Corrugated velocity patterns in the spiral galaxies: NGC 278, NGC 1058,
  NGC 2500 \& UGC 3574
  We address the study of the \Ha\ vertical velocity field in a sample of four
nearly face-on galaxies using long slit spectroscopy taken with the ISIS
spectrograph attached to the WHT at the Roque de los Muchachos Observatory
(Spain). The spatial structure of the velocity vertical component shows a
radial corrugated pattern with spatial scales higher or within the order of {
one} kiloparsec. The gas is mainly ionized by high-energy photons: only in some
locations of NGC~278 and NGC~1058 is there some evidence of ionization by
low-velocity shocks, which, in the case of NGC~278, could be due to minor
mergers. The behaviour of the gas in the neighbourhood of the spiral arms fits,
in the majority of the observed cases, with that predicted by the so-called
hydraulic bore mechanism, where a thick magnetized disk encounters a spiral
density perturbation. The results obtained show that it is { difficult to
explain the \Ha\ large scale velocity field without the presence of a
magnetized, thick galactic disk}. Larger samples and spatial covering of the
galaxy disks are needed to provide further insight into this problem.
