We explore correlations between visual extinction and polarization along the western side of the IRAS Vela Shell using a published polarimetric catalog of several hundreds of objects .
Our extinction maps along this ionization front ( I-front ) find evidence of clumpy structure with typical masses between 1.5 and 6 M _ { \sun } and a mean length scale L \sim 0.47 pc .
The polarimetric data allowed us to investigate the distribution of the local magnetic field in small ( \sim pc ) scales across the I-front .
Using the dispersion of polarization position angles , we find variations in the kinetic-to-magnetic energy density ratio of , at least , one order of magnitude along the I-front , with the magnetic pressure generally dominating over the turbulent motions .
These findings suggest that the magnetic component has a significant contribution to the dynamical balance of this region .
Along the I-front , the mean magnetic field projected on the sky is [ 0.018 \pm 0.013 ] mG .
The polarization efficiency seems to change along the I-front .
We attribute high polarization efficiencies in regions of relatively low extinction to an optimum degree of grain alignment .
Analysis of the mass-to-magnetic flux ratio shows that this quantity is consistent with the subcritical regime ( \lambda < 1 ) , showing that magnetic support is indeed important in the region .
Our data extend the overall \lambda - N ( H _ { 2 } ) relation toward lower density values and show that such trend continues smoothly toward low N ( H _ { 2 } ) values .
This provides general support for the evolution of initially subcritical clouds to an eventual supercritical stage .