There are extensive studies of the optical polarization of stars in the Galaxy. The pioneering work of Hiltner (1949) and Hall (1949) was followed by large scale surveys of Behr (1959) and Mathewson and Ford (1970). These data were reanalyzed by Ellis and Axon (1978). The general conclusion is that this method gives us at most information about our local neighborhood. The magnetic field is aligned in general along the galactic plane, but in the direction of l = 45°. Beyond a circle of 600 pc the magnetic field is directed towards l = 70°. This field configuration is attributed to a local bubble or a single loop of a more general field.
The direct mapping of the polarized radio continuum emission gave us insight into the magnetic fields of galactic objects. The early observations by Mayer et al. (1957) were the first to give information about the magnetic field in the Crab Nebula, a supernova remnant. Galactic radio polarization was discovered by Westerhout et al., 1962) and Wielebinski et al., (1962). The surveys of Berkhuijsen and Brouw (1963), Wielebinski and Shakeshaft (1964) and Mathewson and Milne (1965) at 408 MHz show the local fields only. The direction of l = 140° b = 10° is a direction of a unique singularity where we are looking perpendicular to the local magnetic field. Higher frequency surveys (e.g. Spoelstra, 1984; Junkes et al., 1987) show that more distant magnetic fields could be traced at higher radio frequencies.
The studies of Rotation Measures of extragalactic radio sources have given us a some understanding of the large-scale magnetic field of the Galaxy (e.g. Simrad-Normandin et al., 1981). With this method we get information about B|| (field component parallel to line of sight) only. There is a large scale field with numerous 'local' features. The study of RM's should be improved further using a much larger sample of sources. A study of different zones (MacLeod et al., 1988) offers a possibility of understanding some of the details of the magnetic field structure. One of the interesting results from RM studies is that sudden field reversals occur on scales of a few degrees.
Pulsars offer the most direct method of determining of B||. The reason for this is the fact that we can measure both the RM and the Dispersion Measure. From these two pieces of information the value of B|| can be derived. Recent reanalysis of all the available pulsar data by Lyne and Graham Smith (1989) confirmed a magnetic field in the Galaxy of B|| ~ 3µG, directed towards l = 90° (i.e. along the local spiral arm). Sudden field reversals (indicated by high positive and negative adjacent rotation measures) are seen in a number of directions.
The measurements of the Zeeman effect have succeeded in HI clouds (e.g. Verschuur, 1979), in OH molecular clouds (e.g. Crutcher et al., 1987) and more recently in H2O sources (Fiebig & Güsten, 1989). All the Zeeman measurements, maybe the most direct magnetic field determinations, can be made only in a small number of sources. The fields that have been measured are B > 10 µG, with values of ~ 100µG in some objects. In H2O maser regions magnetic field values are in the milligauss range.
All the data discussed so far gives us the picture that the magnetic fields in the disc of the Galaxy is azimuthal. A recent analysis of Vallee (1988) shows that any deviations of pitch angle of the magnetic field, from the spiral arm are slight, possibly less then 6°. Vallée also deduced a field reversal in the Sagittarius arm. This could support the analysis of Sofue and Fujimoto (1983) who claimed that the magnetic field of the Galaxy is bisymmetric.
The field in the center of our Galaxy is in the Z-direction. The earlier 2.8cm observations (Seiradakis et al., 1985) have been substantiated by new 9 mm observations (Reich, 1989). The magnetic field in the central nucleus area runs perpendicular to the galactic plane, which may be a part of a more extended poloidal field. This non-thermal emission has also an anomalous (positive), spectral index (Reich et al., 1988).
A model of the magnetic field in the Galaxy is shown in figure 1. The fields in the disc have a uniform component Bu and a turbulent component Br. Since Bu|| (from pulsar rotation measures) is ~ 3µG, we can expect Bu ~ 5µG. Since Bu ~ Br the total magnetic field in the plane could have the value of Bt ~ 7µG or more.
Figure 1. A model of the magnetic field in the Galaxy.