Results are reported from the HST High Resolution Spectrograph observations of absorption features of C IV and Si IV in the spectrum of the high-latitude O star HD 93521 (l = 183 deg; b = 62 deg). A comparison of Si IV and C IV profiles showed that the FWHM of both features is about 50 km/sec, in contrast to the 7 km/sec found for one of the several S II features. The line centers for C IV and Si IV are at v = -67 km/sec and -60 km/sec, respectively. As the interval velocity decreases from 90 to 50 km/sec, the Si IV/C IV ratio of the column density per unit velocity interval increases from about 0.2 to 0.4. The result is qualitatively consistent with a fountain model of Shapiro and Benjamin (1991) if the slower gas has cooled and recombined more than the faster gas.
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Lyman et al. (1992) studied this question.