  A single measurement of linear polarization of a nonthermal source provides
direct information about the mean direction and level of ordering of the
magnetic field. Monitoring of the polarization in blazars, combined with
millimeter-wave VLBI imaging in both total and polarized intensity, has the
potential to determine the geometry of the magnetic field. This is a key probe
of the physical processes in the relativistic jet, such as ordered field
components, turbulence, magnetic reconnections, magnetic collimation and
acceleration of the jet flow, particle acceleration, and radiative processes
that produce extremely luminous, highly variable nonthermal emission.
Well-sampled monitoring observations of multi-waveband flux and radio-optical
polarization of blazars show a variety of behavior. In some cases, the observed
polarization patterns appear systematic, while in others randomness dominates.
Explanations involve helical magnetic fields, turbulence, and perhaps particle
acceleration that depends on the angle between the magnetic field and shock
fronts that might be present. Simulations from the author's TEMZ model, with
turbulent plasma crossing a standing conical shock in the jet, show that a
mixture of turbulent and toroidal magnetic field can produce the level of
polarization variability that is observed, even when the two field components
are roughly equal.
