For many purposes, the F-Corona is considered a nuisance whose contribution
must be removed before one can study something interesting, such as the
solar wind. In the UV, the F-Corona will be faint because of the
faintness of the Sun at UV wavelengths. It may be detectable,
however, and observations of the F-Corona in the
ultraviolet can contribute to the understanding
of the size distribution of dust grains and the spatial distribution of
dust in the inner solar system. The dust is believed to originate in comets,
and we may be able to observe variations in the F-Corona brightness when
comets pass close to the Sun. At optical wavelengths the F-Corona declines
from about 
of the disk brightness near the limb to about
at 5 
. Small particles tend to scatter short
wavelength light effectively, and interstellar dust shows a power law
distribution of grain sizes which leads to an order of
magnitude more effective scattering in the UV than in the optical.
In the UV, the F-Corona should show a spectrum similar to that of the disk,
modified by gradual changes in albedo and
scattering cross section with wavelength. This spectrum can be separated
from that of the coronal lines by the low ionization species in the disk
spectrum. C III 
977 and Si III 
1206 are among the very
bright lines formed at temperatures below 
K which are absent
from the coronal spectrum. These species may occasionally be present in
the corona, but only in transient phenomena such as plasmoids or CMEs.
Electron-scattered low ionization disk lines will also be present in
the K-Corona spectrum, but they will be broadened so much as to be
easily distinguishable, and only Ly 
is likely to be detectable.
Distinguishing between the F-Corona and instrumental scattering will be more difficult. The dust is concentrated toward the ecliptic plane, and LASCO can measure the degree of flattening at optical wavelengths. Any instrumental scattering should be independent of roll angle so a series of observations like the synoptic series with longer integration times should separate the F-Corona and instrumental contributions. Another approach would be to point at a specific position in the corona, then repeat the observation with a different combination of offset pointing and mirror position chosen to reach the same position in the corona. That should change the instrumental scattering contribution without affecting the F-Corona contribution. A series of such observations might help to disentangle the contributions.
The Table gives as an example a pointing at 2 
assuming a scattered
intensity of 
of the disk intensity
(corresponding to a size distribution 
) and
the Quiet Sun disk intensities of Vernazza and Reeves (1978). The
assumed brightness gives about 80 counts in the C III line in a 2 hour
integration. A wider slit would reduce the required integration time.
The F-Corona contribution to Ly 
is only 1-2% of that expected
from a coronal hole at 2 
UV Observations of the F-Corona
