On plasma rotation and drifting subpulses in pulsars; using aligned pulsar B0826-34 as a voltmeter

TL;DR

This paper derives an exact formula for plasma drift velocity in pulsar polar caps, explaining observed subpulse phenomena and applying it to PSR B0826-34 to constrain potential variations, thus improving pulsar models.

Contribution

It provides the first exact derivation of plasma drift velocity in pulsars, explaining complex subpulse behaviors within the standard model.

Findings

Drift velocity depends on potential variation, not absolute value.

Observed drift rate variations and reversals are explained by the model.

Potential variation in PSR B0826-34 is about 10^{-3} of the vacuum potential.

Abstract

We derive the exact drift velocity of plasma in the pulsar polar cap, in contrast to the order-of-magnitude expressions presented by Ruderman & Sutherland (1975) and generally used throughout the literature. We emphasize that the drift velocity depends not on the absolute value, as is generally used, but on the variation of the accelerating potential across the polar cap. If we assume that drifting subpulses in pulsars are indeed due to this plasma drift, several observed subpulse-drift phenomena that are incompatible with the Ruderman & Sutherland family of models can now be explained: we show that variations of drift rate, outright drift reversals, and the connection between drift rates and mode changes have natural explanations within the frame of the "standard" pulsar model, when derived exactly. We apply this model for drifting subpulses to the case of PSR B0826-34, an aligned pulsar with two separate subpulse-drift regions emitted at two different colatitudes. Careful measurement of the changing and reversing drift rate in each band independently sets limits on the variation of the accelerating potential drop. The derived variation is small, ~10^{-3} times the vacuum potential drop voltage. We discuss the implications of this result for pulsar modeling.