Introductory and Background Material - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 25

of the auroral oval. As the magnetic field lines reconnect in the tail and jet back
toward the earth, the average electron energy is boosted to about a kilovolt in
the region termed the plasma sheet. The plasma in this region provides a source
of steady precipitation that corresponds visually to the widespread emissions
called diffuse aurora. This precipitation also acts as an ionization source for the
ionosphere. The hot flowing plasma has an associated electric field [see (1.2b)]
that is impressed on the ionosphere. Electrical currents flow in response to this
electric field. For nighttime conditions the precipitating plasma itself provides
most of the conductivity, so an electrical feedback can develop between the
plasma sheet and the ionosphere as currents flow back and forth between them.
This feedback is most complex in the region of the discrete aurora, which
contains the very bright, active, auroral displays. Here, at some point along
the magnetic field (usually above 3000 km), electric fields develop parallel to
the magnetic field lines that accelerate electrons down into the atmosphere and
ions out into the magnetosphere. Typical field-aligned potential drops associated
with this parallel electric field range from 100V to 10 kV. The electron density
in the ionospheric E region can exceed 10 6 cm − 3 below a strong auroral display
(see Fig. 1.5), and the horizontal electrical current in the ionosphere can create
perturbations in the surface magnetic field of up to 3000 nanotesla—nearly 5%
of the earth's surface field at high latitudes. Many other acceleration processes
besides parallel electric fields operate in the magnetosphere and are beyond our
scope to discuss in detail.

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