Magnetic fields in the spiral arms of galaxies

Ordered magnetic fields observed in the Galaxy are approximately aligned along the axis of the spiral arms. The magnetic field strength is 3 microgauss but the total energy of the galactic magnetic field is as high as 1055ergs. The magnetic field direction is observed to reverse between adjacent spiral arms and this configuration requires an electric current sheet to loop each arm. Mechanisms for creating an electric current are discussed and attention is focused on the possibility of a net proton current. This requirement may be compatible with the indications that partially ionized gas is rolling round the spiral arms as they orbit the Galaxy. There is evidence which suggests that the direction of roll of the gas also reverses between adjacent spiral arms. There is a well known coincidence between the energy density of the galactic magnetic field and the energy density of turbulent gas motions, both are close to 1eVcm–3. Possible origins for these energies and the implications of this coincidence are discussed.