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Limits on Turbulent HI Fluctuations Towards PSR B0329+54 On Scales Between 0.0025 and 12.5 AU

We have measured the HI absorption towards pulsar B0329+54 using the Green Bank Telescope during 18 epochs between 2002 June 30 and 2003 October 10. Three observing epochs consisted of a continuous period of 20 hours each, while fifteen epochs were 1-2 hours each. We calculate the structure function of HI absorption variations toward the pulsar on time scales of 10 minutes to 16 months which, using the proper motion of 95 km/s and the parallactic distance of 1.03 kpc measured towards B0329+54 (Brisken et al. 2002), corresponds to angular scales of 0.37 micro-arcseconds to 23.8 milli-arseconds and samples structures between 0.0025-12.5 AU assuming HI gas half way to the pulsar and ignoring scintillation effects. We find no evidence for any turbulent HI absorption fluctuations towards B0329+54, with the following upper limits on the change in opacity for various absorption features: 0.026 at -31, -21, -18, and +4 km/s, 0.12 at -11 km/s, and 0.055 at -1 km/s.