Variable Na I absorption toward ρ Leonis:: Biased neutral formation in the diffuse interstellar medium?

We present multiepoch KPNO coude feed telescope observations of interstellar Na I and Ca II absorption toward the bright star rho Leo. Comparisons of the Na I profiles observed over a period of 8 yr reveal significant temporal variations in the Na I column in at least one component, implying that there is structure at scales on the order of the proper motion (similar to 12 AU). Archival Hubble Space Telescope (HST) Goddard High Resolution Spectrograph observations of the C I fine-structure excitation in the variable component suggest that the density is less than or similar to20 cm(-3), significantly lower than the densities inferred in past H I 21 cm and Na I studies. We suggest that the bulk of the trace neutral species are in the density peaks within an interstellar cloud. The patchy distribution of these species naturally gives rise to the large fluctuations seen on scales of 10 - 1000 AU in past temporal and binary studies. This picture predicts that the scales over which fluctuations will be observed vary as a function of the ionization rate of a species.