Local HI filaments driven by a small-scale dynamo. Update on the
  velocity decomposition algorithm
  Context. HI filaments are closely related to dusty magnetized structures that
are observable in the far infrared (FIR). Recently it was proposed that the
coherence of oriented HI structures in velocity traces the line of sight
magnetic field tangling. Aims. We study the velocity-dependent coherence
between FIR emission at 857 GHz and HI on angular scales of 18 arcmin. Methods.
We use HI4PI HI data and Planck FIR data and apply the Hessian operator to
extract filaments. For coherence, we require that local orientation angles
{\theta} in the FIR at 857 GHz along the filaments be correlated with the HI.
Results. We find some correlation for HI column densities at |v_LSR | < 50 km/,
but a tight agreement between FIR and HI orientation angles {\theta} exists
only in narrow velocity intervals of 1 km/s. Accordingly, we assign velocities
to FIR filaments. Along the line of sight these HI structures show a high
degree of the local alignment with {\theta}, as well as in velocity space.
Interpreting these aligned structures in analogy to the polarization of dust
emission defines an HI polarization. We observe polarization fractions of up to
80%, with averages of 30%. Orientation angles {\theta} along the filaments,
projected perpendicular to the line of sight, are fluctuating systematically
and allow a characteristic distribution of filament curvatures to be
determined. Conclusions. Local HI and FIR filaments identified by the Hessian
analysis are coherent structures with well-defined radial velocities. HI
structures are also organized along the line of sight with a high degree of
coherence. The observed bending of these structures in the plane of the sky is
consistent with models for magnetic field curvatures induced by a Galactic
small-scale turbulent dynamo.
