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