On the origin of the bi-drifting subpulse phenomenon in pulsars

Abstract

The unique and highly unusual drift feature reported for PSR J0815+0939, wherein one component's subpulses drift in the direction opposite of the general trend, is a veritable challenge to pulsar theory. In this paper, we observationally quantify the drift direction throughout its profile, and find that the second component is the only one that exhibits "bi-drifting", meaning that only second component moves in the direction opposite of the others. We here present a model that shows that the observed bi-drifting phenomenon follows from the insight that the discharging regions, i.e. sparks, do not rotate around the magnetic axis per se, but rather around the point of electric potential extremum at the polar cap: minimum in the pulsar case ($\mathbf{\Omega} \cdot \mathbf{B} < 0$) and maximum in the antipulsar case ($\mathbf{\Omega} \cdot \mathbf{B} > 0$). We show that a purely dipolar surface magnetic field cannot exhibit bi-drifting behaviour, though certain non-dipolar configurations can. We can distinguish two types of solutions, with relatively low ($\sim 10^{12} \,{\rm G}$) and high ($\sim 10^{14} \,{\rm G}$) surface magnetic fields. Depending on the strength of the surface magnetic field, the radius of the curvature of magnetic field lines ranges from $10^{5} \,{\rm cm}$ to $10^{7}\,{\rm cm}$. Pulsar \J~allows us to gain an understanding of the polar-cap conditions essential for plasma generation processes in the inner acceleration region, by linking the observed subpulse shift to the underlying spark motion.