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