Geoscience Reference
In-Depth Information
by gravity, production, loss, and transport rather than classical perpendicular
diffusion.
We still need to discuss how the patches shown in Fig. 10.1 are formed. We
postpone for the moment structured sources of ionization to consider only elec-
tric field effects. For example, large-scale ( k
1000 km) organization of the F-
region plasma can occur when the flow field has spatial variations in that same
scale. Such plasma structure can even evolve out of a uniform sunlit region if
different portions of the plasma move into dark areas at different speeds. As
shown in Fig. 2.2, this advection effect can cause the local plasma density to
vary if
n
/∂
t
=− (
V
·
n
) =
0
(10.1)
even when the production and loss terms vanish. Equation (10.1) holds for an
incompressible fluid, a valid approximation for the F region. For sunlit conditions
n is determined by the solar depression angle, and the typical perpendicular
gradient scale length L
) ] 1
=[ (
1
/
n
)(
dn
/
dx
is several hundred kilometers. In
B 2 where E is the applied electric field. A graphic illustration
of plasma structuring by a flow field is shown in Fig. 10.2. Here an initially
cylindrical plasma blob is placed in a spatially varying flow characteristic of the
duskside auroral oval. With time, the structure becomes very elongated in the
magnetic east-west direction. Since the flow is incompressible, the elongation
results in a very short scale size in the meridional direction. If the flow field is
turbulent, as it usually is, patches may be formed from eddies in the flow and be
carried off by the mean flow into the polar cap.
Plasma loss through recombination can also create structure when coupled to
a spatially varying flow field. This process was discussed in detail in Chapter 9,
where the midlatitude plasma trough and the polar hole phenomena were shown
to occur when the plasma remains for a long time on flux tubes with no sunlight
and no particle precipitation. This can occur on flux tubes that flow west at the
same speed as the earth rotates (the trough) or that circulate around a vortex that
is entirely in darkness (the polar hole). Then even a slow decay rate is sufficient
to deplete the plasma density.
The midlatitude nightside trough can become particularly deep during mag-
netic storms, when very large electric fields have been observed to build up at the
equatorward edge of the plasma sheet-ring current system in the magnetosphere
(Smiddy et al., 1977). This electric field points radially outward from the earth
in the equatorial plane. Mapped to the ionosphere, this localized electric field
is poleward and causes intense ionospheric flow toward the dusk terminator. In
fact, the largest ionospheric electric field ever reported—350 mV/m—occurred
in just such an event (Rich et al., 1980). Satellite data taken during such an
event are reproduced in Fig. 10.3a. The top panels show the intense localized
electric field (250mV/m) near L
(10.1) V
=
E
×
B
/
4 (60 invariant latitude) with magnetic field,
density, and energetic particle influx data plotted following. The large pulse cor-
responds to a potential drop of about 25 keV across the boundary. Notice that
=
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