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 VLBA observations of Galactic H i absorption toward the quasar 3C 138 with resolutions of 20 mas ( \sim 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 et al . ( 18 ) .
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 \pm 0.05 .
The typical size scale of the optical depth variations is \sim 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 seven year time span of our three epochs of data .
We also attempted 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 \sim 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 \sim 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 line widths of the milliarcsecond sizescale H i features are comparable to those determined from previous single dish measurements toward 3C 138 , suggesting that the opacity variations can not 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 .