Estimating the Evolution of Sparks in the Partially Screened Gap of Pulsars from Subpulse Drifting

A novel scheme has been developed to show that the observed phase behavior associated with subpulse drifting from two pulsars, J1034-3224 and J1720-2933, can be used to obtain the magnetic field configuration in the partially screened gap (PSG). The outflowing plasma along the open magnetic field line region of pulsars is generated as a result of spark discharges in an inner acceleration region (IAR) above the polar cap. The IAR has been modeled as a PSG with a steady supply of positively charged ions emitted from the heated polar cap surface dominated by strong non-dipolar magnetic fields. In a PSG the sparks are tightly packed and constrained to be present along the polar cap boundary. The sparks lag behind the rotation of the star during their lifetimes. As a result, the sparking pattern evolves along two different directions in a clockwise and counterclockwise manner around a stationary central spark and can be associated with the observed phenomenon of subpulse drifting. PSR J1034-3224 has four prominent components and exhibits bi-drifting where alternate components show the opposite sense of drifting, while PSR J1720-2933 has a single component profile and shows systematically coherent drift bands. We show that the differences in their drifting behavior can be directly linked to different natures of the non-dipolar surface magnetic field configurations.