  We present multi-epoch KPNO Coude Feed observations of interstellar NaI and
CaII absorption toward the bright star rho Leo. Comparisons of the NaI profiles
observed over a period of 8 years reveal significant temporal variations in the
NaI column in at least one component, implying that there is ``structure'' at
scales of order of the proper motion (~12 AU). Archival HST Goddard High
Resolution Spectrograph observations of the CI fine-structure excitation in the
variable component suggest that the density is <20 cm-3, significantly lower
than the densities inferred in past HI 21 cm and NaI 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--1,000 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.
