layer, are influenced
by plasmaspheric fluxes, they are controlled in first order by
ionizing radiation from the Sun, by electric fields, and by
thermospheric winds. The electric fields can be due to
magnetospheric coupling or they can be the result of E- and F-region
dynamo processes. The magnetospheric fields map into the
high-latitude ionosphere and drive the ambient plasma into
rotating cell-like motion. Under quiet conditions, magnetospheric fields
are primarily confined to high latitudes, but under dynamic
storm-time transitions the coupling can extend to low and
equatorial latitudes through penetrating fields and
joule-heat-driven thermospheric winds. The morphological
descriptions of the high-latitude cell-like electric-field patterns
continue to be an intense subject of research, focusing on the question
of two-cell, four-cell, or distorted two-cell configurations, along
with attendant IMF (interplanetary magnetic field) controls. De
la Beaujardiere et al. [1991] carried out a study of seasonal variations
of the large-scale convection patterns using a five-year radar database
from the Sondrestrom facility. They found that the changes involved
the overall shape of the convection pattern, the electric field
intensity, and the dawn-to-dusk cross polar cap potential. Reiff and
Heelis [1994] scrutinized the four-cell concept using Atmospheric Explorer
C data and offered arguments that a four-cell configuration was
oftentimes required to explain observations that could not be explained
in terms of a two-cell or a distorted two-cell geometry. Knipp et al.
[1991] also looked into dynamic transitions in which the IMF underwent
a rapid southward-to-northward turning. By combining
ground- and satellite-based measurements and the AMIE semi-empirical
modeling procedure (Assimilative Mapping of Ionospheric
Electrodynamics) they found that the pattern changed from a
conventional two-cell geometry to a contracted four-cell pattern
with reversed convection cells in the high-latitude dayside.
The patterns, regardless of two- or four-cell, have an important impact on the high latitude ionosphere and on the thermosphere as well. In terms of ionospheric structures and plasma transport they are the fundamental drivers for ``tongues of ionization'' and ``polar cap patches'' with their plasma source-term being either dayside photoionization [ Sojka et al., 1993; Foster, 1993] or particle precipitation and flow channel events in the cusp [ Rodger et al., 1994]. In addition, the directed motion of the ions in these cells transfers momentum and energy to the thermosphere via ion-drag, modifying the thermospheric wind patterns, temperatures, and compositions [ Killeen et al., 1991]. The combination of Joule- and collisonal heating can generate equatorward winds, gravity waves, and penetrating electric fields that affect ionospheric dynamics and chemistry at low and mid-latitudes [ Fejer et al., 1990; Fuller-Rowell, 1994; Millward et al., 1993; Jing and Hunsucker, 1993].
Quiet-time electric fields are produced by diurnal E- and F-region
dynamo mechanisms and conjugate point effects. Prescriptions for
these fields have primarily relied on the empirical model of Richmond
et al. [1980] built on radar drift measurements during solar minimum
and heavily weighted by observations from stations near the 75
west longitude. The model limitations have been recognized for some
time, and efforts are underway to provide a more accurate specification
that includes seasonal and solar cycle variabilities along with
longitudinal controls. In this regard Buonsanto et al. [1993] have
studied
E x B drift patterns at Millstone Hill for summer, winter and equinox
under solar minimum and maximum conditions covering the period February
1984 through February 1992. They constructed average drift patterns
for equinox for both extremely quiet and disturbed periods, with
the quiet-time patterns discussed in terms of E- and F-region
dynamo processes. They found that conjugate-point electric fields
are important in winter when the conjugate ionosphere is sunlit for much
of the night. Fejer [1993] carried out similar efforts for
mid-to-low latitudes by analyzing F-region plasma drifts at the
Arecibo Observatory from 1981 to 1991 and by developing an empirical
model for the drifts with dependence on solar activity and
geomagnetic conditions. Oliver et al. [1993] compiled results for
seasonal and solar cycle controls of plasma drifts using the middle
and upper atmospheric (MU) radar data collected in Japan over the
period September 1986 to January 1991. They found strong
resemblances between the MU radar and Arecibo results for the
time-of-day and solar cycle trends in the daily-maximum drift
amplitudes, but the seasonal trends in the MU data, in the Arecibo data,
and in the Richmond et al. [1980] model strongly disagreed.
The electric fields associated with the observed ionospheric drifts,
along with thermospheric winds, chemistry and diffusion, play an important
role in the control of the global F-region morphologies. Abdu et al.
[1990 and 1993] and Walker et al. [1991] studied the interplay of the
electric field and neutral wind forces on the control of the
equatorial anomaly under quiet and disturbed conditions using a low
latitude network of ionosonde and magnetometer stations.
Another low-latitude study was carried out by Crain et al. [1993]
who presented a model simulation of the global dynamo compared to
ion drift observations at the Jicamarca Observatory. Using simple
diurnal winds and tides and enforcing self-consistency between the
plasma and potential distributions they found reasonable agreement
between the calculated E x B results and the observed drifts.