Limits on Turbulent H I Fluctuations toward PSR B0329+54 on Scales between 0.0025 and 12.5 AU

We have measured the H I absorption toward 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 hr each, while 15 epochs were 1-2 hr each. We calculate the structure function of H I absorption variations toward the pulsar on timescales of 10 minutes to 16 months, which using the proper motion of 95 km s-1 and the parallactic distance of 1.03 kpc measured toward B0329+54 (Brisken et al.), corresponds to angular scales of 0.37 μas to 23.8 mas and samples structures between 0.0025 and 12.5 AU, assuming H I gas halfway to the pulsar and ignoring scintillation effects. We find no evidence for any turbulent H I absorption fluctuations toward B0329+54, with the following upper limits on Δτ for various absorption features: 0.026 at -31, -21, -18, and +4 km s-1; 0.12 at -11 km s-1; and 0.055 at -1 km s-1.