3.5.1. ASS for the Milky Way

Observations show that the large scale magnetic field in the Milky Way can be matched by predictions of the turbulent dynamo theory. The dynamo theory predicts, in cylindrical coordinates, an azimuthal mode m_azim predominantly of an axisymmetric shape (m_azim = 0), a radial mode n that ensures a few reversals (n = 0, 1, 2) of the azimuthal mode with increasing galactic radius, and a vertical mode p (perpendicular to the galactic disk) with the magnetic field structure being symmetric above and below the galactic plane (p = 0). The limited knowledge of the radial gravitational potential of the Milky Way can theoretically allow two or three magnetic field reversals with galactic radius, and 2 are observed (Vallée, 1991b).

As viewed from the top of the Galaxy (North Galactic Pole), the magnetic field lines run clockwise near the Sun (r_gal = 8 kpc), near the Perseus arm (r_gal = 10 kpc), and near the Scutum arm (near r_gal = 5.5 kpc), but they run counterclockwise near the Sagittarius arm (near r_gal = 6.5 kpc). Going out from the galactic nucleus, a total of 2 magnetic field reversals have thus been seen, near r_g of 5.5 kpc and 7.5 kpc.

When it comes to the magnetic field in the Milky Way galaxy, our position in the Milky Way's galactic disk hinders our attempts at interpreting the observational data. Some recent papers have attempted to fit magnetic field models to spiral galaxies, and in particular to the Milky Way galaxy. Magnetic field reversals in the Milky Way are crucial to all interpretations, be they axisymmetric (ASS) or bisymmetric (BSS) global magnetic field models. Only a few magnetic field reversals can be found in ASS models, but 4 or more radial field reversals can only be found in tightly-wound BSS models.

Magnetic field observations in the Milky Way have already been fitted to an ASS model with a few radial reversals (Fig. 3 in Vallée, 1991b; Fig. 4a in Poezd et al., 1993). Dynamo theories already predict an ASS magnetic field model with 2 or 3 radial reversals (e.g., Fig. 1f in Beck et al. 1994a; Fig. 4a in Poezd et al. 1993; Fig. 5 in Ruzmaikin et al. 1985). Thus there is no need for a BSS model a priori for the Milky Way.

Statistics of the observed magnetic fields in a dozen nearby spiral galaxies indicate a net preference for the ASS model (about 3/5, including the Milky Way galaxy), a minority of cases with the BSS mode (about 2/5 of cases), and no galaxy with a quadrisymmetric spiral magnetic field QSS mode (e.g., Wielebinski & Krause 1993; Vallée 1994a).