Electromagnetic Effects Resulted from Explosions - Ultra and Extremely Low Frequency Electromagnetic Fields - page 451

explosions (Daniels et al. 1960 ;Lawrieeta . 1961 ; Stoffregen 1962 , 1972 ;
Gassmann 1963 ; Kotadia 1967 ; Breitling et al. 1967 ; Baker and Davies 1968 ;
Baker and Cotten 1971 ; Kanellakos and Nelson 1972 ; Lomax and Nielson 1972 ),
surface (Barry et al. 1966 ; Najita et al. 1975 ) and underground nuclear explosions
(Blanc 1984 , 1985 ; Pokhotelov et al. 1995 ). SW-induced oscillations in the lower
thermosphere followed by the ionospheric perturbations have been observed after
a 5 kt chemical explosion (Jacobson et al. 1988 ). Among the other sources of the
strong acoustic waves in the atmosphere and ionosphere are volcano eruptions,
spacing flights of supersonic airplanes (Marcos 1966 ) and rocket launch (Rao 1972 ;
Karlov et al. 1980 ).
In standard geophysical practice the techniques of vertical, oblique, and Doppler
sounding are used in order to examine the ionospheric response to natural and
anthropogenic forcing on the Earth's ionosphere. In addition, the technique of
continuous VLF electromagnetic transmission probing of the Earth-ionosphere
waveguide is in routine use (e.g., see Surkov 2000 ; Molchanov and Hayakawa
2008 ). The experimental evidences on the impact of surface and underground
explosions on the ionospheric F , E , and D layers have been demonstrated on
the basis of these techniques (e.g., see Barry et al. 1966 ; Broche 1977 ; Blanc
1984 , 1985 ). In the epicentral region the effect of explosion on the ionosphere
is predominantly due to the upward-propagating atmospheric acoustic wave. This
wave is generated when the underground SW reflects from the ground surface.
The amplitude of mass velocity in the aerial wave increases with altitude due
to the exponential fall off of the atmospheric density. Even a weak upward
propagating wave can be converted into an SW because of nonlinear properties of
the atmosphere. This nonlinear transformation of the wave shape and wave-front

breaking occurs at the altitude H 2P 0 = g g 20 km where P 0 and g
are the pressure and air density at the sea level (e.g., see Whitham 1974 ). As the
altitude is larger than H , the wave profile becomes universal. The wave front has
a triangular shape. It is imperative that the compression phase is followed by the
rarefaction phase in such a way that the wave profile resembles a letter N, as shown
in Fig. 11.13 . It follows from the principle of conservation of momentum that the
positive and negative phases of the N-wave are equal in square (e.g., Landau and
Lifshitz 1959 ). In the bottom of the ionosphere; that is, in the altitude range of 90-
100 km, the amplitude of mass velocity and length of the N-wave can reach several
tens m/s and several km, respectively. The exponential increase of the SW amplitude
ceases at the altitudes over 100 km. The cause of this effect is the enhancement of
the gas viscosity at these altitudes due to the increase of mean free path of molecules
(Enstrom et al. 1972 ). Then the dissipative processes result in both the decrease in
the pressure and mass velocity gradients at the SW front and the increase in the
wavelength which can reach a few tens km in F -region of the ionosphere.
The numerical stimulations have shown that the effect of SW impact on the
lower ionosphere is maximal in the circular area with radius of the order of 100 km
(Orlov and Uralov 1984 ), and the influence of shock upon the ionosphere diminishes
essentially outside this area. This effect is due to the refraction of sound in the