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Many
ionospheric structures and virtually all ionospheric dynamics
are controlled by kinetic and chemical coupling to the thermosphere
and electrodynamic coupling to the magnetosphere and between
ionospheric domains (electric fields have been discussed in Section
2.1.2). The thermosphere provides the neutrals for photoionization.
The winds transport ions up and down field lines, and their
kinetic cross-field forces on the ionosphere create dynamo fields in
the overall electrodynamic circuit. In the lower thermosphere,
tides represent the dominant motion, with the advent of multi-day
LTCS campaigns (Lower Thermospheric Coupling Study) only recently
advancing the basic understanding of their latitudinal, seasonal, and
solar cycle variations [e.g., Fesen et al. 1993; Hagan,
1994]. Fesen
[1992] reviewed the tidal effects on the thermosphere, while
Buonsanto [1991] studied tidal components above Boulder, CO over a
full solar cycle using ionosonde measurements and servo-model techniques.
He found significant components with wave periods of 24, 12, 8, and 6
hours and inferred the importance of ion drag in causing seasonal and
solar cycle variations in the winds. K.L. Miller et al. [1993] in a
global SUNDIAL campaign studied the components of the meridional winds
and their effects on F-region heights and densities. He developed
a climatology for the winds during the solar minimum solstitial period
of June 1987 and compared it with daily variations under quiet
and disturbed conditions. Hagan [1993] also studied climatological
aspects of thermospheric winds, focusing on seasonal and solar
cycle averages of Millstone Hill data over the 1984--1990 period.
Her analysis suggests that high-latitude circulation cells are needed
to interpret thermospheric variability over Millstone Hill even
during geomagnetically quiet periods. On a more global scale, Hedin et
al. [1991] revised the HWM87 global model of horizontal thermospheric
winds using satellite and ground-based observations, providing an
improved empirical framework for global-scale specification of
the winds.
Next: 3.2. Summary of
Up: 3.1. Understanding Solar
Previous: 3.1.1. Ionization and
U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33
Suppl., © 1995 American Geophysical Union