Some Constraints on Magnetic Fields in Galactic Spiral Arms

After a short summary of existing contradicting theories and observations of the longitudinal and vertical components of the magnetic fields in galactic spiral arms, the authors present some constraints on the nature of this field in the light of recent investigations of the rolling motion phenomenon. These vertical structures of H I spiral arms are explained in the context of the galactic fountain model and of vertical magnetic field components. The corrugation structure of the spiral arms also sheds light on the magnetic field structure. Simple energetic requirements for the field are derived. Various components of galactic spiral arms show equal scale properties, which can be understood in terms of the energy equipartition.