Spatial and temporal variations in small-scale Galactic HI structure toward 3C 138

Several recent studies of Galactic H I absorption toward background quasars and pulsars have provided evidence that there are opacity changes in the neutral Galactic interstellar medium on size scales as small as a few AU. The nature of these opacity variations has remained a matter of debate but could reflect a variety of physical processes, including changes in the H I spin temperature or gas density. We present three epochs of Very Long Baseline Array (VLBA) observations of Galactic H I absorption toward the quasar 3C 138 with resolutions of 20 mas (similar to 10 AU). This analysis includes VLBA data from observations in 1999 and 2002 along with a reexamination of the 1995 VLBA data, reported by Faison and coworkers. Improved data reduction and imaging techniques have led to an order of magnitude improvement in sensitivity compared to previous work. With these new data we confirm the previously detected milliarcsecond-scale spatial variations in the H I opacity at the level of Delta tau(max) = 0.50 +/- 0.05. The typical size scale of the optical depth variations is similar to 50 mas or 25 AU. In addition, for the first time we see clear evidence for temporal variations in the H I opacity over the 7 yr time span of our three epochs of data. We also attempt to detect the magnetic field strength in the H I gas using the Zeeman effect. From this analysis we have been able to place a 3 sigma upper limit on the magnetic field strength per pixel of similar to 45 mu G. We have also been able to calculate for the first time the plane-of-sky covering fraction of the small-scale H I gas of similar to 10%. This small covering fraction suggests that the filling factor of such gas is quite low, in agreement with recent optical observations. We also find that the linewidths of the milliarcsecond- scale H I features are comparable to those determined from previous single-dish measurements toward 3C 138, suggesting that the opacity variations cannot be due to changes in the H I spin temperature. From these results we favor a density enhancement interpretation for the small-scale H I structures, although these enhancements appear to be of short duration and are unlikely to be in equilibrium.