2023ApJ...958..192M

Interstellar turbulence shapes the H I distribution in the Milky Way (MW). How this affects large-scale statistical properties of H I column density across the MW remains largely unconstrained. We use the ∼13,000 deg² GALFA-H I survey to map statistical fluctuations of H I over the ±40 km s^–1 velocity range. We calculate the spatial power spectrum (SPS) of the H I column density image by running a 3° kernel and measuring the SPS slope over a range of angular scales from 16' to 20°. Due to GALFA's complex observing and calibration strategy, we construct detailed estimates of the noise contribution and account for GALFA beam effects on the SPS. This allows us to systematically analyze H I images that trace a wide range of interstellar environments. We find that the SPS slope varies between ∼ -2.6 at high Galactic latitudes and ∼ -3.2 close to the Galactic plane. The range of SPS slope values becomes tighter when we consider H I optical depth and line-of-sight length caused by the plane-parallel geometry of the H I disk. This relatively uniform, large-scale distribution of the SPS slope is suggestive of large-scale turbulent driving being a dominant mechanism for shaping H I structures in the MW and/or the stellar feedback turbulence being efficiently dissipated within dense molecular clouds. Only at latitudes above 60° do we find evidence for the H I SPS slope being consistently more shallow. Those directions are largely within the Local Bubble, suggesting that the recent history of this cavity, shaped by multiple supernova explosions, has modified the turbulent state of H I and/or fractions of H I phases.