  The GPS derived anomalous TEC disturbances before earthquakes were discovered
in the last years using global and regional TEC maps, measurements over
individual stations as well as measurements along individual GPS satellite
passes. For strong mid-latitudinal earthquakes the seismo-ionospheric anomalies
look like local TEC enhancements or decreases located in the vicinity of the
forthcoming earthquake epicenter In case of strong low-latitudinal earthquakes
there are effects related with the modification of the equatorial F2-region
anomaly: deepening or filling of the ionospheric electron density trough over
the magnetic equator. We consider that the most probable reason of the NmF2 and
TEC disturbances observed before the earthquakes is the vertical drift of the
F2-region ionospheric plasma under the influence of the zonal electric field of
seismic origin. To check this hypothesis, the model calculations have been
carried out with the use of the Upper Atmosphere Model. The electric potential
distribution at the near-epicenter region boundary required for the electric
field maintenance has been proposed. The upper atmosphere state, presumably
foregone a strong earthquake, has been modeled. The results of the
corresponding numerical model calculations have revealed a fine agreement with
TEC anomalies observed before strong earthquakes at the middle and low
latitudes both in spatial scales and in amplitude characteristics.
