Drifting subpulses and inner acceleration regions in radio pulsars

The classical vacuum gap model of Ruderman & Sutherland, in which spark-associated sub-beams of subpulse emission circulate around the magnetic axis due to the drift of spark plasma filaments, provides a natural and plausible physical mechanism explaining the subpulse drift phenomenon. Moreover, this is the only model with quantitative predictions that can be compared with observations. Recent progress in the analysis of drifting subpulses in pulsars has provided a strong support for this model by revealing a number of sub-beams circulating around the magnetic axis in a manner compatible with theoretical predictions. However, a more detailed analysis revealed that the circulation speed in a pure vacuum gap is too high when compared with observations. Moreover, some pulsars demonstrate significant time variations in the drift rate, including a change of the apparent drift direction, which is obviously inconsistent with the drift scenario in a pure vacuum gap. We attempted to resolve these discrepancies by considering a partial flow of iron ions from the positively charged polar cap, coexisting with the production of outflowing electron-positron plasmas. The model of such a charge-depleted acceleration region is highly sensitive to both the critical ion temperature K (above which ions flow freely with the corotational charge density) and the actual surface temperature Ts of the polar cap, heated by the bombardment of ultra-relativistic charged particles. By fitting the observationally deduced drift-rates to the theoretical values, we managed to estimate polar cap surface temperatures in a number of pulsars. The estimated surface temperatures Ts correspond to a small charge depletion of the order of a few percent of the Goldreich-Julian corotational charge density. Nevertheless, the remaining acceleration potential drop is high enough to discharge through a system of sparks, cycling on and off on natural time-scales described by the Ruderman & Sutherland model. We also argue that if the thermionic electron outflow from the surface of a negatively charged polar cap is slightly below the Goldreich-Julian density, then the resulting small charge depletion will have similar consequences as in the case of the ions outflow. We thus believe that the sparking discharge of a partially shielded acceleration potential drop occurs in all pulsars, with both positively (“pulsars”) and negatively (“anti-pulsars”) charged polar caps.