EXCITATION OF O-I LINES IN THE SOLAR CHROMOSPHERE

Observations of O I lines in the solar spectrum are examined to determ ine whether differences in behavior of lines of the quintet and triple t term systems are consistent with collisional excitation and/or photo excitation of both quintets and triplets. Intensities, I(IR), in near- infrared emission lines observed above the limb at total eclipse decre ase exponentially with height h. The inverse scale heights (d ln I(IR) /dh) for the triplet lines at 844.6 nm and quintet lines at 777.2 nm a re found to be in the ratio of 1.45. Ultraviolet O I emission-line int ensities I(UV) observed on the solar disk show strong variations, and the distributions of triplet (130.4 nm) and quintet line intensities a bout the means are different. Variances in ln I(UV) are found to have a triplet-to-quintet ratio of 1.50, in close agreement with the ratio of d ln I(IR)/dh. It is shown that the simple assumption of collisiona l excitation of quintets and triplets coupled with collisional de-exci tation of the quintets leads to the correct ratios for both the UV var iances and d ln I(IR)/dh. Also, under this assumption d ln I(IR)/dh fo r the quintet lines is predicted to have the same value as d ln I/dh a t the head of the hydrogen Balmer continuum, which, in fact, it does. On the other hand, Carlsson & Judge (1993) have shown that collision r ates computed from the Vernazza, Avrett, & Loeser (1981, hereafter VAL ) model chromosphere using current estimates of O I collision strength s are too low to produce the observed mean intensity in O I 130.4 nm. In a similar sense, we find that the predicted intensity of O I 130.4 nm is much too weak relative to O I 135.6 nm, and that the VAL mean mo dels A-F cannot reproduce the observed behavior of these lines, even i ncluding photoexcitation by H Lybeta. These difficulties are removed b y increasing specific electron-atom collision rates. Such increases co uld reflect (unacceptably?) large errors in atomic cross sections clos e to threshold and/or the inadequacy of the assumptions made by VAL fo r predicting line intensities. The nonlinear dependence of line intens ities on temperature and density, especially for far-UV lines, makes t he latter alternative a likely factor. We conclude that the O I UV lin es are very sensitive to inhomogeneities, much more so than more tradi tional chromspheric lines (e.g., Mg II k) which are formed over simila r regions of the chromosphere. Such lines could therefore provide valu able diagnostics of departures of the chromospheric plasma from mean m odels and thereby place constraints upon heating mechanisms, once accu rate atomic data become available.