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Our understanding
and specification of the ionosphere can be viewed from its
average behavior (i.e., its climatology) and from its
hour-to-hour, day-to-day, and week-to-week variability (i.e., its
weather). Global approaches to ionospheric weather and climatology may
be empirical or they may include first principle modelling activities
like those embodied in the NCAR TIE-GCM (National Center for
Atmospheric Research, Thermosphere-Ionosphere Electrodynamic
Global Circulation Model), the Utah State TDIM (Time-Dependent
Ionospheric Model), and the University of Alabama FLIP
(Field-Line Integrated Plasma) model. These have been reviewed recently
by Schunk and Sojka [1992] and will be discussed below within the context
of specific investigations. None are rigorously first-principle
models, since they rely to one degree or another on empirically-specified
input conditions, including specifications of
solar radiation, auroral particles, magnetospherically-imposed
electric fields, chemical reaction
rates, and in some cases
thermospheric winds with associated tidal components and dynamo
fields. The accuracy of these inputs limits the first-principle models
and their abilities to develop accurate specifications of
ionospheric weather and climate. Hence the need for a more
comprehensive database upon which to build our understanding of
the ionosphere and test the models.
Currently the most-complete and widely-tested climatological
specification of the ionosphere is provided by the empirically-derived
model called the International Reference Ionosphere. It provides
a monthly-averaged specification of the diurnally-variable
laminar ionosphere driven only by the season (i.e., the month) and
the sunspot number. Work has continued on improving its specification
in the topside domain [ Bilitza, 1994], of plasma temperatures and
ion composition [ Bilitza, 1992a], and of global and mesoscale
electron density distributions [ Bilitza, 1992b; and Szuszczewicz et
al., 1993b]. In its current form it provides no information on
sporadic-E and intermediate and descending layers, and its inaccuracies
at high latitudes can be mitigated by inclusion of the
NOAA/TIROS (Television and Infra-Red Observing Satellite), DMSP
(Defense Meteorological Satellite Program), or Feldstein auroral oval
models [ Szuszczewicz, et al., 1993b].
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Up: 2.1. Climatology
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U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33
Suppl., © 1995 American Geophysical Union