Geoscience Reference
In-Depth Information
increase of the F2 region electron concentration maximum (NmF2) and TEC as
a result of the plasma transport up to regions with lower concentration of the neutral
molecules O
2
and N
2
and, consequently, with lower loss rate of the dominating ions
O
C
in the ion molecular reactions (Brunelli and Namgaladze
1988
). An electric
field of the opposite direction (westward) creates the opposite—negative—effect
in NmF2 and TEC. In the low-latitude regions (near the geomagnetic equator) an
increased eastward electric field leads to a deepening of the Appleton anomaly
minimum (“trough” over the magnetic equator in the latitudinal distribution of
electron concentration) from the intensification of the fountain effect.
To check this hypothesis of the zonal electric field as the most probable cause for
the observed TEC disturbances before earthquakes, model calculations were carried
out with the UAM, the global numerical model of the Earth's upper atmosphere
(Namgaladze et al.
1988
,
1990
,
1991
,
1998a
,
b
).Theaimstobesolvedwere(1)to
check in principle the ability to set additional electric potentials of seismic origin
to create the required zonal electric field at the near-epicenter area; (2) to calculate
this electric field and the corresponding TEC disturbances; and (3) to estimate the
efficiency of the proposed mechanism and to compare simulated TEC deviations
with those, which are usually treated as precursors at low- and mid-latitudinal
sectors.
First, the upper atmosphere state, presumably foregoing a strong earthquake, was
modeled (Namgaladze et al.
2009a
,
b
,
2008a
) by means of switching on additional
sources of electric field in the UAM electric potential equation, which was solved
numerically jointly with all other UAM equations (continuity, momentum, and
heat balance) for neutral and ionized gases. These sources were switched on and
maintained permanently for 24 h in the form of additional positive and negative
potentials with values of 2 and 5 kV (in case of the low-latitudinal sources) and of
10 kV (in case of the mid-latitudinal source) on the western and eastern boundaries
of near-epicenter areas, respectively.
We investigated two near-epicenter areas (Namgaladze et al.
2009b
) with sizes
of 10
ı
in latitude and 30
ı
in longitude. The epicenters were situated at these points
with the following magnetic coordinates: (1) (45
ı
N, 90); (2) (15
ı
S, 210). These
sizes approximately correspond to the horizontal sizes of those regions of increased
TEC values, which were found by Pulinets et al.
2003
and Zakharenkova et al.
(
2007b
), for example. The first region is a typical mid-latitude ionosphere, and the
second region is a near-equatorial ionosphere, for which the electric field effects are
more essential than at middle latitudes. For the simulations we chose a magnetically
quiet day of the June solstice at high solar activity.
In Fig.
4.12
the numerical grid of geomagnetic coordinates used in the model
calculations is shown; its mesh points, in which additional potentials were set, are
marked by circles (dark circles correspond to the positive potential, light circles
correspond to the negative potential).
The calculated electric potential pattern and horizontal electric field vectors
for the sources of 5 and 10 kV per mesh point are shown in Fig.
4.13
for the
quiet and seismo-disturbed conditions. The figure illustrates the regions where an
eastward electric field above the prospective epicenters of the future earthquakes
