Geoscience Reference
In-Depth Information
latitudes. We term this region midlatitude as opposed to tropical. Until recently
most of our electrodynamical information has come from vector F-region plasma
drift measurements made with the incoherent scatter method at these sites. As
discussed in Chapter 3, drifts measured perpendicular to
B
in the F region can
be interpreted unambiguously and yield the ambient electric field via the rela-
tionship
E
B
. The parallel drift component is much more complex,
however, since gravity, neutral winds, and pressure gradients all contribute. The
availability of neutral wind measurements using optical techniques at Arecibo,
Fritz Peak, and other midlatitude sites has greatly helped interpretation of the
incoherent scatter parallel drift data. Satellite data on neutral and plasma densi-
ties and temperatures have been very useful but the dynamics of the midlatitude
region are difficult to study at orbital speeds.
The roles of the forces that act on the ionospheric plasma in the direction
parallel to
B
is clear from (2.36a), which we reproduce here:
=−
V
×
V
j
||
=
b
j
E
−
+
D
j
/
H
j
ˆ
g
·
B
D
j
∇
n
/
n
(5.1)
The primes indicate that the quantities aremeasured in the frame of reference in
which the neutral wind velocity vanishes. Transforming to the earth-fixed frame
where the neutral wind has a value
U
, the parallel component of the velocity
V
j
is given by
·
B
V
j
||
V
j
||
=
U
+
(5.2)
The plasma is thus closely coupled to the neutral gas motion along
B
, but its
velocity is modified by the term
V
j
||
. For simplicity, in (5.1) we have continued to
ignore temperature gradients, which must be included in a complete treatment
of the total pressure gradient. For now, we ignore the neutral wind and external
electric fields.
During the day, plasma production or loss by photoionization and recombi-
nation dominates the plasma profiles in the E and lower F regions, creating the
horizontally stratified, slowly varying plasma content of the ionosphere. Rishbeth
and Garriott (1969) and Schunk and Nagy (2000) describe the relevant physics
and chemistry in great detail, and we refer the reader to their discussion. The
ionization profile increases sharply in the 90-100 km range and then more slowly
up to a peak, which is typically near 300 km.
Above the F peak we expect the plasma to be in a state something akin to
diffusive equilibrium in the gravitational field. Because of its light mass, the
electron gas diffuses much faster than the ions down a pressure gradient, a process
that would tend rapidly to destroy any gradient in electron pressure. However,
the resultant charge separation is accompanied by an electric field that restrains
the electrons and enhances the ion diffusion. Quantitatively, we can argue as
follows. In diffusive equilibrium there should be no net flow of the electron gas
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