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accumulate on the western boundary of the near-epicentral region, and negative
charges on its eastern boundary, or vice versa. We showed only what kinds of
electric field were able to produce TEC and NmF2 disturbances observed before
strong earthquakes.
The next step was to reproduce TEC variations before concrete earthquake
events instead of synthetic model cases and to investigate other types of electric
field sources (dipole-like, positive, and negative) and their spatial configurations.
Numerical simulations were carried out for (1) the 26 September 2005 earthquake
in Peru (Zolotov et al.
2008b
) and (2) the Kythira seismic event on 8 January 2006
in Greece, at mid-latitudes (Zakharenkova et al.
2008
; Zolotov et al.
2008a
).
It was shown that in case of the Kythira earthquake that a simulated dipole-like
source agreed better with the experimental GPS TEC data than in cases of monopole
(positive) additional sources. Variations of the TEC disturbances for the low-latitude
region (Peru earthquake) had the form and the structure of the equatorial anomaly
(“trough”). The increase of the eastward electric field leads to the deepening of the
equatorial anomaly minimum (“trough” over the magnetic equator in the latitudinal
distribution of electron density) from an intensification of the fountain effect.
The dipole-like and single sign (positive) additional electric field sources generate
similar disturbances in ionospheric TEC (in contrast to the mid-latitudinal cases).
A noticeable difference appears in the magnitude of these variations: the dipole-
like sources generate a deeper equatorial trough. The amplitudes of the peaks (the
“camel case” maxima) remain nearly at the same level or are slightly larger. That
behavior agrees well (at least qualitatively) with the observed Peru pre-earthquake
TEC variations.
Thus, both cases reveal a good agreement with the observations; the dipole-
like and monopole sources have fewer differences in their manifestation at low
latitudes.
It should be noticed that the applied “manually fixing” technique has two
drawbacks: (1) it is hard to explain the nature of the “manually set” electric
potential sources (which should be charges) on the borders of the near-epicenter
area, especially their accumulation; and (2) the approaching sunrise terminator
and its corresponding changes of the ionospheric conductivity caused by the well-
conducting sunlit ionosphere should depress the additional electric potential and the
corresponding disturbances up to full disappearance down to zero.
A more physical approach was realized by Sorokin et al. (
2005a
,
2006
,
2007
)
by setting vertical electric currents flowing between the Earth and ionosphere at the
considered region. This 'electric current technique' was used for the case of the
Haiti earthquake of 12 January.
4.3.5.2
TEC Disturbances Before the Haiti Earthquake of 12 January
2010: GPS Observations and UAM Modeling
This section is aimed at (1) the determination of pre-earthquake TEC modifications
and their description for the case of the 12 January 2010, 21:53 UT (16:53 LT) Haiti
