4.4. Magnetic field B in galactic disks and neutral hydrogen HI
4.4.1. B shapes and HI masses
The type of global magnetism (either ring/axisymmetric, or spiral/bisymmetric) has been studied as a function of 21 different physical parameters for 9 spiral galaxies (Vallée, 1986). The first type (ring/axisymmetric) of magnetism appears to occur more frequently in a spiral galaxy if that galaxy possesses a large amount of HI gas mass (MHI a few × 109 M). Conversely, the second type (spiral/bisymmetric) appears to occur more freqently in a spiral galaxy possessing a lesser amount of HI gas mass. On the theoretical side, nonlinear dynamo models predict a mixture of modes, usually with a predominent mode and a weaker secondary mode.
When looking at the suggested triggers for the appearance of the bisymmetric magnetic field in nearby galaxies, many suggested triggers (mostly from the dynamo theory) were: macroscopic shocks from density waves, or rotational shears in differential rotation, or half-thickness of the ionised gas layer, or the effects of a single warp in the outer galaxy. Again, the most likely trigger for bisymmetric magnetism, measured in terms of differences in average values for mazim = 0 galaxies versus average values for mazim = 1 galaxies, was the smaller amount of HI gas mass in a spiral galaxy (Vallée, 1993a).
What is the observational possibility of seeing another azimuthal mode, mazim = 2 (quadrisymmetric magnetic field shape) ? A prediction was obtained (e.g., Equation 2 in Vallée 1992) that a lower HI gas mass in a spiral galaxy could allow for a higher magnetic field mode mazim, i.e., the observation of a galaxy with mazim = 2 would contain an amount of HI gas mass < 108 M.
A more recent study on the possible triggers of the global magnetic field shapes, using substantial corrections published on the galactic magnetic values, was made in Vallée (1993a). The updated data confirm the correlation between the HI gas mass and the azimuthal magnetic mode, i.e., the mazim = 0 galaxies have more galactic HI mass (Vallée, 1993a). Yet the best dynamo theories to date still do not include the amount of neutral HI gas, preferring to use the amount of ionised HII gas. The best argument for theoreticians to include neutral HI gas parameters in future dynamo models is the confirmed observational link between HI shapes (MD, see below) and ASS or BSS magnetic field shapes (mazim, see below).
Using the observed distribution of HI data in galaxies, and the critical HI mass separating mazim = 1 galaxies from mazim = 0 galaxies, a prediction of 1-to-1 equal predominance of mazim = 0 galaxies versus and mazim = 1 galaxies has been proposed (Vallée, 1993b).