Measuring turbulence in the ISM by comparing N(HI;Lya) and N(HI;21cm)
  We present a study of the small-scale structure of the interstellar medium in
the Milky Way. We used HST STIS data to measure N(HI) in a pencil-beam toward
59 AGNs and compared the results with the values seen at 9 arcmin - 36 arcmin
resolution in the same directions using radio telescopes (GBT, Green Bank
140-ft and LAB survey). The distribution of ratios N(Lya)/N(HI) has an average
of 1 and a dispersion of about 10%. Our analysis also revealed that spectra
from the Leiden-Argentina-Bonn (LAB) all-sky HI survey need to be corrected,
taking out a broad gaussian component (peak brightness temperature 0.048 K,
FWHM 167 km/s, and central velocity -22 km/s). The column density ratios have a
distribution showing similarities to simple descriptions of hierarchical
structure in the neutral ISM, as well as to a more sophisticated 3D MHD
simulation. From the comparison with such models, we find that the sonic Mach
number of the local ISM should lie between 0.6 and 0.9. However, none of the
models yet matches the observed distribution in all details, but with many more
sightlines (as will be provided by COS) our approach can be used to constrain
the properties of interstellar turbulence.
