(deviation from background conditions) is often reported from several days or
hours to a couple of weeks before the earthquake release moment. TEC precursor
evolution might be in form of sign change to opposite ones. Before the earthquake
release moment or a few hours earlier, the anomaly depression up to its full
decay could take place. The lifetime of the seismo-induced deviations of the same
sense is from about 4-6 h up to 12 h in case of very strong earthquakes. (3)
The vertical projection of the epicenter position does not mandatorily coincide
with the maximum phenomenon's manifestation location. (4) Similar effects are
often reported at the magnetically conjugated area. (5) In the case of strong
low-latitudinal earthquakes, there are effects related to the modification of the
ionospheric F2-region equatorial anomaly: increase or decrease of the equatorial
(Appleton) anomaly with trough deepening or filling (Depueva and Ruzhin 1995 ;
Depueva et al. 2007 ).
Many authors (e.g., (Pulinets 1998 ; Sorokin and Chmyrev 1999 ; Sorokin et al.
2005a , b , 2006 , 2007 ) strongly rely on the hypothesis of a seismogenic electric field
related to the vertical turbulent transportation of injected aerosols and radioactive
particles (radon isotopes). The increase of the atmospheric radioactivity level during
the earthquake formation leads to the enlargement of the ionization and electric
conductivity of the near-ground atmosphere. The joint action of these processes
leads to an intensification of an electric field in the ionosphere up to the value of units
of tens of mV/m (Chmyrev et al. 1989 ). Freund ( 2011 ) proposed another mechanism
of the near-ground atmosphere layer ionization based on so-called positive holes.
Most crustal rocks contain dormant electronic charge carriers in the form of peroxy
defects; when rocks are stressed, peroxy links break, releasing electronic charge
carriers, known as positive holes. The positive holes are highly mobile and can flow
out of the stressed subvolume. Freund expects this mechanism to be significantly
more efficient than the aforementioned radon-related ones.
Total electron content (TEC) is formed mainly by the F2-layer plasma density.
NmF2 and TEC disturbances related with magnetic activity are created by the
neutral atmosphere (neutral gas composition, internal gravity waves, and winds)
and electric field variations. However, it is impossible to localize neutral atmosphere
disturbances at a limited area as they will propagate away from the source. There-
fore, there are strong arguments in favor of the idea of an electric field of seismic
origin as the main cause for the TEC anomalies observed before earthquakes: (a) the
geomagnetically conjugate ionospheric precursors (Pulinets et al. 2003 ); (b) effects
related to the ionospheric F2-region equatorial (Appleton) anomaly controlled by
the zonal electric field (Depueva and Ruzhin 1995 ; Pulinets and Legen'ka 2002 );
and (c) “ban-time” at the near-noon hours (Namgaladze et al. 2011 ).
It was proposed (Namgaladze et al. 2007 ) that the main reason for the appearance
of local TEC anomalies in the form of an increased (decreased) total electron
content of the ionosphere, observed on the base of GPS (Global Positioning System)
signal measurements, is the vertical drift of F2-region ionospheric plasma upward
(or downward) under the influence of a zonal electric field of seismogenic origin
directed to the east (or west). At middle latitudes the vertical upward component
of the electromagnetic drift, created by an eastward electric field, leads to an