Arecibo 430 MHz Pulsar Polarimetry: Faraday Rotation Measures and Morphological Classifications

We have measured Faraday rotation measures (RMs) at Arecibo Observatory for 36 pulsars, 17 of them new. We combine these and earlier measurements to study the Galactic magnetic field and its possible temporal variations. Many RM values have changed significantly on several-year timescales, but these variations probably do not reflect interstellar magnetic field changes. By studying the distribution of pulsar RMs near the plane in conjunction with the new NE2001 electron density model, we note the following structures in the first Galactic longitude quadrant: (1) the local field reversal can be traced as a null in RM in a 0.5 kpc wide strip interior to the Solar circle, extending ~7 kpc around the Galaxy. (2) Steadily increasing RMs in a 1 kpc wide strip interior to the local field reversal, and also in the wedge bounded by 42° < l < 52°, indicate that the large-scale field is approximately steady from the local reversal in to the Sagittarius arm. (3) The RMs in the 1 kpc wide strip interior to the Sagittarius arm indicate another field reversal in this strip. (4) The RMs in a final 1 kpc wide interior strip, straddling the Scutum arm, also support a second field reversal interior to the Sun, between the Sagittarius and Scutum arms. (5) Exterior to the nearby reversal, RMs from 60° < l < 78° show evidence for two reversals, on the near and far side of the Perseus arm. (6) In general, the maxima in the large-scale fields tend to lie along the spiral arms, while the field minima tend to be found between them. We have also determined polarized profiles of 48 pulsars at 430 MHz. We present morphological pulse profile classifications (Rankin 1983) of the pulsars, based on our new measurements and previously published data.