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We present a study of the small-scale structure of the interstellar medium inthe Milky Way. We used HST STIS data to measure N(HI) in a pencil-beam toward59 AGNs and compared the results with the values seen at 9 arcmin - 36 arcminresolution in the same directions using radio telescopes (GBT, Green Bank140-ft and LAB survey). The distribution of ratios N(Lya)/N(HI) has an averageof 1 and a dispersion of about 10%. Our analysis also revealed that spectrafrom 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 adistribution showing similarities to simple descriptions of hierarchicalstructure in the neutral ISM, as well as to a more sophisticated 3D MHDsimulation. From the comparison with such models, we find that the sonic Machnumber of the local ISM should lie between 0.6 and 0.9. However, none of themodels yet matches the observed distribution in all details, but with many moresightlines (as will be provided by COS) our approach can be used to constrainthe properties of interstellar turbulence.

Measuring turbulence in the ISM by comparing N(HI;Lya) and N(HI;21cm)