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Since the ionosphere
is produced mainly by solar X-ray, EUV and Ly-alpha fluxes,
information about this radiation is necessary for studies of
both ionospheric weather and climatology. Work continues to improve
the accuracy in the specification of these fluxes and the
relationship between the long wavelength radiation (10.7 cm) and
the shorter ionizing components which drive the chemistry and dynamics
of the lower thermosphere. Since reliable day-to-day, month-to-month,
and season-to-season measurements of the shorter wavelengths are
not available, the 10.7 cm flux (which can be monitored from the ground)
is traditionally used as the index for solar ionization intensity. This
is particularly important to modelers who use 10.7 cm fluxes as proxies
for ionization radiation. Studies of solar wavelength variabilities
have shown that proxy indicators of solar ionizing radiation, like the
10.7 cm flux, can be a source of error. On a long-term basis (i.e.,
solar cycle) short-wavelength fluxes have been found to vary linearly
with the 10.7 cm flux, with deviations from linearity only at solar
minimum and maximum [ Lean, 1991, Balan et al., 1994: and
Barth et al.
1990]. On a short-term basis (i.e., a solar rotation) the relationship
is irregular and unpredictable.
Efforts continue to measure the radiative output of the Sun
[ White et
al., 1994] and to validate and improve solar flux models used
in ionospheric/thermospheric investigations [ Buonsanto et al.
1992b; Richards et al., 1994; Fuller-Rowell, 1993; and
Tobiska,
1993]. Problems include accuracies in the absolute levels of
various portions of the solar spectrum and the inability of solar
flux models to satisfactorily reproduce measured
photoelectron observations. However, Richards et al. [1994]
recently developed a solar flux module for aeronomic applications in
the FLIP model which reproduces photoelectron spectra that are in
reasonable agreement with spectra observed at F-region altitudes
during solar minimum and solar maximum periods. Important problems
related to the solar spectrum include determination of photoionization
and photoabsorption cross sections, oscillator strengths, and
transition probabilities. These not only affect ionospheric
and thermospheric models but also impact the accuracy of
inversion techniques using optical remote sensing to determine
state variables. For example, Chang et al. [1993], in a study of
O
(
P) reaction rate coefficients at F-region altitudes, found that
15% uncertainties in the solar EUV flux caused uncertainties of up to
a factor of 1.5 in the rate of O
(
P) quenching
via O
(
P)
O 
O
(
S,
D)+O.
Next: 3.1.2. Thermospheric influences.
Up: 3.1. Understanding Solar
Previous: 3.1. Understanding Solar
U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33
Suppl., © 1995 American Geophysical Union