Ionospheric Response to Electric Fields - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 449

and its neutral atmosphere along the magnetic field lines. This in turn reduces
recombination of O + , which requires interaction with a neutral molecule for
efficient recombination, thereby increasing the plasma density. Similarly, pene-
trating zonally eastward electric fields associated with an increase in the dawn-
to-dusk interplanetary electric field in the earth's reference frame (IMF B z south
conditions) causes an uplift of the daytime equatorial ionosphere, removing ions
from the recombination zone and allowing for further production of plasma in
sunlight. This uplift eventually leads to an enhanced fountain effect and an expan-
sion of the equatorial anomaly (equatorial arcs) to higher latitudes. If, in turn,
this plasma is ripped away from the midlatitude zone by meridional penetrating
fields, a SED (Storm Enhanced Density) event may occur, leading to channels of
dense plasma stretching even over the polar zone (Foster and Burke, 2002; Vlasov
et al., 2003; Foster et al., 2005).
Negative storms are thought to be due to changes in the composition of the
neutral atmosphere at thermosphere/ionosphere heights. This in turn is due to
heating and upwelling of the lower atmosphere with its molecular-rich compo-
sition. In turn, this decreases the [O]/[N 2 ] ratio and enhances the recombination
of the O + plasma, resulting in a negative ionospheric storm. The molecular-rich
atmosphere can be transported for vast distances by disturbance winds (Fuller-
Rowell and Rees, 1981). Large scale TIDs can also affect the density as the
associated oscillating winds drive the ionosphere into the neutral atmosphere
in one phase, decreasing the local density, while in the equatorward wind phase,
to first order, the plasma merely rises without changing its content, something
of a half-wave rectifier effect.

9.3 Electrodynamic Forcing of the Neutral Atmosphere

9.3.1 J B Forcing

The fact that the plasma is often in motion with high velocities in the polar region
has important consequences for thermospheric dynamics. We cannot hope to
treat this fascinating topic in the detail it deserves but will only outline some of
its most important aspects.
In Chapters 3 and 5 we pointed out that the plasma acts as a drag on the
thermosphere at low and midlatitudes. We showed that this drag force was
equivalent to a J

×

× B force on the neutral gas where

J = σ · ( U × B ).

If this current is not divergence free, we found that electric fields build up
and usually reduce the total current with the result that the J B force decreases.

Neutral winds in such a case therefore create electric fields via a dynamo
process.

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