The ionosphere can be viewed as the culmination of all coupling processes in the system of solar-terrestrial plasmas, including such diverse phenomena as coronal mass ejections, magnetospherically-imposed electric fields, and solar-driven thermospheric tides. It follows therefore that the accurate and self-consistent specification of ionospheric plasma distributions represents a critical test of our understanding of the ionosphere itself and the myriad of controls and coupling mechanisms from the Sun to the Earth.
There is, however, neither a code nor a model that can accurately specify the quiet- or storm-time characteristics of ionospheric plasma distributions. Nor is there an adequate database to constrain the models and enforce unique quantitative solutions. Among the challenges are the ionospheric structures themselves which span eight orders of magnitude in size. Their theoretical and experimental investigation requires a broad computational grid and unique combinations of ``in situ'' and remote sensing techniques to define the hierarchy of sizes and cause-effect terms. These structures range from tens-of-thousands of kilometers to tens-of-centimeters, with scale lengths that can be horizontal or vertical in the Earth's gravitational field, or parallel or perpendicular to the geomagnetic field depending on the causal mechanisms.
A multitude of structures populate various ionospheric regions at
different times, with their causality having been the focus of
intense research over the past four years. At meso- and macroscales (i.e.,
50-10
km) the interest has been in such phenomenologies as
the auroral oval, the equatorial anomaly, the mid-latitude trough, the
cusp, intermediate and descending layers, and polar cap patches. At
smaller scale sizes (tens of kms to cms) attention has been on
plasma instabilities which play a role in the distribution of
ionospheric irregularities. Through wave-particle interactions
the instabilities and associated irregularities can influence currents
and energize ions that ultimately populate the magnetosphere [e.g.,
Peterson et al., 1993; Ganguli et al., 1994]. Many processes, like
the Rayleigh-Taylor,
ExB,
current convective, and universal drift
wave modes, depend upon ionospheric structures and their density
gradients as energy sources to drive the plasma unstable [e.g., Aggson
et al., 1992; Hysell et al., 1994; Swartz and Farley, 1994].
Clearly, an understanding of these structures will provide a hierarchical
perspective on all of the ionosphere and develop insights into the
processes that control and maintain the structures themselves and
influence the coupling to other geospace domains.