The dispersion of polarization position angles can provide an estimate of the ratio of kinetic energy density to magnetic energy density (ρkin/ρmag) and the local magnetic field strength in the interstellar medium, as first suggested by Chandrasekhar and Fermi (CF). Nevertheless, a relatively large number of data points are necessary in order to obtain a reasonable estimate. We use a recently published catalog of several hundreds of objects toward the western side of IRAS Vela Shell in order to investigate the distribution of the local magnetic field in small (∼pc) scales across an ionization front (I-front). We find variations of ρkin/ρmag of one order magnitude along the I-front, with the magnetic pressure generally dominating over the turbulent motions. In a few cases the polarization angle appears to change randomly, suggesting that the kinetic energy may dominate. These findings suggest that the magnetic component has a significant contribution to the dynamical balance of this region. Along I-front, the mean magnetic field on the plane of the sky is (0.3 ± 0.2) mgauss and the full range of variation is (0.2 - 1.1) mgauss. © 2005 American Institute of Physics.