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along the geomagnetic field lines (FACs), which conduct well; exceptions are
plasma drift processes in the magnetosphere, like pressure gradient-driven currents.
Because of the high electrical conductivity of the geomagnetic field lines, they
can be assumed to be electrical equipotentials, so that electric fields along these
lines are zero or very small. Electric fields at heights >170 km are therefore almost
always perpendicular to the geomagnetic field lines. These mutually perpendicular
electric (
E
) and magnetic (
B
) fields force the ionospheric plasma to move with the
so-called
E
B
plasma drift velocity (equal to
E
B
/
B
2
), that is, in the direction
perpendicular to both. In this drift motion, ions and electrons move together with the
same velocity; that is, there is no charge separation and they do not create electric
currents.
Furthermore, there exists the so-called co-rotation electric field induced by the
rotation of the Earth and its atmosphere around the geodetic axis. In the equatorial
plane, this electric field is directed toward the Earth's center and causes the
magnetized ionospheric plasma to rotate together with the Earth (to drift with the
Earth's rotation velocity).
Global electric field patterns obtained mainly from satellite measurements are
usually presented as polar maps of electric potential distributions for different
magnetic activity levels and/or different IMF orientations. We show and discuss
some of the recent data in Sect.
4.3
of this chapter.
Ionospheric
E
B
plasma drifts cause many important peculiarities of the
spatial and time variations of the ionospheric plasma, such as the formation of the
equatorial anomaly, the main ionospheric trough, the plasmasphere, and ionospheric
disturbances related to magnetic storms and substorms.
At high and subauroral latitudes (where the geomagnetic field inclination
I
is near 90
ı
-60
ı
), the ionospheric plasma drifts affect electron density mainly
because of the divergence/convergence of the horizontal plasma flows. The daytime
plasma, having high density, can thus flow across the polar caps to the night side,
increasing the density there. Plasma motion will stagnate where the magnetospheric
convection and the co-rotation are oppositely directed with about the same velocity
magnitudes. If this occurs outside the sunlit region, the plasma will be lost because
of recombination processes to very low density values, forming the so-called main
ionospheric trough in the dark winter ionosphere at subauroral latitudes.
At middle to low latitudes, the plasma co-rotation with the Earth leads to the
formation of the plasmasphere. At these latitudes (where inclination
I
is small) the
vertical plasma drifts also become important. Close to the equator, they cause the so-
called fountain effect, forming the equatorial anomaly. At middle latitudes, they can
create increases or decreases of electron density by lifting the plasma up or down
the geomagnetic field lines to heights where the ion loss rate is lower or higher,
respectively.
Both the ionospheric plasma drifts and the electric currents influence ion and
neutral temperatures (Joule heating) via ion-neutral collisions. Thermospheric wind
circulation changes the thermospheric gas composition, which in turn influences the
ionospheric ion composition and electron density and so forth. The plasma physical
processes within the near-Earth space environment as well as the interactions
