  We present a study of the filamentary structure in the atomic hydrogen (HI)
emission at the 21 cm wavelength toward the Galactic plane using the
observations in the HI4PI survey. Using the Hessian matrix method across radial
velocity channels, we identified the filamentary structures and quantified
their orientations using circular statistics. We found that the regions of the
Milky Way's disk beyond 10 kpc and up to roughly 18 kpc from the Galactic
center display HI filamentary structures predominantly parallel to the Galactic
plane. For regions at lower Galactocentric radii, we found that the HI
filaments are mostly perpendicular or do not have a preferred orientation with
respect to the Galactic plane. We interpret these results as the imprint of
supernova feedback in the inner Galaxy and Galactic rotation in the outer Milky
Way. We found that the HI filamentary structures follow the Galactic warp and
that they highlight some of the variations interpreted as the effect of the
gravitational interaction with satellite galaxies. In addition, the mean scale
height of the filamentary structures is lower than that sampled by the bulk of
the HI emission, thus indicating that the cold and warm atomic hydrogen phases
have different scale heights in the outer galaxy. Finally, we found that the
fraction of the column density in HI filaments is almost constant up to
approximately 18 kpc from the Galactic center. This is possibly a result of the
roughly constant ratio between the cold and warm atomic hydrogen phases
inferred from the HI absorption studies. Our results indicate that the HI
filamentary structures provide insight into the dynamical processes shaping the
Galactic disk. Their orientations record how and where the stellar energy
input, the Galactic fountain process, the cosmic ray diffusion, and the gas
accretion have molded the diffuse interstellar medium in the Galactic plane.
