shock event (18.46 ı N, 72.5 ı W) magnitude (M) 7.0 and (2) the numerical simulation
of the ionosphere TEC response on electric fields generated by external seismo-
related electric currents.
The upper atmosphere is very sensitive to influences of solar activity changes
and to the impacts from outer space. Thus, changes in geomagnetic activity such as
magnetic storms and substorms can lead to the generation of different positive and
negative ionospheric disturbances at mid- to low latitudes (increases or decreases of
the NmF2 and TEC) that may mask pre-earthquake disturbances, triggered possibly
by seismo-ionosphere coupling processes, because the magnetic activity effects are
of similar or even stronger magnitudes.
The geomagnetic situation was relatively quiet for the considered period: the Kp
index was in general less than 2 during 1-12 January 2010, reaching distinct maxima
values less than 3 for a short period. The Ap index took values of about 5 nT and
did not exceed 10 nT for the considered period. The D st index variations were in the
range from 15 nT up to 15 nT. Therefore, the observed TEC disturbances could not
be explained by solar and geomagnetic activity; they must be caused by processes
originating from the underlying near-Earth atmosphere impact.
To estimate pre-earthquake TEC variations we have built differential (%) TEC
maps relative to the quiet background conditions. We used global ionospheric maps
(GIM) of the TEC (Dow et al. 2009 ) provided by NASA in IONEX format as
the initial data for the analysis ( ftp://cddisa.gsfc.nasa.gov/pub/gps/products/ionex/ ) .
The spatial resolution of those TEC data is 5 ı in longitude and 2.5 ı in latitude.
Corresponding time resolution is 2 h. In contrast to other ground-based and satellite
systems, GNSS-based networks provide almost global coverage and near-permanent
(i.e., almost without time gaps) data.
We defined and calculated background TEC values (i.e., undisturbed conditions)
as 7-day UT-grouped running observation medians before the current calculation
moment. The resulting TEC difference (%) maps are presented in Fig. 4.16 .
Anomalous TEC enhancements were observed during 06 UT-14 UT of 9 January
2010, 12 UT-14 UT of 10 January 2010, 08 UT-16 UT of 11 January 2010,
and 04 UT-10 UT of 12 January 2010, reaching more than 50% by magnitude
relative to the background values (Zolotov et al. 2011a , b ). The amplitudes of the
positive anomalies were significantly larger at the magnetically conjugated region.
They were strongly fixed at the near-epicenter area. Their lifetime was limited
to the nighttime hours. The terminator and subsolar point positions (indicated by
continuous lines and circle symbols, respectively) have apparently some influence
on the behavior of the observed positive structures. Approaching of the sunlit
ionosphere is correlated with a disappearance of the anomalies. Such a behavior of
the positive TEC disturbances could be explained by the corresponding disappear-
ance (from increased conductivity) of the external electric field generated by the
external electric current. Therefore, we consider positive structures as evidence of
the “lithosphere-atmosphere-ionosphere” coupling via an electric current system. It
should be noticed that there exist also negative TEC disturbances around the positive
ones but with smaller magnitudes (about 15%).