Electromagnetic Effects Resulted from Explosions - Ultra and Extremely Low Frequency Electromagnetic Fields - page 426

the magnitude of horizontal component of magnetic perturbations decreased from
40 pT at the distance of 5 km to 10 pT at the distance of 11 km from the detonation
point (Sweeney 1989 ). At the same time the vertical component of the magnetic
perturbations exceeded several hundreds pT.
The natural ULF electromagnetic background due to the atmospherics, the
ionospheric and magnetospheric micropulsations and others restricts the possibility
for detection of the EMP. As the distance is of the order of or much greater
than 10 km, it appears that the EMP becomes undetectable because of small value
of the signal-to-noise ratio. Moreover, the instability of the signal polarization
produces the additional difficulty in the utilization of the cross-correlation technique
to separate the signals from the background noise (Sweeney 1989 , 1995 , 1996 ).
However the duration of initial part of the EMP can be utilized in order to estimate
the energy of nuclear underground explosions (Gorbachev et al. 1999a ,b).

11.1.2 Physical Mechanisms of EMP Caused by Atmospheric

and Space Nuclear Explosions

At first let us consider possible physical mechanisms of the EMP under the
atmospheric explosion. This effect can be due to the generation of radial cur-
rents of Compton recoil electrons originated from the short-term interaction (
0:25s) between the gamma-quantums of nuclear detonation and environment
(Kompaneets 1958 ). It is known that approximately 0:03 % of the whole energy
of the explosion is transformed into gamma-quantum radiation (Karzas and Latter
1962a , b ). The average gamma-quantum energy is 1 MeV, and about 7:5 10 21
gamma-quantums are generated per 1 kt of the TNT equivalent of the explosion. The
gamma-quantums interact with the matter of nuclear device and with the molecules
of air that causes the electron fluxes due to the Compton effect. On average the
vectors of the electron velocities coincide with the directions of gamma-quantums
motion. The radial electric current is also attributed to the photoelectrons resulted
from X-rays emitted by the heated matter of the nuclear device.
Every Compton's electron ionizes the medium that leads to the generation of a
great number of ion pairs. For example, the electron with kinetic energy of 2 MeV
gives rise to approximately 3 10 4 pairs of ions in the air. In the air the free electrons
are captured by molecules of O 2 having great chemical affinity with electrons. Under
normal conditions the characteristic time of the electron attachment to molecules
O 2 is about 0:01s. As a result, the reverse ionic current is developed thereby
producing the relaxation of the dipole moment and radiation of electromagnetic
waves in the frequency range around 10 kHz (Troitskaya 1960 ; Latter et al. 1961 b;
Gilinsky 1965 ; Gilinsky and Peebls 1968 ; Medvedev et al. 1980 ).