Spatial and Temporal Variations in Small-Scale Galactic HiStructure toward 3C 138

We present three epochs of VLBA observations of Galactic HI absorption towardthe quasar 3C~138 with resolutions of 20 mas (~ 10 AU). This analysis includesVLBA data from observations in 1999 and 2002 along with a reexamination of 1995VLBA data. Improved data reduction and imaging techniques have led to an orderof magnitude improvement in sensitivity compared to previous work. With thesenew data we confirm the previously detected milliarcsecond scale spatialvariations in the HI opacity at the level of Delta(tau_{max}) =0.50 \pm 0.05.The typical size scale of the optical depth variations is ~ 50 mas or 25 AU. Inaddition, for the first time we see clear evidence for temporal variations inthe HI opacity over the seven year time span of our three epochs of data. Wealso attempted to detect the magnetic field strength in the HI gas using theZeeman effect. From this analysis we have been able to place a 3 sigma upperlimit on the magnetic field strength per pixel of ~45 muG. We have also beenable to calculate for the first time the plane of sky covering fraction of thesmall scale HI gas of ~10%. We also find that the line widths of themilliarcsecond sizescale HI features are comparable to those determined fromprevious single dish measurements toward 3C~138, suggesting that the opacityvariations cannot be due to changes in the HI spin temperature. From theseresults we favor a density enhancement interpretation for the small scale HIstructures, although these enhancements appear to be of short duration and areunlikely to be in equilibrium.Comment: 34 pages, 8 figures. Figures 3 & 4 are in color. Accepted to A