On the Excess Dispersion in the Polarization Position Angle of Pulsar Radio Emission

The polarization position angles (PA) of pulsar radio emission occupy adistribution that can be much wider than what is expected from the averagelinear polarization and the off-pulse instrumental noise. Contrary to ourlimited understanding of the emission mechanism, the excess dispersion in PAimplies that pulsar PAs vary in a random fashion. An eigenvalue analysis of themeasured Stokes parameters is developed to determine the origin of the excessPA dispersion. The analysis is applied to sensitive, well-calibratedpolarization observations of PSR B1929+10 and PSR B2020+28. The analysisclarifies the origin of polarization fluctuations in the emission and revealsthat the excess PA dispersion is caused by the isotropic inflation of the datapoint cluster formed by the measured Stokes parameters. The inflation of thecluster is not consistent with random fluctuations in PA, as might be expectedfrom random changes in the orientation of the magnetic field lines in theemission region or from stochastic Faraday rotation in either the pulsarmagnetosphere or the interstellar medium. The inflation of the cluster, andthus the excess PA dispersion, is attributed to randomly polarized radiation inthe received pulsar signal. The analysis also indicates that orthogonalpolarization modes (OPM) occur where the radio emission is heavily modulated.In fact, OPM may only occur where the modulation index exceeds a critical valueof about 0.3.Comment: Accepted for publication in Ap