  We present a stationary, axisymmetric, self-similar semi-analytic model of
magnetically dominated jet plasma based on force-free regions of a relativistic
magnetohydrodynamic simulation. We use this model to illustrate how the
composition of relativistic jet plasma can be determined, with special
attention to the example of M87. In particular, we compute synthetic Stokes
maps in e-e+p plasmas with various positron-to-proton ratios using synchrotron
emission models scaling the partial pressure of electrons and positrons
emitting at the observed frequency to the magnetic pressure, taking into
account Faraday rotation and conversion. The lepton-dominated models produce
bilaterally asymmetric radio intensity profiles with strong linear polarization
and Stokes Q and U maps that are bilaterally asymmetric (but strongly up-down
correlated) and antisymmetric (and sometimes up-down anticorrelated),
respectively. The hadronic models produce more centrally brightened intensity
and polarization maps. Circular polarization provides the cleanest
observational tool for distinguishing the plasmas, as it increases outward from
the jet core and central axis for highly ionic plasma, and is suppressed for
pair dominated plasma. We find a measurable degree of circular polarization V/I
of O(10e-3) for sub-equipartition hadronic jet plasmas. Our stationary model
predicts that the intensity-normalized autocorrelation functions of Q and U
increase and decrease with frequency, respectively. On the other hand, the
autocorrelation of V is less sensitive to the frequency. Multi-band
polarimetric observations could therefore be used as a novel probe of the
composition of jet plasma.
