  The alignment of striated intensity structures in thin neutral hydrogen (HI)
spectroscopic channels with Galactic magnetic fields has been observed.
However, the origin and nature of these striations are still debatable. Some
studies suggest that the striations result solely from real cold-density
filaments without considering the role of turbulent velocity fields, i.e., the
velocity caustics effect in shaping the channel's intensity distribution. To
determine the relative contribution of density and velocity in forming the
striations in channel maps, we analyze synthetic observations of channel maps
obtained with simulations that represent realistic magnetized multi-phase HI.
We vary the thickness of the channel maps and apply the Velocity Decomposition
Algorithm to separate the velocity and density contributions. In parallel, we
analyze GALFA HI observations and compare the results. Our analysis shows that
the thin channels are dominated by velocity contribution, and velocity caustics
mainly generate the HI striations. We show that velocity caustics can cause a
correlation between unsharp-masked HI structures and far-infrared emission. We
demonstrate that the linear HI fibers revealed by the Rolling Hough Transform
(RHT) in thin velocity channels originate from velocity caustics. As the
thickness of channel maps increases, the relative contribution of density to
fluctuations in channel maps also increases. As a result, more RHT-detected
fibers tend to be perpendicular to the magnetic field. Conversely, the
alignment with the magnetic field is the most prominent in thin channels. We
conclude that similar to the Velocity Channel Gradients (VChGs) approach, RHT
traces magnetic fields through the analysis of velocity caustics in thin
channel maps.
