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electrons carry current away from the ionosphere, this relationship suggests that
an arc should occur when the vorticity is positive. This dependence has indeed
been reported for polar cap arcs by Burke et al. (1982). It is of interest also
to note that, from Poisson's equation, this result is equivalent to the statement
that electron precipitation occurs into regions of net negative charge density (see
Section 2.4), which seems counterintuitive but is correct for the usual case that
the ionosphere acts as an electrical load. The multitude of polar arcs shown in
Fig. 10.11a suggests a turbulent flow with many sign changes of the vorticity.
This vorticity in the magnetospheric flow or solar wind creates charge separation
in a low impedance source, which leads to field-aligned currents. These currents
then become unstable at some altitude, which results in particle acceleration via
parallel electric fields. The experimental situation is complicated by the conduc-
tivity gradients which are created by the particle precipitation. In fact, detailed
observations near a winter polar cap arc show that both conductivity gradients
and structured electric fields are important in the horizontal current divergence
(Weber et al., 1989).
So far we have discussed the production of F-region plasma in the cusp (dayside
oval), in the polar cap, and at the edge of the nightside auroral oval. In the heart
of the nightside oval the situation is very chaotic, due in part to the role of
substorm activity in the midnight sector. The series of
Dynamics Explorer 1
images of the auroral oval in Fig. 10.12 shows some of the dynamical features in
a typical substorm. As a crude first approximation there are three classes of oval
precipitation, whichmay be described by their optical and plasma signatures. The
diffuse aurora is characterized by a widespread, nearly uniform particle influx
from the plasma sheet. Since the diffuse aurora mirrors the hot plasma sheet, it is
also characterized by the same flow pattern, which is roughly zonally westward
before midnight and zonally eastward after midnight. Imbedded in this plasma
are regions of discrete auroral arcs that are usually aligned east-west and are
often associated with potential drops along the magnetic field which accelerate
electrons into the atmosphere. This acceleration zone has been located by satellite
and rocket techniques at altitudes ranging from 2000 to 8000 km. This process
occurs throughout the auroral oval. The auroral arcs in the dayside oval are
associated with acceleration zones having several hundred to a thousand volts
of potential drop. In the midnight sector the potential is higher and most of
the plasma in this nighttime sector is produced by accelerated electrons striking
neutrals in the E region. The intense E-region plasma density increase shown in
Fig. 1.5 is of this type. Quiet auroral arcs are usually east-west aligned, that is,
much smaller in latitudinal extent than in longitude. The active auroral forms
associated with substorms comprise a third rough class of auroras.
Other than the obvious comment that more plasma is produced in a bright
aurora, active auroral forms are often extremely contorted. The examples in
Fig. 1.4 and Fig. 10.12 show this very clearly. The plasma density produced by
particle precipitation in the E and F regions will then also be quite structured.
So many factors contribute to the horizontal structuring in the nightside oval
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