Geoscience Reference
In-Depth Information
(UAM, Upper Atmosphere Model) was used for the presented investigations.
Ionospheric electric field patterns were calculated by solving numerically Eq.
4.20
,
taking into account both the dynamo action of neutral winds and the external current
drivers at the lower and upper boundaries of the ionospheric dynamo region. These
currents transfer electric charges from the magnetosphere to the dynamo region
upper boundary and from the near-Earth's lower atmosphere to the lower boundary.
We should keep in mind, however, that the original sources of both kinds of electric
fields (of magnetospheric and seismogenic origin) are motion: the solar wind plasma
generator flow across the magnetopause and the displacement of tectonic plates.
Electric fields generated by the interaction of the solar wind and its embedded
interplanetary magnetic field with the geomagnetic field, as well as geomagnetic
field-aligned currents, have already been studied for many years; their existence is
beyond doubt. Very high electrical conductivity along the geomagnetic field lines
ensures reliable electrical coupling between the magnetosphere and the ionosphere.
Another situation takes place with the seismogenic electric fields because of the
very low (in comparison with the ionized layers above
80 km) conductivity of
the neutral air between the Earth and the ionosphere. Nevertheless, it is not zero,
and varies significantly from place to place, for example, from “good weather”
regions to thunderstorm areas. The average vertical electric current density between
the Earth and the ionosphere is about 2-3 pA/m
2
, and the corresponding average
electric potential difference amounts to about 250-300 kV. In thunderstorm areas,
the vertical electric current density increases by several orders in magnitude. The
same takes place, presumably, near tectonic fault regions before earthquakes as a
consequence of the motion of (or accumulated stresses between) tectonic plates.
Some of the recent results presented here on TEC variations before earthquakes,
which were obtained using GPS data, give strong evidence in favor of the hypothesis
on seismogenic electric fields as the main cause for these variations. The persistence
of the near-epicenter disturbance regions, their geomagnetically conjugate occur-
rence, and their tendency to avoid sunlit time periods constitute these evidences.
The UAM calculations presented above show that a vertical electric current of about
10
8
A/m
2
at an area of about 1,000 km
4,000 km can create electric fields of
several mV/m in the nighttime ionosphere, which in turn can produce the TEC
variations of up to 50%, which turns out to be very similar to those observed.
The electric coupling between the tectonic faults and the ionosphere requires
thepresenceofemergingelectriccharges (or ionization sources) near the faults.
Two mechanisms were considered in recent years: ionization by radioactive radon
emission and the “positive hole” mechanism by Freund. Possibly, they are not
competing but both acting. The detailed physics and chemistry of the lower (near
the Earth surface) atmosphere in presence of these ionization sources should be
developed in the future.
Electric fields of magnetospheric origin, on the other hand, have proved to be
very important for the “space weather” tasks. One should necessarily notice their
strong dependence on the solar wind parameters, primarily on the interplanetary
magnetic field strength and orientation. The coupling of the magnetosphere with the
ionosphere via field-aligned currents and their use in electric field modeling should
