Inverse Depolarization: A Potential Probe of Internal Faraday Rotation
  and Helical Magnetic Fields in Extragalactic Radio Jets
  Motivated by recent observations that show increasing fractional linear
polarization with increasing wavelength in a small number of optically thin jet
features, i.e. "inverse depolarization", we present a physical model that can
explain this effect and may provide a new and complementary probe of the low
energy particle population and possible helical magnetic fields in
extragalactic radio jets. In our model, structural inhomogeneities in the jet
magnetic field create cancellation of polarization along the line of sight.
Internal Faraday rotation, which increases like wavelength squared, acts to
align the polarization from the far and near sides of the jet, leading to
increased polarization at longer wavelengths. Structural inhomogeneities of the
right type are naturally produced in helical magnetic fields and will also
appear in randomly tangled magnetic fields. We explore both alternatives and
find that, for random fields, the length scale for tangling cannot be too small
a fraction of the jet diameter and still be consistent with the relatively high
levels of fractional polarization observed in these features. We also find that
helical magnetic fields naturally produce transverse structure for inverse
depolarization which may be observable even in partially resolved jets.
