Spectra for PSR B1557−50. First panel: H I emission scaled to match GASS in the direction of the pulsar with a velocity resolution of 0.1 km s⁻¹. Second panel: velocity-binned H I absorption spectra taken at E1 (black) and E2 (red) with a channel width of ∼1.7 km s⁻¹. I(v)/I₀ is as defined in Equation (2). Third panel: distance vs. radial velocity curve, calculated from Mróz et al. (2019) using the best-fit linear model with R₀ = 8.09 kpc, Θ₀ = 233.6 km s⁻¹. Fourth panel: velocity-binned difference H I absorption profiles (﹩{\rm{\Delta }}I(v)/{I}_{0}={[I(v)/{I}_{0}]}_{{\rm{E}}2}-{[I(v)/{I}_{0}]}_{{\rm{E}}1}﹩) with a channel width of 1.7 km s⁻¹, before (black) and after (red) mean-filtering. Red and black dashed lines represent ±2σ and 3σ noise envelopes where the contribution from H I emission has been taken into account. The blue channel represents the TSAS we have identified with an S/N of 3.2. The blue horizontal line represents 0 difference. Fifth panel: spatial scale as a function of velocity. The dashed–dotted line represents the maximum spatial scale corresponding to the transverse distance traveled by the pulsar between the two epochs.