As a dynamic plasma environment which envelops the Earth, the ionosphere
and its distribution of structures are known to control, limit, and in
some cases threaten the performance of space and Earth-based systems
[e.g., Hruska et al., 1992; and Goodman, 1993]. Its
refractive characteristics and current-carrying capabilities can
(1) interrupt transionospheric command, control and communication
systems, (2) compromise global positioning networks, and (3)
induce damaging currents in land-based power grids and
transcontinental pipelines. While much has been written about such
adverse effects, there are important ways in which the ionosphere is
an asset, and knowledge of its behavior can contribute significantly
to other disciplines. For example, the refractive property of the
ionosphere at high frequencies (5--30 MHz) has opened the possibility
for continuous large-area monitoring of the oceans (e.g., the entire
North Atlantic with an approximate area of 6(10)
sq. mi.) using
the over-the horizon (OTH) radar technique for studies of sea state,
ocean currents, air-sea interactions, and severe storm tracking
[e.g., Georges et al., 1993; Szuszczewicz et al., 1993a].
The accuracy of the derived parameters (rms waveheights, ocean wave
spectra, surface winds, and surface currents), and of a storm track
in severe weather applications, depends on the accuracy of the
ionospheric specification. This includes the specification of
the three-dimensional time-dependent profile of ionospheric densities
under quiescent and disturbed conditions. It follows, therefore, that
the fundamental goals of ionospheric research not only mitigate some
of the deleterious effects on man-made systems but can often contribute
a supporting role to other disciplines. This includes the potential
for important advances in predicting atmospheric weather and climate
through the OTH radio-oceanographic study of air-sea interactions and
the influence of related processes on the Earth's hydrologic cycle.
In the sections which follow, ionospheric research activities for the period 1991 through mid-1994 will be presented with a few references to earlier work where appropriate for traceability. The perspective will be on emerging interests in ionospheric weather and climatology and on the roles and limitations of data and models in developing a predictive capability. The approach will be selective and illustrative rather than comprehensive, in order to accommodate page limitations and allow for expansion of some ideas without telegraphic reporting of all published papers.