5. Global structure of the regular magnetic field
In this section we derive the strength and the direction of the regular magnetic field, and we discuss the global properties of these parameters.
Results of the fits presented in Table 4 can be converted into the amplitudes of the regular magnetic field (see Eq. (7)) using the estimates of and given in Table 2. The results are compiled in Table 5 (of course, the pitch angles and phases of individual modes remain the same as given in Table 4). A note of caution is appropriate here: the resulting amplitudes were obtained assuming that is independent of azimuthal angle. For each ring we also give the strength of the regular magnetic field averaged over the azimuth, , where with for the disk and for the halo. Uncertainties were calculated with allowance for errors in both and . The large errors, especially at 12-15 kpc, are mainly due to the errors in .
Table 5. Amplitudes of magnetic modes and the regular magnetic field averaged along azimuth in
In Fig. 7 we show the radial variation of and that of the total and regular magnetic field strengths, B and , obtained from the total nonthermal emission and the observed degree of polarization as described in Sect. 3.1 and given in Table 1. We note that for each ring there is a close agreement between the two values of the regular magnetic field which were obtained from completely independent physical parameters and methods. Between and 15 kpc the exponential radial scale length of the regular magnetic field obtained from the synchrotron emission is .
Apart from the comparison of the ring averages in Fig. 7 one could also compare the azimuthal variation of the transverse magnetic field as given by the fits with the azimuthal distribution of the polarized intensity in each ring. However, as this would require the knowledge of and as functions of azimuthal angle, we postpone this to a later paper.
As can be seen from Table 4 the magnetic fields in the halo and in the disk inside kpc are horizontal. In the halo we have , and , which means that the radial component of the regular magnetic field is directed inwards, with the azimuthal component directed counterclockwise; that is, and - see Eq. (8). Meanwhile, in the disk we have and both are negative together with the pitch angles. Therefore, for -9 kpc the radial field in the disk is directed outwards, with directed clockwise at almost all . We conclude that the regular magnetic fields in the disk and the halo have almost opposite directions everywhere within kpc except in the northwestern part of the ring at -6 kpc.
In Fig. 8 the direction of in each sector is shown for the disk and the halo separately. The length of the vectors is proportional to with scaling factors specified in the caption.
In the outer ring, 12-15 kpc, the magnetic field structure is distorted. The values of and differ considerably from those for and are even positive. Inspection of the polarization map in Fig. 1a confirms that in the northern part the magnetic pattern at these radii is plagued by strong distortions still having a rather large spatial scale.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998