Geoscience Reference
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when projected parallel to
B
, due to the high conductivity in that direction,
which keeps other sources of
E
small. The force on the ions associated with this
electric field, plus that of gravity, is countered by the plasma pressure gradient.
Perpendicular to
B
, this electric field is trivial.
Since the assumed dynamic equilibrium is between electrons and the dominant
ion species, light minor ions such as He
+
and H
+
can be accelerated outward
from the ionosphere by the parallel electric field. A considerable literature has
developed concerning the effect of this
E
||
on minor light ions and a complete
theory of the topside ionosphere requires its consideration. We take up this topic
in some detail in the polar case treated in Chapter 9, but for midlatitudes the
reader is referred to Schunk and Nagy (2000).
Since hydrogen atoms and molecules can escape the earth's gravitational field,
the earth is surrounded with a hydrogen gas “geocorona,” which interacts with
oxygen ions via the charge exchange reaction
||
H
→
←
O
+
+
H
+
+
O
which is a very rapid process. The result is that a transition occurs between
an oxygen and a hydrogen plasma between 500 and 1000 km, depending on
seasonal and other effects (Vickrey et al., 1979; Gonzalez, 1994). Since light
ions such as hydrogen can escape gravity, our simple assumption of diffusive
equilibrium breaks down and a net upward flux of plasma is possible during
the day at the “top” of the ionosphere. The closed dipole magnetic flux tubes at
tropical and midlatitudes act as a reservoir for plasma, called the plasmasphere,
which is created during the day by photoionization and carried upward by light
ions. At night the plasma can flow back down, tending to maintain the density
in the ionosphere. The result is a complex interaction between the ionosphere
and a region of hydrogen plasma trapped by the dipole magnetic field.
To summarize, without winds and electric fields, photoionization coupled with
diffusion, recombination, and charge exchange with the geocorona would com-
pletely determine the properties of the ionosphere. Rishbeth and Garriott (1969)
discuss this situation in great detail, deriving the so-calledChapman layer formfor
the ionosphere and discussing the various ionospheric layers, E
F, and so on.
A diurnal variation in peak plasma density would be expected, and some balance
would arise between the low-altitude production and outflow of plasma along
magnetic field lines during the day and low-altitude recombination and inflow
of plasma at night. We explore simple models for these conditions following.
The situation is much more complicated than this, however, since neutral
winds and electric fields also move the plasma. These forces greatly affect the
altitude of ionospheric plasma, particularly in the F region. Since the density of
molecular ions determines the speed with which O
+
recombination occurs and
since these densities are exponentially dependent on altitude, dynamic processes
also affect the plasma content. As an example of the variability of the midlatitude
ionosphere, data showing the change in altitude of the F-region peak density
,
F
1
,
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