2017ApJ...845...95S

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 the second component moves in the direction opposite of the others. We 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 maximum potential at the polar cap. We show that a purely dipolar surface magnetic field cannot exhibit bi-drifting behavior. though certain non-dipolar configurations can. We can distinguish two types of solutions, with relatively low (∼10¹² G) and high (∼10¹⁴ G) surface magnetic fields. Depending on the strength of the surface magnetic field, the radius of the curvature of magnetic field lines ranges from 10⁵ to 10⁷ cm. Pulsar J0815+0939 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.