3.7.2. Future trends in Galactic Dynamos
Some criticism of the current dynamo theories has been voiced (e.g., Kim et al. 1996; Subramanian 1995; Kulsrud & Anderson 1992; Schramlowski & Achtenberg 1993) over some aspects of dynamo theory. Will the galactic dynamos survive the test of time ? Supporters of the galactic dynamos have started to look more closely at persistent problems (e.g., Parker 1981; Parker 1987, Field 1995; Subramanian, 1998). This subject is currently in a state of intense scrutiny, and improvements are needed in many areas. More complete theoretical treatments can be found in Cattaneo and Vashtein (1991), Vainshtein and Cattaneo (1992), Kulsrud and Anderson (1992), Tao et al. (1993), Kim et al. (1996), among others. More details on recent dynamo theories are reviewed in Wielebinski & Krause (1993), Kronberg (1994), and Beck et al. (1996).
As stated above, any primordial magnetic field may diffuse out of the disk through the halo and out into the intergalactic medium on a short time scale (~ 108 yrs), perhaps shorter than the time scale of the galaxy (Parker, 1971; Parker, 1976), or perhaps not that short (e.g., Kulsrud and Howard 1997). In more complex dynamo models, a weak primordial magnetic field may be found (e.g., Kulsrud & Anderson 1992) or may not be found (e.g., Field 1995) in the galactic disk.
Current dynamo models cannot yet explain the presence of strong regular interarm magnetic fields (e.g., Moss 1996a). Dobler et al. (1996) studied dynamo systems in which the thickness of the galactic disk grows with time, becoming so large as to suppress the dynamo action. A theoretical study by Fan and Lou (1996), about the locations of magnetic arms with respect to the locations of the stellar arms, used fast MHD density waves to give a coincidence of magnetic arms with stellar arms in disks with differential rotation, and used slow MHD density waves to give magnetic arms interlaced with stellar arms.
More recent developments include non-linear terms, which take account of the back reaction of the magnetic field on the motions. The full non-linear dynamo problem requires the solution of a complex non-linear system of partial differential equations, which is still beyond the access of even the best computers (e.g., Schultz et al. 1994). The magnetic field strength of the dynamo eventually exceeds the equipartition magnetic field by about a factor 2 (Rüdiger et al., 1993). Cross-helicity dynamos are being explored (Yokoi, 1996).
Thus to obviate the long length of time needed to activate the dynamo. Parker (1992) and others have since suggested a cosmic-ray driven galactic dynamo, requiring only minor changes to the basic equations for galactic dynamos as used to date. This suggestion needs to be followed up with numerical calculations. This model would replace the 'slow-acting' mean field dynamo driven by interstellar turbulence over a time scale of 3 × 109 years or so (e.g., Equ. 9 in Lesch 1993). The cosmic-ray-driven dynamo would operate over a shorter time scale of 3 × 107 years (e.g., Parker 1992). Parker (1992) used cosmic rays going out of the galactic plane to inflate the magnetic field lines into loops (cosmic-ray driven dynamo), providing a natural way for rapid reconnection between opposite vertically oriented magnetic field lines. Near the center of a loop one can expect a stellar association with stellar winds and supernovae that would generate the cosmic rays. The importance of cosmic-ray-driven dynamos comes from the discovery of strong galactic magnetic fields in quasars at an earlier epoch, requiring the need to build up quickly the galactic magnetic field there (Perry et al., 1993).
An important but perhaps not quite the same type of cosmic-ray-driven dynamo is pursued by Ferriére (1993a), Ferriére (1993b), and Ferriére (1993c).
There is a bit of confusion since Parker (1992) refered to his cosmic-ray-driven dynamo as 'fast-acting' dynamo, yet the term 'fast-acting' has been used with another meaning elsewhere, namely 'a dynamo in which the magnetic field is amplified exponentially in the limit that the plasma resistivity goes to zero' (e.g., Childress, Gilbert, and Ott 1996).
Using a global smoothing, Han and Qiao (1994) suggested a S1 dynamo (mazim = 1; p = 0) in the Milky Way, while Han et al. (1997) suggested an A0 dynamo (mazim = 0; p = 1) in the Milky Way halo, inside the solar orbit. In both studies, they neglected the strong RM of nearby superbubbles ( 90 rad./m2) and the huge size of Loop I in HI (Heiles et al., 1980; Heiles, 1996a). In Figure 3 of Han et al. (1997), the pulsar RM does not increase with distance from the sun (above 1 kpc), and pulsar RM are not the best indicator of B (Heiles, 1996b).