Electromagnetic Effects Resulted from Natural Disasters - Ultra and Extremely Low Frequency Electromagnetic Fields - page 396

Hayakawa et al. ( 2007b ) have suggested that the over-the-horizon VHF wave
propagation in seismo active regions can be due to the tropospheric ducting via the
wave reflection below the tropopause. In this case the VHF waves are guided in
the same way as in a metallic waveguide. The ducting in the troposphere followed
by the ray distortion builds up as a result of refraction index changes with altitude.
In the geometric optics approach the curvature radius, R, of the ray can be expressed
via the refraction index n as follows (e.g., see Landau and Lifshits 1982 ):

R D N r n

1

n ;

(10.46)

where N denotes the unit vector of a principal normal to the curve/ray. The refraction
index of air depends on meteorological conditions including the atmospheric pres-
sure, temperature and the partial pressure of water vapor, which is contained in the
air. Under a standard meteorological condition the refractive index of air decreases
with height z , and the typical value of its derivative is dn=d z D 4 10 8 m 1 .The
ducting takes place under the requirement that the refractive index falls off more

rapidly with height so that

dn
d z <

1
R e ;

(10.47)

where R e is the Earth's radius. This implies that the rays will follow the curvature of
the Earth. The major cause of radio ducting is humidity and temperature inversion,
which occasionally occurs in the coastal region during anticyclone conditions. The
tropospheric ducting requires low-angle entry and exit of the ray into the duct that is
consistent with the elevation angle (smaller than 10 ı ) observed by Fukumoto et al.
( 2002 ).
Hayakawa et al. (2007) have pointed a possible connection between the abnormal
variations of refractive index and the increase in the ground surface temperature in
seismo-active regions. As it follows from simple estimates, the weak heating of the
ground surface (T D 1-3 K) for several days or weeks can produce a large amount
of water vapor. If the favorable meteorological conditions occur in the near-surface
layer over seismo-active regions, it can produce the air humidity and temperature
inversion followed by the abnormal variations of the air refractive index.
The enhancement of radon concentration in the air due to its penetration through
soil into the atmosphere has been occasionally observed before an EQ (e.g., see
Pulinets et al. 1997 ; Yasuoka et al. 2012 ). The radioactive radon is a source of Ǜ-
particles with a mean kinetic energy about 6 MeV. This effect can result in additional
ionization of narrow near-surface layer of 1-3 m thickness. Every particle can
produce about 2 10 5 electron-ion pairs. The electrons are captured by oxygen and
nitrogen molecules for a short time and then the ions are absorbed by dusty particles
and by aerosols, which in turn climb at the upper layer of the air. The amplitude