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

Abstract

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 R₄≡ΣI(844.6 nm)/ΣI(777.4 nm)=0.20+/-0.02, very 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(N_e)>11.5 and that collisional excitation is more important than the photoexcitation by accidental resonance mechanism.