3.3.2. Location of zero B_reg

Where does the magnetic field reverse, i.e., where can one find the field strength B_reg close to zero ? The radio observations of synchrotron emission (~ B_tot) from nearby spiral galaxies at radio frequencies (100 MHz to 1 GHz, say) show a stronger radio emission within the arms, than in the interarm regions (e.g., Fig. 4 in Segalovitz 1977). Thus B_tot is maximum within the spiral arms.

The thermal electron density is observed to be strongest within the spiral arms, not in the interarm regions (e.g., Fig. 4 in Taylor & Cordes 1993). For thermal densities n > 100 cm^-3, the observed magnetic field is tied to the observed thermal electron density by the statistical relation B_tot ~ n^0.5 , supported by many observations (e.g., Troland and Heiles 1986; Heiles 1987; Fig. 4 in Vallée 1990b; Fig. 1 here), and both B_tot and n are thus found to be strongest in the spiral arms.

In nearby spiral galaxies, B_reg is highly tangled in the arms (yielding a low degree of polarization there), and B_reg is strongest in the interarm regions (yielding B lines parallel to the optical arms, yet offset by a distance from the arms) (e.g., Beck 1991a; Beck 1991b).

The observed magnetic field reversal B_reg nearest to the Sun occurs about 600 pc radially inwards toward the galactic center (e.g., Fig. 5 in Thomson & Nelson 1980; Fig. 6 here). This field reversal occurs at a location within an interarm region (about halfway between the Sagittarius arm and the Perseus arm). This large-scale magnetic field reversal near r_g = 7.5 kpc has been observed through the RM data from QSOs and galaxies, as well as from the RM data of pulsars - it is not limited to the local Orion spur in which the Sun is placed.

Also in the Milky Way, the second nearest observed magnetic field reversal occurs near r_g = 5.5 kpc (with the Sun at r_g = 8.0 kpc), i.e., located about 2.5 kpc from the Sun toward the galactic center (e.g., Fig. 1 of Vallée et al. 1988b; Fig. 5 of Rand and Lyne 1994; Fig. 9 here). This magnetic field reversal also occurs within an interarm region, about halfway between the Scutum arm and the Sagittarius arm (e.g., not within a spiral arm). This large scale magnetic field reversal has been observed through the RM data of QSO and galaxies, and independently from the RM data of pulsars.

Table 2 summarizes what we know and when we first detected the various orientations of the magnetic field in the Milky Way. In our Milky Way, the regular galactic magnetic field seems to follow the spiral arms, and to have the axisymmetric magnetic field shape m_azim = 0. Between galactic radii 5.5 and 7.5 kpc from the galactic center, the regular magnetic field reverses in direction (going anticlockwise) compared to the rest of the Galaxy (where it goes clockwise). Future trends: more observations are needed in the Perseus + II spiral arm, and more external spiral arms, to test more possible magnetic field reversals, as well as toward the center of the Galaxy (from the Galactic center up to the Scutum spiral arm).

The dynamo theories show that the location of a magnetic field reversal (B_reg = 0) can move slowly in radial direction with time (e.g., Fig. 1a-1f in Beck et al. 1994; Fig. 4a in Poezd et al. 1993).