Polarised Views of the Drifting Subpulse Phenomenon

Abstract

I review recent results concerning the shape of drifting subpulse patterns, and the relationship to model predictions. While a variety of theoretical models exist for drifting subpulses, observers typically think in terms of a spatio-temporal model of circulating beam-lets. Assuming the model is correct, geometric parameters have been inferred and animated "maps" of the beam have been made. However, the model makes very specific predictions about the curvature of the drift bands that have remained largely untested. Work so far in this area indicates that drift bands tend not to follow the prediction, and in some cases disconti-nuities are seen that are suggestive of the superposition of out of phase drift patterns. Recent polarimetric observations also show that the drift patterns in the two orthogonal polarisation modes are offset in phase. In one case the pattern in one of the modes shows a discontinuity suggesting no less than three superposed, out-of-phase drift patterns! I advise caution in the interpretation of observational data in the context of overly simplistic models. 1 THE CAROUSEL MODEL Although the drifting subpulses phenomenon has now been approached from a number of different theoretical perspectives (e.g. Fung et al. 2006), most observational studies have assumed the applicability of the so-called carousel model (Ruderman 1972). This model postulates that the modulations have a spatio-temporal origin: the subpulse structure is due to the passing of the line of sight over multiple "beamlets", while the temporal, pulse-to-pulse modulation is due to the slow circulation of the beamlet system (Figure 1(a)). In the usual physical model, the beamlets consist of radiation beamed tangentially to local magnetic field lines, within "tubes" of plasma, each flowing outward from a localised breakdown ("spark") of a potential gap over the magnetic pole (Ruderman 1972). The circulation of the sparks is attributed to E × B drift. Edwards & Stappers (2002) showed that the carousel model makes very specific predictions regarding the shape of drift bands. Specifically, assuming that the beamlets are uniformly spaced in magnetic azimuth, the variation of subpulse phase with pulse longitude obeys a geometric relationship. This is because it is linearly tied via the number of beamlets (N) to the magnetic azimuth of the observer (ψ), which in turn obeys the following relation: tan ψ = sin φ sin ζ cos ζ sin α − cos φ sin ζ cos α , (1) where φ is pulse longitude, ζ is the angle between the spin axis and the line of sight, and α is the angle between the magnetic and spin axes (see Figure 1(b)). The reader may recognise the similarity between this relation and the common "rotating vector model" for the position angle of linear polarisation, χ. The reason for this similarity is that all of the pertinent angles are related according to the spherical triangle of ⋆