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3.1. Magnetic fields and Interstellar Superbubbles

In addition to the large-scale (regular and random) components of the galactic magnetic field, there are localized deviations (or detours) of the magnetic field, occasioned by interstellar magnetized superbubbles around OB associations and supernovae, with typical superbubble diameters of approx 200 - 250 pc.

Figure 6 shows the local deviations of the regular large scale magnetic field lines due to the presence of local superbubbles with a known supershell magnetic field.

Figure 6

Figure 6. Face-on view of the local area of the Milky Way disk, showing the nearby superbubbles with a detected shell magnetic field. Arrows show the direction of the local galactic magnetic field lines. The expansion of the superbubbles caused local deviations or detours of the regular component of the galactic magnetic field. Two nearby spiral arms of stars are shown (Perseus arm, Sagittarius arm). The Sun's location is shown (circled dot). The location of the nearest magnetic field reversal is shown (dashes). See Vallée (1996) for more details.

The pioneering theories of galactic dynamos were made by Parker, 1971a; Parker, 1971b; Parker, 1973; Parker, 1976, Parker, 1979, and by others since (e.g., Ruzmaikin et al. 1985; Ruzmaikin et al. 1988). These early theories proved exciting and of great potential. They already could explain many diverse features in the Milky Way (e.g., Vallée 1991b).

Large interstellar magnetic bubbles are becoming necessary ingredients in the theories of galactic dynamos. More recent theories of galactic dynamos use cosmic-ray-driven dynamos (e.g., Parker 1992). Cosmic-ray-driven dynamos are dynamos powered by cosmic rays which originate in fast stellar winds of O-type stars inside interstellar bubbles, and in exploding supernovae (inside or outside interstellar bubbles).

The observations of the properties of large interstellar magnetic bubbles are compatible with the recent predictions of galactic dynamos (Vallée, 1993e), notably the mean shell magnetic field strength, the mean k = 1 exponent in the shell, and the mean shell expansion speed.

Zweibel (1996) also studied large scale fluctuations of the magnetic fields for the equilibrium and stability of the interstellar medium. The local magnetic field direction within 1000 pc was found some 30 years ago to differ according to the method used, and that difference was elucidated first by Vallée (1973) and Vallée and Kronberg (1973) as being almost entirely due to the effect of the nearby North Polar Spur a.k.a. Loop I. Others since, notably Heiles (1996a), have confirmed the strong effect of Loop I. Simonetti and Cordes (1986) and Simonetti et al. (1984) have studied enhanced magnetic turbulences along lines of sight going through various superbubbles. The HI shell is outside the synchrotron shell in Loop I (Heiles et al., 1980), both contributing to the rotation measure.

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