Reversals in the Direction of Polarization Rotation in OJ 287
  We have obtained a smooth time series for the Electric Vector Position Angle
(EVPA) of the blazar OJ 287 at centimeter wavelengths, by making $\pm n\pi$
adjustments to archival values from 1974 to 2016. The data display rotation
reversals in which the EVPA rotates counter-clockwise (CCW) for 180 deg and
then rotates clockwise (CW) by a similar amount. The time scale of the
rotations is a few weeks to a year, and the scale for a double rotation,
including the reversal, is one to three years. We have seen four of these
events in 40 years. A model consisting of two successive outbursts in polarized
flux density, with EVPAs counter-rotating, superposed on a steady polarized
jet, can explain many of the details of the observations. Polarization images
support this interpretation. The model can also help to explain similar events
seen at optical wavelengths. The outbursts needed for the model can be
generated by the super-magnetosonic jet model of Nakamura et al. (2010) and
Nakamura and Meier (2014), which requires a strong helical magnetic field. This
model produces forward and reverse pairs of fast and slow MHD waves, and the
plasma inside the two fast/slow pairs rotates around the jet axis, but in
opposite directions.
