ger reflect the details of the bolides as launched but rather their interactions with the (magnetised) medium through which they flow. This magnetic field behaviour appears to dominate for distances from the launch-point exceeding $\approx$2arcsec.

Very highly polarised emission is observed away from the jet trace on the East side of the source (labelled A in Fig \[fig:fracpolvectors\]). Both @Roberts2008 and @Miller-Jones2008 have drawn attention to these off-ridgeline regions being significantly more polarised than the jet ridgeline itself in VLA data at 15GHz and 8GHz respectively. Our 230GHz data show significantly higher fractional polarisation values (0.6 – 0.7) in these regions. Comparison of all these data suggest the degree and direction of polarisation may change with precession period and possibly also distance from the nucleus.

The interaction of bolides into larger coalescences has been reasoned above to occur where there is a change in polarisation behaviour namely at approximately 0.35arcsec from the nucleus. We note that this coincides with the region reported by @Migliari2002 [@Migliari2005] to show distinct X-ray variability which we suggest arises from shocks formed by the coalescence. Such a mechanism would naturally give rise to the stochastic nature of the X-ray variations reported by @Migliari2002 [@Migliari2005].

![Panel (a) shows a colour scale depicting the fractional polarisation, $p$, of SS433 averaged over the ALMA observing band. Points are shown blanked (grey) wherever the total intensity $I < 5\sigma_I$. Panel (b) shows vectors whose lengths are proportional to $p$ (with the scale indicated by the labelled bar) and whose directions are along the apparent ${\bf B}$-field direction (i.e. rotated by 90$^\circ$ from the ${\bf E}$-vector direction with no correction for Faraday rotation: see text). The vectors are plotted where $I > 5\sigma_I$ and $P > 3\sigma_I$ and are superimposed on a grey-scale of total intensity, as indicated by the wedge labelled in mJy/beam. The inset show