AAS 203rd Meeting, January 2004
Session 110 Interstellar Medium I
Poster, Thursday, January 8, 2004, 9:20am-4:00pm, Grand Hall

## [110.05] Time Variations in Small-Scale Galactic HI Structure toward 3C 138

B.A. Zauderer (U. of Maryland), T.J. Lazio (Naval Research Laboratory), C. Brogan, W.M. Goss (NRAO), M.D. Faison (Reed College)

Several recent VLBI studies of Galactic HI absorption toward background quasars have shown that there are small-scale opacity changes in the neutral Galactic interstellar medium on size scales as small as a few AU (e.g. Faison et al. 1998; Faison & Goss 2001). These opacity changes could reflect changes in the HI spin temperature, the gas density, or both. In the current work we present three epochs of VLBA observations of Galactic HI absorption toward 3C 138. These data include two new VLBA epochs from 1999 and 2002 along with a reexamination of the 1995 VLBA data reported in Faison et al. (1998). 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 size scale spatial variations in the HI opacity at the level of \Delta\tau =0.52±.05. In addition, we see clear evidence for temporal variations in the HI opacity over the seven year time span of our three epochs of data. We also attempted to detect the magnetic field strength in the HI gas using the longitudal Zeeman effect (via the Stokes V parameter). From this analysis we have been able to place a three sigma upper limit on the magnetic field strength of 60-90\mu G.

Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.