2023MNRAS.526.4345K

We characterize the kinematic and magnetic properties of H I filaments located in a high Galactic latitude region (165° < α < 195° and 12° < δ < 24°). We extract three-dimensional filamentary structures using fil3d from the Galactic Arecibo L-Band Feed Array H I (GALFA-H I) survey 21-cm emission data. Our algorithm identifies coherent emission structures in neighbouring velocity channels. Based on the mean velocity, we identify a population of local and intermediate velocity cloud (IVC) filaments. We find the orientations of the local (but not the IVC) H I filaments are aligned with the magnetic field orientations inferred from Planck 353 GHz polarized dust emission. We analyse position-velocity diagrams of the velocity-coherent filaments, and find that only 15 per cent of filaments demonstrate significant major-axis velocity gradients with a median magnitude of 0.5 km s^–1 pc^–1, assuming a fiducial filament distance of 100 pc. We conclude that the typical diffuse H I filament does not exhibit a simple velocity gradient. The reported filament properties constrain future theoretical models of filament formation.