ionospheric slab, which is then converted into the energy of the
shear and fast modes. Some of this energy is lost/dissipated due to the ionospheric
Joule heating and wave energy leakage through the upper boundary of IAR into the
magnetosphere.
Consider again an idealized slab geometry that ignores the magnetic field dip
angle but includes conductivity variations with height along the magnetic field
direction. The IAR excitation is supposed to be only due to the ionospheric wind
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