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The powerful explosions can generate not only acoustic but also internal gravity
waves (IGWs) in the atmosphere. It is well known that IGWs develop in media
whose density varies with altitude and, in particular, in the stratified media. Basi-
cally, these waves propagate horizontally along the Earth surface at the velocities
up to 400-500 m/s. At the epicentral distance of 1,200 km the period of IGW is
about 7 min while the period of the acoustic wave is approximately equal to 1-2 min
(Broche 1977 ).
The satellite observations have shown the increase of the electric noises in
the frequency range of 0.1-1 kHz 6-7 min after the surface detonation MASSA-
1 (Galperin and Hayakawa 1996 ). The enhanced noises were detected within
˙ 200 km around the magnetoconjugate tube with L 1:5. The field-aligned
electric components exhibited the greatest noise amplitude while the most spectral
intensity is related to the frequency region below 100 Hz. Taking the notice of weak
magnetic perturbations in this region, the observed effect is assumed to be the result
of electrostatic turbulence induced by Alfvén waves propagating along the magneto-
conjugate paths (Pokhotelov et al. 1994 ). The similar effect has been observed in the
vicinity of the magnetoconjugate tube during the experiment MASSA-2 (Galperin
and Hayakawa 1996 ). The region of the electric noise expanded at the velocity about
0.6 km/s up to the altitudes about 10 3 km.
Of interest in the analysis of satellite observations is the strong Alfvén pulses
(with amplitudes 117 and 50 nT) measured by AUREOL-3 with onboard magne-
tometers and electric field sensors several minutes after HE detonations MASSA-1
and MASSA-3. The above estimates have shown that the acoustic channel of the
explosion energy transfer to the ionosphere cannot be so effective in order to excite
the pulses with so high amplitudes. It has been speculated that this effect can
be attributed to the electric discharge generated at the SW front (Galperin and
Hayakawa 1996 , 1998 ; Surkov and Galperin 2000 ). The thermal ionization and
changes in constants of chemical and ionization equilibrium can lead to an increase
of the conductivity at the SW. In this notation, the SW surface with the enhanced
conductivity and the bottom of the ionosphere form a peculiar kind of capacitor
which can be charged by chance. For example, as the aerial SW propagates through
the thundercloud or dust cloud or the wave flank crosses them, then a portion of the
charge can flow from the cloud to the wave surface. Assuming for the moment that
the total charge captured from the cloud is about 20 C and considering the SW as a
hemisphere with radius of 60 km, the average surface charge density has to be about
0:9 nC=m 2 which corresponds to the electric field 10 2 V=m. This value is close to
the air breakdown threshold, 130-250 V/m, at the altitudes 60-70 km. So one might
expect the generation of the electric discharges between the SW and the ionosphere
such as BJs or so on. It was hypothesized that this kind of discharge can be initiated
by a meteor-burst channel of ionization. Certainly, this is only the maximal estimate
of the effect because the charge decreases continuously due to the atmospheric
conductivity. In addition, the favorable circumstances such as appropriate meteor
path are desirable to explain this exotic phenomenon.
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