Geoscience Reference
In-Depth Information
However, electrodynamic processes are important in the lower and middle atmo-
sphere (troposphere, stratosphere, and mesosphere) as well because of the existence
of such lower atmosphere electricity sources as cosmic rays, thunderstorms, soil
radioactivity, and seismodynamic effects. The lower atmosphere electric currents
are connected with the ionospheric electric currents so that they are closely related to
the magnetospheric currents, creating a common global electric circuit. This circuit
consists of currents flowing upward from thunderstorm current generators, through
the ionosphere, and down to the Earth's surface as fair-weather currents. The vertical
fair-weather current between the ionosphere and the Earth is about 2-3 pA/m
2
and
the total global current is about 1 kA, producing a potential difference of about
250-300 kV between the ionosphere and the Earth.
The horizontal potential difference in the ionosphere may reach 100-150 kV
(usually 30-50 kV) between the morning and evening sides of the polar cap bound-
aries at the polar edges of the auroral zones (at about 75
ı
magnetic latitude) in both
Northern and Southern Hemispheres. The cross-polar cap potential difference (or
drop) is the commonly used characteristic of the high-latitude ionospheric electric
fields. Its dependences on solar and geomagnetic activity have been investigated by
many authors and used as input in electric field calculations of the model.
This potential difference is generated at the magnetopause under the solar wind-
magnetosphere interaction and transported into the polar ionosphere along the
geomagnetic field lines via so-called field-aligned currents (FACs) of Region-1.
They flow down into the ionosphere at the dawn side and up from the ionosphere at
the dusk side, creating electric fields of several tens or even hundreds of mV/m in the
polar caps directed from dawn to dusk. FACs of opposite signs flow at the equatorial
edges of the auroral zones (Region-2 FACs). These currents shield the inner
magnetosphere and the mid-latitude ionosphere from penetrating magnetospheric
electric fields. This shielding and the presence of internal atmospheric dynamo
processes results in smaller mid-latitude electric fields (of about several mV/m) in
comparison with the high-latitude and subauroral ones.
Region-0 FACs are found during intervals of a strictly positive
B
z
-component
of the interplanetary magnetic field (IMF). They may exist in the vicinities of the
cusps. The IMF
B
y
-component is responsible for the asymmetry of the morning and
evening cells of the ionospheric convection (plasma drift trajectories). The intensity
of the FACs and the corresponding cross-polar cap potential drops (or the electric
field strength) are well correlated with the southward IMF
B
z
-component, which can
trigger magnetospheric substorm and storm generation processes.
Another source of ionospheric electric fields is the dynamo action of the
thermospheric winds pushing ions across the geomagnetic field lines at ionospheric
E- and F1-region heights (100-170 km), where electrons are magnetized (electron-
neutral collision frequency
en
˝
e
- electron cyclotron frequency) but ions are
not (ion-neutral collision frequency
in
˝
i
- ion cyclotron frequency). For this
reason, this height region is referred to as the dynamo region. Horizontal ionospheric
currents of the upper atmosphere flow predominantly in this height range.
In the ionospheric F2-region and above it (heights >170 km), both electrons and
ions are magnetized (
en
,
in
˝
e
,˝
i
) and electric currents flow first and foremost
