Toward An Empirical Theory of Pulsar Emission VIII: Subbeam Circulation and the Polarization-Modal Structure of Conal Beams

The average polarization properties of conal single and double profiles directly reflect the polarization-modal structure of the emission beams which produce them. Conal component pairs exhibit large fractional linear polarization on their inside edges and virtually complete depolarization on their outside edges; whereas profiles resulting from sightline encounters with the outer conal edge are usually very depolarized. The polarization-modal character of subbeam circulation produces conditions whereby both angular and temporal averaging contribute to this polarization and depolarization. These circumstances combine to require that the circulating subbeam systems which produce conal beams entail paired PPM and SPM emission elements which are offset from each other in both magnetic azimuth and magnetic colatitude. Or, as rotating subbeam systems produce (on average) conal beams, one modal subcone has a little larger (or smaller) radius than the other. However, these PPM and SPM “beamlets” cannot be in azimuthal phase, because both alternately dominate the emission on the extreme outer edge of the conal beam. While this configuration can be deduced from the observations, simulation of this rotating, modal subbeam system reiterates these these conclusions. These circumstances are also probably responsible, along with the usual wavelength dependence of emission height, for the observed spectral decline in aggregate polarization. A clear delineation of the modal polarization topology of the conal beam promises to address fundamental questions about the nature and origin of this modal emission—and the modal parity at the outer beam edges is a fact of considerable significance. The different angular dependences of the modal “beamlets” suggests that the polarization modes are generated via propagation effects. This argument may prove much stronger if the modal emission is fundamentally only partially polarized. Several theories now promise quantitative comparison with the observations. Subject headings: MHD — plasmas — pulsars: general, individual (B0301+19, 0329+54, 0525+21, 0809+74, 0820+02, 0943+10, 1133+16, 1237+25, 1923+04, 2016+28, 2020+28 and 2303+30), radiation mechanism: nonthermal On leave from Physics Department, A405 Cook Bldg., University of Vermont, Burlington, VT 05405 USA; email: [email protected] Current address: Astronomy Department, 601 Campbell Hall, University of California, Berkeley, CA 94720-3411 USA; email: [email protected] The Outer-Edge Depolarization Phenomenon A humble fact about pulsar radio emission, which to our knowledge has attracted virtually no notice or comment, is the following: The extreme outer edges of virtually all conal component pairs are prominently, and apparently accurately, depo-