2013MNRAS.433..445R

Arecibo observations of the conal triple pulsar B1918+19 at 0.327 and 1.4 GHz are used to analyse its subpulse behaviour in detail. We confirm the presence of three distinct drift modes (A, B, C) plus a disordered mode (N) and show that they follow one another in specific cycles. Interpreting the pulsar's profile as resulting from a sightline traverse which cuts across an outer cone and tangentially grazes an inner cone, we demonstrate that the phase modulation of the inner cone is locked to the amplitude modulation of the outer cone in all the drift modes. The 9percent nulls are found to be largely confined to the dominant B and N modes, and, in the N mode, create alternating bunches of nulls and emission in a quasi-periodic manner with an averaged fluctuation rate of about 12 rotation periods (P1). We explore the assumption that the apparent drift is the first-order alias of a faster drift of subbeams equally spaced around the cones. This is shown to imply that the drift modes A, B and C have a common circulation time of 12P1 and differ only in the number of subbeams. This time-scale is on the same order as predicted by the classic ExB drift model of Ruderman & Sutherland and also coincides with the N-mode modulation. We therefore arrive at a picture where the circulation speed remains roughly invariant while the subbeams progressively diminish in number from modes A to B to C, and are then re-established during the N mode. We suggest that aliasing combined with subbeam loss may be responsible for the apparently dramatic changes in drift rates in other pulsars.