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New Measurements of the Solar Limb Emission in the O I 777.4 and 844.6 Nanometer Lines

Slit spectra of the solar limb emission in the 777.4 and 844.6 nm O i lines are calibrated against the solar disk center brightness. Data from CCD spectrograph instruments at two different observatories show limb emission with identical line intensities and measure a value of , very R4 { O I(844.6 nm)/ O I(777.4 nm) 5 0.20 5 0.02 different from the ratios computed from early photographic measurements of these lines. Implied by this new value of R4 is that O i lines in the solar chromosphere are formed in regions with electron densities log (Ne ) 1 11.5 and that collisional excitation is more important than the photoexcitation by accidental resonance mechanism.

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