  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 solar masses and a mean length scale L ~ 0.47pc. The polarimetric data
allowed us to investigate the distribution of the local magnetic field in small
(~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 +/- 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(H2) relation
toward lower density values and show that such trend continues smoothly toward
low N(H2) values. This provides general support for the evolution of initially
subcritical clouds to an eventual supercritical stage.
