-, and 
-region
heights of 110,
150, and 300 km, respectively. Accumulating data point to an
additional group of layered structures in the 95-200 km region that do
not fit classical ionospheric model descriptions. These layers have
gained attention because of their relevance to dynamo fields, their
kinetic interactions with thermospheric winds, their enhanced
conductivity and associated controls of E-region current systems, and
their potential role as a tracer of wind-shear nodes and tidal
components. The work of Wilkinson et al. [1992] first suggested the
possible ubiquitous nature of these layers, while the month-long
SUNDIAL campaign study of Szuszczewicz et al. [1994] confirmed the
regular occurrence of these intermediate and descending layers
at low-to-high latitudes. The campaign investigation compared
numerical simulations of the NCAR TIE-GCM [ Richmond et al., 1992]
against the SUNDIAL observations with results that suggested
that windshears from diurnal, semi-diurnal, and even terdiurnal
tidal components were the primary mechanisms giving rise to the
layer formation and subsequent dynamics. The investigation also
included the influences of electric fields but definitive results on
their overall effect were not apparent. Related work has been continuing
in the radar studies of Mathews et al. [1993] who have shown that the
layers are a regular and persistent feature of the lower ionosphere
above Arecibo. Their study also provided evidence of a coupled neutral
and ion-layer mechanism that can deposit lower-ionospheric metallics
into the upper mesosphere as a result of descending layers and
the associated wind-shear nodes which converge and transport the ions
[ Kane et al., 1993].
It is generally agreed that wind shears are likely to be the primary
causal mechanism at low-to-mid latitudes, with electric fields
contributing to the overall process in regions above 125 km.
However, Bristow and Watkins [1991] showed that intermediate layers
can be formed at high latitudes due to electric fields alone, and that
the layers can be stationary in altitude or descending in
nature, depending on the prevailing electric fields.
In addition to the roles of winds and electric fields, chemistry
continues to be important in the study of the lower ionosphere. With
regard to intermediate and descending layers, N.J. Miller et al.
[1993] have analyzed low perigee passes (140-200 km) of the Atmospheric
Explorer E ion mass spectrometer data. They found ion composition waves,
intermediate layers of enhanced ionization, and ionization
depletions similar to equatorial ionization bubbles. NO
and
O
dominated the enhanced ionization layers without
significant metallic ions. This suggests that metallic ions are not
required to produce the intermediate layers at altitudes above 140 km.
Because of its importance to the total plasma density in the
lower ionosphere, the NO
/O
ratio is a subject of continuing
research. The ratio depends on the neutral nitric oxide
concentration which is a minor, but important, constituent of
the thermosphere up to approximately 150 km. Within the thermosphere,
NO is an important cooling agent, and charge exchange with O
provides the major NO
production term at altitudes below 150 km. The
NO concentration has also been found to be responsible for enhancements
in the quiet sunrise E-region at high latitudes during winter [ Swider
and Keneshea, 1993]. In addition, a combination of the long NO lifetime
and a prevailing downward circulation can transport NO into
the stratosphere, where it depletes the ozone concentration. In studying
NO and its transport properties, Fuller-Rowell [1993] used a
one-dimensional globally-averaged model to calculate the solar cycle
change in nitric oxide in the thermosphere and upper mesosphere
with favorable comparisons with measurements from the Solar
Mesosphere Explorer (SME) satellite.