4.5. Magnetic fields in Halos of spiral galaxies
Most normal galaxies have little or no radio halo, with the radio surface brightness perpendicular to the disk being best described by exponential functions with a median scale height of ~ 1 kpc (Beck, 1997b).
Bright, extended radio halos around spiral galaxies are very rare - exceptional, rather than normal. One of these rare galaxies with an extended halo is NGC 4631, with a scale height of ~ 2 kpc, magnetic field lines going vertically out of the galactic disk, possibly due to a galactic fountain or galactic wind (Hummel et al., 1991).
Future trends. Little is known for sure on magnetic field strengths and shapes in halos of spiral galaxies. Very preliminary data have been reviewed in Beck et al. (1996) and Beck (1997b), and some early comparisons were made with galactic disks (Beck, 1997a).
In the case of M51, Berkhuijsen et al. (1997) used a multi-layer 3-dimensional model for the position angles of the polarization at 4 different radio wavelengths, smoothed to a beam of 3.5 kpc. Their method involves the Fourier parameterization of the magnetic field structure into many radial, azimuthal, and vertical components inside each layer. They found a small predominence of the bisymmetric (BSS) global magnetic field shape in the galactic disk, notably for a radial distance < 6 kpc. They also found a predominence of the axisymmetric magnetic field shape in the galactic halo facing us, possibly due to a topological pumping of magnetic field by a galactic fountain flow. They could not say anything about the magnetic field in the side of the halo on the other side of the disk, since the nearly face-on galaxy is not transparent at 20cm. The general features of the magnetic fields seem to be in general agreement with predictions from dynamo theory, but detailed dynamo modelling is required to reach definite conclusions.