4.3.4 Magnetic-field structure

Very little polarized flux is either predicted or observed to come from the inner region, and no polarized emission is detected from the inner counter-jet. For this reason, the field ratios are essentially unconstrained there (Table 7).

The toroidal field component is the largest over most of the flaring and outer regions, increasing from $\approx$ 0.6 to $\approx$ 0.9 of the total between the inner boundary and the end of the modelled region (Fig. 21). The longitudinal component, conversely, decreases from $\approx$ 0.7 to $\approx$ 0.4 over the same distance. We found no evidence for any variation of the longitudinal/toroidal field ratio across the shear layer. The component ratios for the spine are not well determined (Table 7) and could quite plausibly be identical to those for the shear layer.

**Figure 21:** Grey-scale images of the rms magnitudes of the magnetic field components as fractions of the total field for the SSL model.
Top: radial component $\langle B_r^2 \rangle^{1/2} / B$ ;
centre: toroidal component $\langle B_t^2 \rangle^{1/2} / B$ ;
bottom: longitudinal component $\langle B_l^2 \rangle^{1/2} / B$ .
The arrows mark the location at the edge of the jet where the three field components are roughly equal, as discussed in the text. The radial component is constrained to be zero for the spine and the streamline in the shear layer. The values for the inner region are poorly determined (Table 7).
$\begin{figure}\epsfxsize =8cm \epsffile{bratios.eps}\end{figure}$

Close to the edge of the jet in the flaring region, the radial component becomes appreciable, reaching a maximum of $\approx$ 0.6 at the edge on the outer boundary (marked by the arrows in Fig. 21). Thereafter, it decreases rapidly with distance along the jet, becoming negligible by 9.5 kpc from the nucleus in the SSL model. The radial component, unlike the other two, increases with radius (Fig. 21). As mentioned earlier, this variation is required in order to achieve even a qualitative fit to the observed polarization in the flaring region.