Direct measurements of the Voyager 2 spacecraft in the heliosheath indicate that the surrounding interstellar magnetic field is 4−5 µG strong and oriented at an angle of about 30∘ from the Galactic plane (Opher et al. 2009), probably because the ISM field twists close to the heliosphere. Voyager 1 crossed into interstellar space in 2012 and measured a smooth increase in field strength to 5.62 ± 0.01 µG (Burlaga et al. 2013). This value is very close to those obtained with other methods (see below) and to those in nearby galaxies with a low star-formation rate (Sect. 4.1).
To deduce information about the strength and structure of the Galactic magnetic field, pulsars are ideal objects because their RMs provide field directions at many distances from the Sun (Noutsos 2012). Since most pulsars are concentrated along the Galactic plane, they sample the field in the disk. Combination of RM and DM data of pulsars (Eq. 11), assuming uncorrelated fluctuations, gives an average strength of the local regular field along the line of sight of 2.1 ± 0.3 µG and about 4 µG at 3 kpc Galacto-centric radius (Han et al. 2006). These are upper limits in the case of correlated fluctuations (Beck et al. 2003), while lower limits in the case of anticorrelated fluctuations or field reversals along the line of sight.
From the dispersion of pulsar RMs, the Galactic magnetic field was found to have a significant turbulent component with a mean strength of 5−6 µG (Rand & Kulkarni 1989; Ohno & Shibata 1993; Han et al. 2004). Magnetic turbulence occurs over a large spectrum of scales, with the largest scale determined from pulsar RMs of lturb ≃ 55 pc (Rand & Kulkarni 1989) or lturb ≃ 10−100 pc (Ohno & Shibata 1993). These values are consistent with the size of turbulent cells of d ≃ 50 pc estimated from beam depolarization and Faraday depolarization in external galaxies (Sects. 3.5 and 3.7). However, turbulence scales of only a few parsecs in spiral arms were derived from RM structure functions of polarized background sources (Haverkorn et al. 2008).
Modeling the surveys of the total synchrotron and γ–ray emission from the Milky Way yield field strengths near the Sun of about 5 µG of the isotropic turbulent field, about 2 µG of the anisotropic turbulent field and about 2 µG of the regular field (Orlando & Strong 2013), adding up to a total field strength of about 6 µG. This is in excellent agreement with the Voyager and pulsar RM data (see above) and the Zeeman splitting data of low-density gas clouds (Crutcher et al. 2010). In the inner Galaxy the total field strength is about 10 µG. In the synchrotron filaments near the Galactic Center the total field strength is about 100 µG (Crocker et al. 2010).
Optical polarization data of about 5500 selected stars in the Milky Way yielded the orientation of the large-scale magnetic field near the Sun (Fosalba et al. 2002), which is mostly parallel to the Galactic plane and oriented along the local spiral arm.
The all-sky maps of polarized synchrotron emission at 1.4 GHz from the Milky Way from DRAO and Villa Elisa and at 22.8 GHz from WMAP, and the Effelsberg RM survey of polarized extragalactic sources, were used to model the regular Galactic field (Sun et al. 2008; Sun & Reich 2010). One large-scale field reversal is required at about 1−2 kpc from the Sun towards the Milky Way's center, consistent with pulsar data.
Figure 25. All-sky map of rotation measures in the Milky Way, constructed from the RM data of about 40000 polarized extragalactic sources from the VLA NVSS survey (Taylor et al. 2009) and other catalogs. Red: positive RM, blue: negative RM (from Oppermann et al. 2012).
RM data from pulsars and extragalactic radio sources (Fig. 25) was used to model the Galactic magnetic field (Nota & Katgert 2010; Van Eck et al. 2011; Jansson & Farrar 2012). A large-scale magnetic field reversal appears to be present between the Scutum-Crux-Sagittarius arm and the Carina-Orion arm (Fig. 26). As distances to most pulsars are uncertain, this result should be taken with some caution. The overall structure of the regular field in the disk of the Milky Way is still uncertain (Noutsos 2009). A larger sample of pulsar RM data and improved distance measurements to pulsars are needed. A satisfying explanation for the large-scale reversal in the Milky Way is still lacking. So far no similar reversals have been detected in external galaxies (Sect. 4.11). Possible reasons are:
(1) The reversal in the Milky Way may be restricted to a thin layer near to the plane and therefore hardly visible in the average RM data of external galaxies along the line of sight.
(2) The reversal in the Milky Way may be of limited azimuthal extent and difficult to observe in external galaxies with present-day telescopes.
(3) The reversal in the Milky Way may be part of a disturbed field structure, e.g. due to interaction with the Magellanic clouds, or a relic from seed fields (Moss et al. 2012).
Figure 26. Model of the magnetic field in the Milky Way (vectors), derived from Faraday rotation measures of pulsars and extragalactic sources. Generally accepted results are indicated by yellow vectors, while white vectors are not fully confirmed. The background image shows the distribution of ionized gas delineating the spiral arms (from Jo-Anne Brown, Calgary).
Even less is known about the halo field of the Milky Way. The vertical full equivalent thickness of the synchrotron emission from the Milky Way is about 3 kpc near the Sun (Beuermann et al. 1985), scaled to a distance to the Galactic center of 8.5 kpc, yielding an exponential scale height of Hsyn ≃ 1.5 kpc and at least 6 kpc of the total field, similar to that in external galaxies (Sect. 4.13).
The signs of RMs of extragalactic sources and of pulsars at Galactic longitudes l = 90∘−270∘ are mostly the same above and below the plane (Fig. 25): the magnetic field in the local disk is symmetric, while the RM signs towards the inner Galaxy (l = 270∘−90∘) are opposite above and below the plane. This could be assigned to an odd-symmetry (antisymmetric) halo field (Sun & Reich 2010). However, according to RM data from extragalactic sources, the local regular Galactic field has no significant vertical component towards the northern Galactic pole and only a weak vertical component of Bz ≃ 0.3 µG towards the south (Mao et al. 2010). This is neither consistent with an odd-symmetry nor with an even-symmetry halo field as found in several external galaxies (Sect. 4.7). The halo field of the Milky Way has a more complicated structure than predicted by dynamo models. Modeling the diffuse polarized emission and RMs gave evidence for an X-shaped vertical field component (Jansson & Farrar 2012), similar to that in external galaxies (Sect. 4.13).
In summary, strength, spiral pattern, and vertical extent of the Galactic magnetic field are similar to those in nearby spiral galaxies. Two major differences still need to be understood: the large-scale field reversal(s) and the antisymmetric pattern of the halo field, both of which are not observed in external galaxies. Either our Milky Way is special, or the different observational methods deliver apparently incompatible results.